# Physics 6 HEAT

Physics: 6. Heat

Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier

Questions to make you think

1. The foil and the cotton towel are both at the same temperature – so why does the foil feel colder?

1. A candle needs oxygen to burn, which is why if we want to resuscitate a dying fire we blow on it.
So why does blowing on a candle put it out?

2. Why does it get colder as you climb up a mountain, even though you’re getting closer to the sun?

3. When pouring boiling water into a glass, it is advised to first place a metal spoon in the glass. Why?

1. Why might a glass crack when you pour boiling water into it?

1. If you lick a frozen pipe why will your tongue stick to the pipe?

1. Why are ice cubes much better at cooling a drink that the equal amount of iced water?

2. Why is a scald from steam much more serious that a burn from boiling water?

3. Why do hurricanes pick up energy when they pass over oceans?

4. Why does a glass beaker often crack when you pour in boiling water?

1. How come a shiny material is both a bad absorber of heat and a bad radiator of heat?

2. When stepping out of bed on a cold morning, why does it feel colder if your feet touch say, a marble floor rather than a floor with carpet even though both are at the same temperature?

3. Why do footpaths/ rail-tracks/ bridges have gaps in them (hint: sometimes you see the gaps in footpaths or roads filled with a piece of rubber)?

4. Why does water rise when heated?

5. How does perspiration (sweating) help to keep us cool?

6. Many Arabs wear dark clothing in warm weather, even though dark clothes are better at absorbing heat than white clothes. Any idea why?

7. Heat can be transferred by conduction, convection or radiation. A thermos flask tries to keep hot liquids hot by preventing heat loss. Any idea how it minimises each of these three methods of heat transfer?

8. When cooking potatoes on a gas cooker you put the gas supply onto ‘max’ to boil the water as quickly as possible.

The water is now boiling.

If you turn the gas down will it take longer for the potatoes to cook or will it not make any difference?

1. This all assumes that there is no cover on the saucepan.
Why would a cover change things?
What is the effect of increased pressure on the boiling point of water?

1. Why does increased pressure affect the boiling point of water?

1. How does an Infra-Red thermometer work, i.e. how does it detect the temperature of what you ‘shine’ it at?

Higher Order Questions

1. What property of oven gloves allow a baker to pick up hot bread?

1. Why do metals expand when heated?

1. What is meant by the term ‘the anomaly of water’?

1. Sublimation occurs when a ________ changes directly to a ________ when heated.

2. What is meant by the term ‘latent heat’?

3. Draw a diagram of a cooling curve for wax and indicate on it all the various states of matter.

4. How come when I do this experiment I don’t get a nice graph like this?

5. Can you think of an application of this (a device which is built on this principle?

Can you think of anyone who might be affected by this?

Hurricanes pick up energy while travelling over the ocean.

Water evaporates and in doing so picks up heat energy when going from a liquid to a gas.

Now later on when it condenses from a gas to a liquid it gives out this heat in the form of molecular energy to nearby molecules which presumably heats them up.

Syllabus

OP22   Recall that heat is a form of energy and that it can be converted into other forms of energy

OP23   Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

OP24   Demonstrate the expansion of water on freezing

OP25   Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water

OP26   Investigate the effect of pressure on the boiling point of water

OP27   Explain the difference between heat and temperature

OP28 Carry out experiments that involve changes of state from

i. solid to liquid and liquid to solid

ii. liquid to gas and gas to liquid

OP29   Plot a cooling curve and explain the shape of the curve in terms of latent heat

OP30   Appreciate that all hot bodies radiate heat

OP31   Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water

OP32   Identify good and bad conductors of heat and compare insulating ability of different materials.

Student Notes

 Temperature is a measure of the hotness of an object

‘Hotness’ in turn is a measure of how much the atoms are vibrating or jiggling.

You may not think that all the atoms in every single object jiggle, but there you go; Physics is mad.

Note: You must use the term ‘hotness’ (how many people do you think actually know this?)

So why can’t we say that “the temperature of an object is a measure of how hot or cold an object is”?

Because ‘hot’ is a vague term; does it refer specifically to temperature (jiggliness of the atoms), or to the amount of heat in the object? After all, a litre of water at 1000 Celsius has twice as much heat as half a litre of water at 1000 Celsius. ‘Hotness’ is the physicists’ way of overcoming this potential confusion.
Personally I think we should just stop using the word temperature altogether and stick with jiggliness.

 Heat is a form of energy and it can be converted into other forms of energy

To show that heat is a form of energy you must be able to show that it can do work (because energy is the ability to do work).

Work is done when you cause an object to move.

So we need to show that heat can move something.

Demonstration

Alcohol in a thermometer expands and moves up the glass when heat is applied.

 Solids, liquids and gases expand when heated, and contract when cooled

Solids expand when heated and contract when cooled

Demonstration

1. Heat the brass ball.
2. Note that he ball fits through the ring when the ball is cold but not when hot.

Liquids expand when heated and contract when cooled

Demonstration

1. Connect a glass tube to the top of a beaker of water (use dye to make the water more visible). I use a Bunsen burner instead of a hair-dryer).
2. Note that the water rises up the tube as it gets heated and drops back down as it cools.

Gases expand when heated and contract when cooled

Demonstration

1. Gently heat the flask of air (much better to use a hair dryer in case the flask breaks).
2. Note that bubbles come out of the tube when the flask is heated and as it cools water from the trough rises back up the tube because of the partial vacuum which has formed.

The anomaly (strangeness) of water

Water is an exception to almost all other substances in that when it drops below 4 0C the water actually expands when cooling rather than contracting.

The explanation for this is a little complicated (it has to do with the arrangement of water molecules, but you don’t need to know it for exam purposes).

Demonstration

1. Fill a glass bottle with water and place it in a plastic bag in a freezer.
2. When you take it out the following day the bottle will be broken because the water has expanded on freezing.

(The purpose of the bag was to ensure that all the pieces of glass get taken out).

This is why water pipes sometimes burst in winter causing flooding in a home.

The effect of pressure on the boiling point of water

 Reduced pressure decreases the boiling point of water

Demonstration

1. Suck up water which is at about 80 0C into a syringe so that the syringe is about one-quarter full.
2. Cover the open end (watch out - it’s hot!) and pull back the handle to create a partial vacuum.
3. Result: The water begins to boil!

Explanation

1. The air acts like a blanket which presses down on the water and makes it difficult for the water molecules to leave (‘jump out of’) the liquid and become part of the air.
2. Higher pressure therefore results in a higher boiling point (the molecules need to have more energy/ move more rapidly to make the transition).

Changes of state, the cooling curve and latent heat

1. Heat a tube of wax to a high temperature (so that it is in liquid form) and as it cools note the temperature.
2. Plot a graph of temperature against time.

Result

1. The temperature decreased at a steady rate until (in this case) it reached 43 0C.

This is when the wax began to change state from a liquid to a solid.

1. It remains at this temperature until all the wax has solidified (in this case it took 4 minutes) and after that it began to drop in temperature again as the solid wax cooled down.

Explanation

As the wax changes state from a liquid to a solid it gives out heat without cooling down.

This heat is called latent heat because latent means ‘hidden’ and in this case it is not obvious where the heat is coming from.

 Latent Heat is the heat taken in or given out whenever a substance changes state

Heat Transfer: Conduction, Convection and Radiation

Heat can be transferred in three different ways - conduction, convection and radiation

 Conduction is the method by which heat travels from particle to particle through a solid

 Convection is the transfer of heat through a liquid or gas when the particles move and carry the heat with them

 Radiation is the transfer of heat from a hot object without the need for a medium

All objects radiate heat, but not all substances conduct or convect heat

 Conduction Convection Radiation Solids Yes No Yes Liquids No Yes Yes Gases No Yes Yes

Conduction

To compare the ability of different metals to conduct heat

1. Use the apparatus shown which consists of a piece of timber with four different strips of metal.
2. Place some candle wax at the end of each metal and stand a match in the wax at the end.
3. Light the Bunsen (or candle) under the middle and note the order in which the matches fall.

The match which falls first was standing in the best conductor.

To show that water is a poor conductor of heat

1. Half fill a boiling tube with water and use piece of  metal gauze to hold down the ice.
2. Holding the boiling tube at an angle with a tongs, heat it at the top using the Bunsen burner for a short period of time.

Result: the water at the top boils which the ice at t he bottom stays frozen.

Convection

To demonstrate convection currents in water

1. Use the apparatus shown and drop in some copper sulphate or potassium permanganate to act as a dye.
2. Place the Bunsen under one of the corners and note the movement of the water around the apparatus.

To demonstrate convection currents in air

Cut tissue paper into narrow strips; tie the strips together at one end using the piece of thread and hang them from a retort stand as shown over a hot-plate.

Result: the tissue paper will begin to move as a result of the convection current generated by the hot-plate.

Demonstration

1. Take two identical metal containers and paint one with one black and the other silver.
2. Fill both with hot water.
3. Using a thermometer and stop-watch note which container cools the quickest.
4. The dark container cooled more quickly because it is a better radiator of heat.

Conductors and Insulators

 A conductor is a substance which allows heat to flow through it easily

 An insulator is a substance which does not allow heat to flow through it easily

To compare the insulating ability of different materials

Demonstration

1. Take two identical containers and wrap one in cotton wool.
2. Fill both with hot water.
3. Note which container cools more quickly than the other.
4. The container which cooled more slowly had better insulating material.

Note: There are no maths problems in this chapter.

A thermos flask keeps hot liquids hot but also keeps cold liquids cold.

Can you see why (look at the diagram on the right for a clue)?

Heat Crossword

Across

2.         Gas changing to liquid. (10)

3.         Apparatus used to demonstrate that reduced pressure decreases the boiling point of water. (7)

5.         You find this in most school thermometers. (7)

6.         Visible evidence that gas expands when heated.(7)

7.         A glass bottle of water might do this when the temperature drops below 4 degrees. (5)

9.         Method by which heat travels from particle to particle through a solid. (10)

12.       What type of materials are good absorbers of heat?  (4)

13.       Transfer of heat from a hot object without the need for a medium. (9)

16.       Liquid changing to gas. (7)

17.       What type of conductor of heat is water?  (4)

18.       Energy is defined as the ability to do this. (4)

19.       Solid changing to liquid. (7)

20.       The heat taken in or given out when a substance is changing state (without changing temperature). (6)

21.       What type of materials are poor radiators of heat?  (5)

22.       Liquid changing to solid. (8)

23.       Solids, liquids and gases do this when heated. (6)

Down

1.         It is a substance which does not allow heat to flow through it easily. (9)

2.         It is a substance which allows heat to flow through it easily. (9)

4.         Transfer of heat through a liquid or gas when the particles move and carry the heat with them. (10)

8.         Heat is a form of this. (6)

10.       It's a form of energy. (4)

11.       Solids, liquids and gas do this when cooled. (8)

14.       The effect of increased pressure on the boiling point of water. (9)

15.       The strangeness of water. (7)

16.       used to demonstrate that solids expand when heated. (11)

From Puzzlemaker.com

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ALCOHOL

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Exam Questions

1. [2006]

Define temperature and give a unit used to express temperature

1. [2008][2010]

Give two differences between heat and temperature.

1. [2007][2010 OL][2007 OL]

The diagram shows a piece of equipment that can be used to investigate the effect of heat on a metal.

The ball will pass through the ring when it is cold.

When the ball is heated it will no longer pass through the ring.

1. Explain why the ball does not pass through the ring when it is heated.
2. How would you get the ball to fit through again?
3. What does this investigation tell us about the effect of heat on metals?

1. [2009 OL]

Describe, with the help of a labelled diagram, how you could carry out an experiment to show that metals expand when heated.

Use the following headings: Labelled diagram, Equipment, Procedure, Result.

1. [2011 OL]

Electric cables made from copper sag or droop in summer as shown in the diagram.

What property of metals does this demonstrate?

1. [2012]

The damage to the railway tracks shown in this image was caused by an environmental factor.

Name the factor and explain how it caused the damage.

1. [2008 OL][2012 OL]

The diagram shows a round-bottomed flask full of coloured water.

1. What would you expect to notice if the flask is heated gently?
2. Give a reason why this should happen.
3. Why is coloured water used during this investigation?
4. A measuring instrument used in this laboratory is based on this behaviour of liquids.

Name this instrument.

1. [2010 OL][2007 OL]

In an investigation to see the effect heating had on gases, a student heated a round-bottomed flask containing air using a hairdryer as shown in the diagram.

1. What would you expect the student to have seen when the flask was heated?
2. What conclusion can you draw from this investigation?

1. [2010]

The apparatus shown in the diagram was used to investigate the expansion and contraction of a gas.

1. What is observed when the flask is heated?
3. What is observed when the flask is allowed to cool?
4. Explain what you observe as the flask cools.
1. [2006]

Describe an experiment to show the expansion of water when it freezes.

You may include a labelled diagram if you wish.

1. [2009]

The boiling point of water can be determined using the apparatus shown in the diagram.

1. Why are boiling (anti-bumping) chips added to the water?
2. At what temperature does water boil, at standard (normal) atmospheric pressure?
3. What effect does the raising of pressure have on the boiling point of water?
4. What effect does the lowering of pressure have on the boiling point of water?

1.  [2011 OL][2007 OL]

Heat is transferred in different ways.

In each case use a word from the list on the right to correctly complete each sentence below.

1. Heat travels through solids by ___________________.
2. Heat travels through liquids and gases by ___________________.
3. Heat travels from the Sun to the Earth by ___________________.

1. [2006 OL]

Choose the method of heat transfer that occurs in each of the following.

1. The boiling of water in a kettle. ___________________________
2. The heating of the Earth by the Sun. ___________________________

1. [2006]

Name the mode of heat transfer from the hot liquid, through the spoon, to the hand.

1. [2009]

Copper, aluminium and iron rods are set-up as shown in the diagram. A metal ball is attached by wax to the end of each rod. Hot water is poured into the beaker. The ball falls from the copper rod first. What conclusion can be drawn from this observation?

1. [2011 OL]

A student set up the investigation shown in the diagram.

The apparatus consisted of a metal box that was filled with boiling water.

A piece of candle wax was placed on the top end of each rod.

The piece of wax on top of the copper rod melted first and the piece of wax on top of the glass rod melted last.

1. What does the result of this investigation tell us about copper, iron and glass?
2. Why was it important to use rods of the same length and thickness and that they dipped into the boiling water to the same depth?

1. [2011]

The experiment shown in the photograph was set up by a student.

1. What changes take place to the water in the beakers A and B as time passes?
2. Explain why these changes occur.
3. What instrument would be used, in this experiment, to monitor the changes?
4. Name a material to replace copper in this experiment that will not allow these changes to occur

1. [2007]
1. What does the experiment shown in the diagram tell us about the transfer of heat energy in water?
2. If you wanted to warm all of the water why would the bottom of the test tube be the best place to heat with the Bunsen flame?

1. [2006]

Heat moves in liquids by convection. Give one difference between convection and conduction.

1. [2009]

The photograph shows a solar panel being installed.

Water passing through the panel is heated by the sun.

1. How does heat from the sun travel, through the vacuum of space, to the earth?

Insulation

1. [2009 OL]

The diagram shows two metal cans equal in size and filled with the same amount of water at 100 °C. Can A is wrapped in cotton wool and can B has no wrapping.

1. After 15 minutes, which can, A or B, would you expect to have the higher temperature?

1. [2012]

An experiment was performed to investigate the effect of pressure on the boiling point of water.

The data from the experiment is given in the table below.

 Pressure (kPa) 100 120 140 160 180 200 Temperature (0C) 100 105 109 14 119 124

1. Draw a graph of pressure against temperature using the grid below.

1. What two pieces of information can be drawn from the graph about the relationship between the boiling point of water and pressure?

1. What effect would reducing the pressure on water below normal atmospheric pressure, about 100kPa, have on its boiling point?

Latent Heat

1. [2006]

The graph is a cooling curve. The substance used in this experiment was naphthalene. Naphthalene has a melting point of 80 0C.

The rate of heat loss was constant throughout the experiment.

1. What is happening to the naphthalene on the horizontal section of the graph?
2. What is the heat loss on the horizontal section of the curve called?

1. [2010]

A substance that is a solid at room temperature was heated above its melting point and then allowed to cool at a steady rate. The temperature was taken at regular intervals. The data is in the graph.

Why is there no drop in temperature between B and C?

1. [2008]

A pupil heated some lauric acid, which is a solid at room temperature, until it turned into a liquid.

The lauric acid was then allowed to cool at a uniform rate. The temperature of the lauric acid was taken every minute.

The data from this experiment is given in the table.

 Temperature (0C) 75 64 54 43 43 43 43 43 32 22 10 Time (minutes) 0 1 2 3 4 5 6 7 8 9 10

1. Draw a graph, using this data, of temperature against time (x-axis) in the grid provided below.

1. Explain the shape of the graph that you obtain.

1. Use the graph to estimate the melting point of lauric acid.

Exam Solutions

1. Temperature is a measure of the hotness of an object.

The unit of temperature is degrees Celsius (0C).

1. Heat is a form of energy (and temperature isn’t).

Temperature is a measure of the hotness of an object (and heat isn’t).

1.
1. Ball expanded / increased in volume
2. Cool the ball / stated cooling method
3. (Metals) expand when heated / contract when cooled
1. Diagram of ball and ring apparatus.

Ball fits through ring when both are at room temperature.

Heat the ball over a Bunsen burner for one minute.

Result: the ball will no longer fit through the ring.

1. Metals expand when heated
2. Heat: solids expand when heated
3.
1. Water rises up the tube
2. Water (liquid) expands (when heated)
3. Easier to see
4. The thermometer
1.
1. Bubbles of air coming from the mouth of the flask into water trough
2. Air (gas) expands when heated
2.
1. Bubbles of air come out of the bottom of the glass tube.
2. The air in the flask expanded.
3. The bubbles stop and water rises up the glass tube.
4. Air in flask contracted therefore the air pressure is less than atmospheric pressure.
1. Fill a bottle with water

Put the bottle in a freezer

The bottle bursts after a few hours.

1.
1. Chips help to ensure that all the water boils at the same temperature{I know – not on the syllabus so shouldn’t have appeared!}
2. 1000 C
3. It raises the boiling point
4. It lowers the boiling point

1.
1. Conduction
2. Convection
1.
1. Convection
1. Conduction
2. Copper is the best conductor
3.
1. Metals conduct (carry) heat, copper is the best conductor and glass is the worst.
2. So that the experiment would be a fair test / compare like with like

1.
1. The temperature of water in A decreases and temperature of water in B increases.
2. Copper conducts (transfers) heat
3.  Thermometer
4. Wood/ plastic/ named plastic e.g. nylon…

1.
1. Water is a poor conductor of heat
2. Because hot water rises (note that ‘heat rises’ alone gets no marks)
1. Convection: particles of liquid move carrying the heat with them.

Conduction: heat is transferred from one particle to another without any overall movement of the particles themselves.

1.
2. Advantage: to reduce fuel bills, reduce CO2 emissions, it is renewable.

Disadvantage: expensive to set up,  less heat is absorbed in winter or on cloudy days.

1. Can A will have a higher temperature after 15 minutes.
2. Because it is insulated so it loses heat more slowly.

1.
1. See graph
2. Boiling point increases with pressure/

The increase in boiling point is proportional to the increase in pressure.

1. It would reduce the boiling point.

Latent Heat

1.
1. It is changing from a liquid to a solid
2. Latent heat of fusion
1. It is changing state (freezing).
2.
1. See graph
2. Initially the temperature of the liquid falls until it reaches 43 0C.

Here it changes state from a liquid to a solid.

Then the solid cools down.

1. 43 0C

Other test questions

1. Draw a labelled diagram of an apparatus used to demonstrate convection in a liquid.

2. Draw a labelled diagram of the apparatus used to compare the conductivity of different metals. How does it work?

3. Give two uses of a bimetallic strip.

4. Draw a labelled diagram of the apparatus you would use to show that water is a poor conductor of heat.

5. How would you demonstrate that solids expand when heated and contract when cooled? Include a fully labelled diagram.

6. How would you demonstrate that liquids expand when heated and contract when cooled? Include a fully labelled diagram.

7. How would you demonstrate that gas expands when heated and contract when cooled? Include a fully labelled diagram.

8. Why is water not a suitable liquid for use in thermometers?

9. Define conduction.

10. Define convection.

11. A student filled a boiling tube with water and placed an ice-cube at the bottom by keeping metal weight on top of it. The student then heated the top of the boiling tube until the water was boiling at the top.
1. What was the student trying to investigate?
 Object Method of Heat Transfer A spoon A saucepan of water Space
1. Why did he put a weight on the cube of ice?
2. Why was it important that the weight was made of metal?
3. What did the student notice?

1. Fill in the table with the words convection, conduction, or radiation where appropriate

2. Why is the heating element of a kettle placed near the bottom?

3. What is the main way in which heat is transferred when the water in an electric kettle is heated?

4. What is the main way in which heat is transferred when the Sun heats the Earth?

Teaching Heat

Syllabus

OP22   Understand that heat is a form of energy and that it can be converted into other forms of energy

OP23   Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

OP24   Demonstrate the expansion of water on freezing

OP25   Measure the temperature of various solids and liquids at, above and below room temperature; determine the            melting point of ice and the boiling point of water

OP26   Investigate the effect of pressure on the boiling point of water

OP27   Explain the difference between heat and temperature

OP28   Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

OP29   Plot a cooling curve and explain the shape of the curve in terms of latent heat

OP30   Understand that all hot bodies radiate heat

OP31   Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation;             investigate conduction and convection in water

OP32   Identify good and bad conductors of heat and compare insulating ability of different materials.

This chapter contains over 15 Experiments / Demonstrations!

It could easily take you four week unless you organise it properly.

Adequate organisation will also help the weaker students considerably.

To enable this I have put together multiple sets of experiments into appropriate boxes.

This enables student to carry out up to three experiments in a single class period.

For example the three experiments, expansion of solids, liquids and gas, can be carried out in about 7 minutes each, and the students can rotate from one to the other after a set period of time (I allow them 10 minutes – if they’re finished early they get to write it up in their blue lab-books).

To facilitate this I have included four sets of each of the three experiments in the relevant tray (to accommodate 24 students, working in pairs).

Therefore at any one time there should be four pairs of students carrying out the same experiment (ideally all on the same row).

To ensure smooth running the students should be familiar with the experiment in advance.

A similar approach can be used for the other experiments.

Finally, bear in mind that there are so many Experiments/Demonstrations in this chapter it must be horrendous for any student - particularly the weaker ones – to remember what each one is for.

One way to alleviate their suffering is to summarise all experiments on one A3 page using diagrams.

I have got my students to do this and have kept some of the neatest for photocopying.

The trays for these are available in the resource room.

Please do not mix up the contents of the various trays.

Please stack the trays back as you found them; they are designed to stack on top of each other if arranged appropriately.

OP22: Understand that heat is a form of energy and that it can be converted into other forms of energy

1. Understand that heat is a form of energy
• It’s best to remind students that all matter is made up of atoms, and that heat is best understood in terms of the movement of these atoms.
• Remember to show that something is a form of energy we need to show that it can move something (from our definitions; energy is the ability to do work, and work in turn is force × distance). Now how can we show that heat can move something (think boiling water)?

1. Understand that heat can be converted into other forms of energy

You can heat a wire until it gives off light

OP23: Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

• There are four sets of each of these experiments which allows a total of 12 experiments (24 students) to be active at any one time. Students rotate after 10 minutes (see introduction above).
• Warn students not to leave hot apparatus lying around their bench; a metal at a temperature of 200 0C looks just like one which is at room temperature!
• For all three expansions remember that you must also observe and record contraction upon cooling down.
• Experiments 2.1, 2.2 and 2.3 are mandatory and must be written up.

2.1 Expansion of solids when heated and contraction when cooled

As described in the text book.

Because the ball comes off its chain regularly (due to expansion of the links when heated) I have opted to have the ball and ring on separate rods.

If possible, encourage the students to figure out for themselves what to do with the equipment, rather than telling them straight off.

It is not necessary to let the ball get red-hot.

Note that the wooden handles sometimes come off the metal rods; they should still be in the relevant box so please don’t lose them.

I have included a set of bimetallic strips in the resource box - not actually on the syllabus still worth investigating.

2.2 Expansion of liquid when heated and contraction when cooled

Here we use boiling tubes with thistle funnels and Bunsen burners.

Warn the students to have the Bunsen on a low flame.

Put a little food colouring or copper-sulphate in the water for visibility.

Don’t fill the tube completely; leave about 1 cm of air at the top – it’s a little more impressive.

The flask is heated and the water gets replaced by each new pair of students (wooden tongs supplied).

2.3 Expansion of gas when heated and contraction when cooled

Round-bottom flasks and hair dryers are in the resource boxed.

Bear in mind that the bubbles of air come out of the tube fairly slowly.

The advantage of this is that there is no fear of the flask imploding.

Be sure to ask the students to observe what happens when the hair dryer is turned off and the flask is cooled.

If using a Bunsen burner only allow it to heat the flask (to bubble) for 10 seconds – explain why.

You should use a round-bottomed flask to reduce the risk of implosion; the strain is shared equally by all points.

Incidentally one of the first major improvements in commercial airline safety was when an engineer realised that many previous tragedies were caused by the square windows cracking at the corners. Now all windows in airplanes are curved.

Demo: Attach a balloon to the top of a boiling tube and heat with a bunsen burner. The balloon inflates!

OP24: Demonstrate the expansion of water on freezing

A nice variation on the standard is to get two different types of lucozade bottles; plastic (from the shop) and glass (from a pub). Why is the glass bottle more likely to burst?

Make sure to put them inside a tied plastic bag or container to prevent broken glass becoming a hazard.

Note that there is no reference to why water expands when it goes below four degrees.

Check with your friendly chemistry teacher for an explanation.

It would be nice to think that the students would demand an explanation for this, but don’t hold your breath.

OP25: Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water

• Students should have already used a thermometer to measure room temperature in first year.

• There is a skill to reading the thermometer; it takes time for heat from the boiling water to transfer through the glass in the thermometer and from there to the alcohol inside, so you may well find that the reading on the thermometer continues to go up even after it has been removed from the boiling water.

This in itself is a valuable lesson and should be covered.

• Note that the temperature of boiling water depends on atmospheric pressure, among other things (see the extract at the end, taken from the book The Golem).

OP26: Investigate the effect of pressure on the boiling point of water

• There is a set of large syringes available in the resource-box.
• Suck up water which is at about 80 0C so that the syringe is about one-quarter full.
• Now cover the open end (watch out- it’s hot!) and pull back the handle to create a partial vacuum; the water boils!
• Make sure students are familiar with the text-book version which is a lot clumsier.
• It would be nice if students were expected to know why the water boils at a reduced pressure (or even that they would ask themselves). I explain it in terms of the air acting like a blanket which presses down on the water and makes it difficult for the water molecules to leave the liquid and become part of the air. Higher pressure therefore results in a higher boiling point (the molecules need to have more energy/ move more rapidly to make the transition).

This is the principle behind the pressure cooker (does anyone use these anymore?) Increased pressure increases the boiling point of water in the cooker so water can reach a temperature of 160 0C and therefore the food cooks more quickly.

OP27: Explain the difference between heat and temperature

Heat is a form of energy while temperature is a measure of the hotness or coldness of that object.

An alternative way to think of temperature is to say that “the temperature of an object is a number (on some manmade scale) that indicates the hotness of the object”.

Hotness’ in turn is a measure of the kinetic energy of the molecules of the material.

So why can’t we say that “the temperature of an object is a measure of how hot or cold an object is”?

‘Hot’ is a vague term. Does it refer specifically to temperature, or to the amount of heat in the object?

After all, a litre of water at 1000 Celsius has twice as much heat as half a litre of water at 1000 Celsius.

Hotness’ is the physicists’ way of overcoming this potential confusion.

Damn tricky for a junior cert student to understand this subtle difference.

It may help to note that heat can be transferred but temperature can’t.

OP28: Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

You can use water, butter or acetic acid for the solid to liquid change, while water will suffice for liquid to gas.

OP29: Plot a cooling curve and explain the shape of the curve in terms of latent heat

Best done with a data-logger, but you need to play with these in advance.

I still haven’t come across ‘the best way’ to this, or the best material to use.

OP30: Understand that all hot bodies radiate heat

This is actually misleading in that the word hot is a very subjective term. In actual fact all objects radiate heat, because all objects have heat energy (their molecules vibrate at all temperatures above absolute zero). There are plenty of videos about this on YouTube if you want to check it out.

Look up infra-red images on google-images. Many students find it difficult to accept that ‘cold’ objects radiate heat.

Purchase an infra-red thermometer and use it to measure the temperature of various objects including ice. The troops find it fascinating.

OP31: Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water

There are four sets of each of experiments 3.1, 3.2 and 3.3

1. To show that some metals are better conductors than others
2. To demonstrate that water is a bad conductor
3. To demonstrate convection in water

These allow for a total of 12 experiments (24 students) to be active at any one time. Students rotate after 10 minutes (see introduction above).

3.4 and 3.5 are quick demonstrations while 3.6 can take the form of a full class investigation.

1. To demonstrate convection in air
2. To demonstrate the transfer of heat by radiation
3. Identify good and bad conductors of heat and compare insulating ability of different materials.

3.1 To demonstrate transfer of heat by conduction, and to show that some metals are better conductors than others

Here we use a circular piece of timber with four different metals set on top, meeting in the middle. Candle wax is placed at the end of each.

A bunsen burner causes the wax to melt too quickly; a night-light will do fine.

It’s actually quite impressive: students can follow the progress of the heat as it travels along the metal, melting the old wax along the way. Bear in mind that it takes five minutes for the apparatus to cool down sufficiently for the wax to solidify.

Students should be encouraged to write up their experiments or answer other questions while waiting. Alternatively put a few drops of water in the indentations instead of wax, and wait for them to boil. The correct order should be Copper, Aluminium, Brass, Iron.

It’s worth pointing out that this is a different apparatus to that shown in the text-books but that they should be familiar with both.

3.2 To demonstrate that water is a bad conductor

Half fill a boiling tube with water and use piece of  metal gauze to hold down the ice.

Holding the boiling tube at an angle with a tongs, heat it at the top using the Bunsen burner for a short period of time.

Ensure boiling tube is facing away from the student.

A nice tip for obtaining crushed ice is to use a plastic bottle to freeze the water and then hammer the bejaysus out of the bottle when you remove it from the freezer. Then cut the bottle in half to access the crushed ice.

3.3 To demonstrate convection in water

Relatively straight-forward.

Use copper sulphate or potassium permanganate and drop it in via a small funnel.

Make sure the Bunsen is under one of the corners and not under the middle.

3.4 To demonstrate convection in air

Cut tissue paper into narrow strips; tie the strips together at one end using the piece of thread. Then hang them from a retort stand over a hot-plate.

3.5 To demonstrate the transfer of heat by radiation

Strictly speaking the syllabus doesn’t specify that the student must know that bright surfaces are poor absorbers and emitters of radiation compared to dark surfaces, only that we need to demonstrate the transfer of heat by radiation.

With the foil flat on the desk, hold the thermometer 50cm. away from the flame and note the temperature

Now hold the foil between the flame and the thermometer and again note the temperature.

OP32: Identify good and bad conductors of heat and compare insulating ability of different materials.

• Straightforward experiment in theory but very problematic in practice.
• It may well be that the class are getting fed up with the chapter at this stage and there isn’t a heck of a lot happening in this one to excite them.
• It’s also quite likely to produce contradictory results if the initial conditions are not exactly the same, e.g. same mass of water and same initial temperature.
• It may be that it works better as a demo.
• It’s also one which is ideal for data-loggers.
• There is plenty of scope here for discussing the concept of a fair experiment; same quantity of water, same initial temperature, same mass of material etc.

The following table may help the troops to revise this section:

 Conduction Convection Radiation Solid Liquid Gas

When discussing each experiment ask the class if it is reasonable to assume that only one form of heat transfer is occurring in each case.

Why can we ignore the other forms?

Technically conduction can and does occur in liquids and gases, but its effect is negligible.

Provisional approach for teaching this chapter

If you break up the chapter into the following topics it should in theory take a total of 8 class periods (5 singles and  2 doubles), or 2 weeks.

This may be a bit ambitious and doesn’t take into account time spent correcting homework.

It certainly will not work if the teacher is unfamiliar with all the apparatus in advance.

Single period

• Understand that heat is a form of energy and that it can be converted into other forms of energy
• Explain the difference between heat and temperature

Double period

• Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled
• Demonstrate the expansion of water on freezing

Single period

• Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water
• Investigate the effect of pressure on the boiling point of water

Single period

Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

Single period

Plot a cooling curve and explain the shape of the curve in terms of latent heat

Double period

• Understand that all hot bodies radiate heat
• Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water
 Conduction Convection Radiation Solid Liquid Gas

Single period

Identify good and bad conductors of heat and compare insulating ability of different materials.

Physics: 6. Heat

Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier

Questions to make you think

1. The foil and the cotton towel are both at the same temperature – so why does the foil feel colder?

1. A candle needs oxygen to burn, which is why if we want to resuscitate a dying fire we blow on it.
So why does blowing on a candle put it out?

2. Why does it get colder as you climb up a mountain, even though you’re getting closer to the sun?

3. When pouring boiling water into a glass, it is advised to first place a metal spoon in the glass. Why?

1. Why might a glass crack when you pour boiling water into it?

1. If you lick a frozen pipe why will your tongue stick to the pipe?

1. Why are ice cubes much better at cooling a drink that the equal amount of iced water?

2. Why is a scald from steam much more serious that a burn from boiling water?

3. Why do hurricanes pick up energy when they pass over oceans?

4. Why does a glass beaker often crack when you pour in boiling water?

1. How come a shiny material is both a bad absorber of heat and a bad radiator of heat?

2. When stepping out of bed on a cold morning, why does it feel colder if your feet touch say, a marble floor rather than a floor with carpet even though both are at the same temperature?

3. Why do footpaths/ rail-tracks/ bridges have gaps in them (hint: sometimes you see the gaps in footpaths or roads filled with a piece of rubber)?

4. Why does water rise when heated?

5. How does perspiration (sweating) help to keep us cool?

6. Many Arabs wear dark clothing in warm weather, even though dark clothes are better at absorbing heat than white clothes. Any idea why?

7. Heat can be transferred by conduction, convection or radiation. A thermos flask tries to keep hot liquids hot by preventing heat loss. Any idea how it minimises each of these three methods of heat transfer?

8. When cooking potatoes on a gas cooker you put the gas supply onto ‘max’ to boil the water as quickly as possible.

The water is now boiling.

If you turn the gas down will it take longer for the potatoes to cook or will it not make any difference?

1. This all assumes that there is no cover on the saucepan.
Why would a cover change things?
What is the effect of increased pressure on the boiling point of water?

1. Why does increased pressure affect the boiling point of water?

1. How does an Infra-Red thermometer work, i.e. how does it detect the temperature of what you ‘shine’ it at?

Higher Order Questions

1. What property of oven gloves allow a baker to pick up hot bread?

1. Why do metals expand when heated?

1. What is meant by the term ‘the anomaly of water’?

1. Sublimation occurs when a ________ changes directly to a ________ when heated.

2. What is meant by the term ‘latent heat’?

3. Draw a diagram of a cooling curve for wax and indicate on it all the various states of matter.

4. How come when I do this experiment I don’t get a nice graph like this?

5. Can you think of an application of this (a device which is built on this principle?

Can you think of anyone who might be affected by this?

Hurricanes pick up energy while travelling over the ocean.

Water evaporates and in doing so picks up heat energy when going from a liquid to a gas.

Now later on when it condenses from a gas to a liquid it gives out this heat in the form of molecular energy to nearby molecules which presumably heats them up.

Syllabus

OP22   Recall that heat is a form of energy and that it can be converted into other forms of energy

OP23   Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

OP24   Demonstrate the expansion of water on freezing

OP25   Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water

OP26   Investigate the effect of pressure on the boiling point of water

OP27   Explain the difference between heat and temperature

OP28 Carry out experiments that involve changes of state from

i. solid to liquid and liquid to solid

ii. liquid to gas and gas to liquid

OP29   Plot a cooling curve and explain the shape of the curve in terms of latent heat

OP30   Appreciate that all hot bodies radiate heat

OP31   Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water

OP32   Identify good and bad conductors of heat and compare insulating ability of different materials.

Student Notes

 Temperature is a measure of the hotness of an object

‘Hotness’ in turn is a measure of how much the atoms are vibrating or jiggling.

You may not think that all the atoms in every single object jiggle, but there you go; Physics is mad.

Note: You must use the term ‘hotness’ (how many people do you think actually know this?)

So why can’t we say that “the temperature of an object is a measure of how hot or cold an object is”?

Because ‘hot’ is a vague term; does it refer specifically to temperature (jiggliness of the atoms), or to the amount of heat in the object? After all, a litre of water at 1000 Celsius has twice as much heat as half a litre of water at 1000 Celsius. ‘Hotness’ is the physicists’ way of overcoming this potential confusion.
Personally I think we should just stop using the word temperature altogether and stick with jiggliness.

 Heat is a form of energy and it can be converted into other forms of energy

To show that heat is a form of energy you must be able to show that it can do work (because energy is the ability to do work).

Work is done when you cause an object to move.

So we need to show that heat can move something.

Demonstration

Alcohol in a thermometer expands and moves up the glass when heat is applied.

 Solids, liquids and gases expand when heated, and contract when cooled

Solids expand when heated and contract when cooled

Demonstration

1. Heat the brass ball.
2. Note that he ball fits through the ring when the ball is cold but not when hot.

Liquids expand when heated and contract when cooled

Demonstration

1. Connect a glass tube to the top of a beaker of water (use dye to make the water more visible). I use a Bunsen burner instead of a hair-dryer).
2. Note that the water rises up the tube as it gets heated and drops back down as it cools.

Gases expand when heated and contract when cooled

Demonstration

1. Gently heat the flask of air (much better to use a hair dryer in case the flask breaks).
2. Note that bubbles come out of the tube when the flask is heated and as it cools water from the trough rises back up the tube because of the partial vacuum which has formed.

The anomaly (strangeness) of water

Water is an exception to almost all other substances in that when it drops below 4 0C the water actually expands when cooling rather than contracting.

The explanation for this is a little complicated (it has to do with the arrangement of water molecules, but you don’t need to know it for exam purposes).

Demonstration

1. Fill a glass bottle with water and place it in a plastic bag in a freezer.
2. When you take it out the following day the bottle will be broken because the water has expanded on freezing.

(The purpose of the bag was to ensure that all the pieces of glass get taken out).

This is why water pipes sometimes burst in winter causing flooding in a home.

The effect of pressure on the boiling point of water

 Reduced pressure decreases the boiling point of water

Demonstration

1. Suck up water which is at about 80 0C into a syringe so that the syringe is about one-quarter full.
2. Cover the open end (watch out - it’s hot!) and pull back the handle to create a partial vacuum.
3. Result: The water begins to boil!

Explanation

1. The air acts like a blanket which presses down on the water and makes it difficult for the water molecules to leave (‘jump out of’) the liquid and become part of the air.
2. Higher pressure therefore results in a higher boiling point (the molecules need to have more energy/ move more rapidly to make the transition).

Changes of state, the cooling curve and latent heat

1. Heat a tube of wax to a high temperature (so that it is in liquid form) and as it cools note the temperature.
2. Plot a graph of temperature against time.

Result

1. The temperature decreased at a steady rate until (in this case) it reached 43 0C.

This is when the wax began to change state from a liquid to a solid.

1. It remains at this temperature until all the wax has solidified (in this case it took 4 minutes) and after that it began to drop in temperature again as the solid wax cooled down.

Explanation

As the wax changes state from a liquid to a solid it gives out heat without cooling down.

This heat is called latent heat because latent means ‘hidden’ and in this case it is not obvious where the heat is coming from.

 Latent Heat is the heat taken in or given out whenever a substance changes state

Heat Transfer: Conduction, Convection and Radiation

Heat can be transferred in three different ways - conduction, convection and radiation

 Conduction is the method by which heat travels from particle to particle through a solid

 Convection is the transfer of heat through a liquid or gas when the particles move and carry the heat with them

 Radiation is the transfer of heat from a hot object without the need for a medium

All objects radiate heat, but not all substances conduct or convect heat

 Conduction Convection Radiation Solids Yes No Yes Liquids No Yes Yes Gases No Yes Yes

Conduction

To compare the ability of different metals to conduct heat

1. Use the apparatus shown which consists of a piece of timber with four different strips of metal.
2. Place some candle wax at the end of each metal and stand a match in the wax at the end.
3. Light the Bunsen (or candle) under the middle and note the order in which the matches fall.

The match which falls first was standing in the best conductor.

To show that water is a poor conductor of heat

1. Half fill a boiling tube with water and use piece of  metal gauze to hold down the ice.
2. Holding the boiling tube at an angle with a tongs, heat it at the top using the Bunsen burner for a short period of time.

Result: the water at the top boils which the ice at t he bottom stays frozen.

Convection

To demonstrate convection currents in water

1. Use the apparatus shown and drop in some copper sulphate or potassium permanganate to act as a dye.
2. Place the Bunsen under one of the corners and note the movement of the water around the apparatus.

To demonstrate convection currents in air

Cut tissue paper into narrow strips; tie the strips together at one end using the piece of thread and hang them from a retort stand as shown over a hot-plate.

Result: the tissue paper will begin to move as a result of the convection current generated by the hot-plate.

Demonstration

1. Take two identical metal containers and paint one with one black and the other silver.
2. Fill both with hot water.
3. Using a thermometer and stop-watch note which container cools the quickest.
4. The dark container cooled more quickly because it is a better radiator of heat.

Conductors and Insulators

 A conductor is a substance which allows heat to flow through it easily

 An insulator is a substance which does not allow heat to flow through it easily

To compare the insulating ability of different materials

Demonstration

1. Take two identical containers and wrap one in cotton wool.
2. Fill both with hot water.
3. Note which container cools more quickly than the other.
4. The container which cooled more slowly had better insulating material.

Note: There are no maths problems in this chapter.

A thermos flask keeps hot liquids hot but also keeps cold liquids cold.

Can you see why (look at the diagram on the right for a clue)?

Heat Crossword

Across

2.         Gas changing to liquid. (10)

3.         Apparatus used to demonstrate that reduced pressure decreases the boiling point of water. (7)

5.         You find this in most school thermometers. (7)

6.         Visible evidence that gas expands when heated.(7)

7.         A glass bottle of water might do this when the temperature drops below 4 degrees. (5)

9.         Method by which heat travels from particle to particle through a solid. (10)

12.       What type of materials are good absorbers of heat?  (4)

13.       Transfer of heat from a hot object without the need for a medium. (9)

16.       Liquid changing to gas. (7)

17.       What type of conductor of heat is water?  (4)

18.       Energy is defined as the ability to do this. (4)

19.       Solid changing to liquid. (7)

20.       The heat taken in or given out when a substance is changing state (without changing temperature). (6)

21.       What type of materials are poor radiators of heat?  (5)

22.       Liquid changing to solid. (8)

23.       Solids, liquids and gases do this when heated. (6)

Down

1.         It is a substance which does not allow heat to flow through it easily. (9)

2.         It is a substance which allows heat to flow through it easily. (9)

4.         Transfer of heat through a liquid or gas when the particles move and carry the heat with them. (10)

8.         Heat is a form of this. (6)

10.       It's a form of energy. (4)

11.       Solids, liquids and gas do this when cooled. (8)

14.       The effect of increased pressure on the boiling point of water. (9)

15.       The strangeness of water. (7)

16.       used to demonstrate that solids expand when heated. (11)

From Puzzlemaker.com

M C P M I C A A S O W M G K G

X L O Q K L D X Y T G Y T V M

S I O N C Z F U R M S N Y A B

M B R O D D E L I O P R U V U

B G H G N U C R N U W L U O L

A O S A N O C B G I W F K B O

L Q S R T J W T E J C A C O M

N W Q F L U A D I A P K S E Q

W T E N R A G X Y O I N T U U

C O N D E N S I N G N E Q O I

C L S L J F D F G I U O M W N

V U F C K H B P Y U M Y L U F

U W Y O M X X A M W B O A D Y

Z S O P O T G E Y I Q D H A P

E S E L B B U B P V D Y M B B

ALCOHOL

BUBBLES

BURST

CONDENSING

CONDUCTION

SYRINGE

Exam Questions

1. [2006]

Define temperature and give a unit used to express temperature

1. [2008][2010]

Give two differences between heat and temperature.

1. [2007][2010 OL][2007 OL]

The diagram shows a piece of equipment that can be used to investigate the effect of heat on a metal.

The ball will pass through the ring when it is cold.

When the ball is heated it will no longer pass through the ring.

1. Explain why the ball does not pass through the ring when it is heated.
2. How would you get the ball to fit through again?
3. What does this investigation tell us about the effect of heat on metals?

1. [2009 OL]

Describe, with the help of a labelled diagram, how you could carry out an experiment to show that metals expand when heated.

Use the following headings: Labelled diagram, Equipment, Procedure, Result.

1. [2011 OL]

Electric cables made from copper sag or droop in summer as shown in the diagram.

What property of metals does this demonstrate?

1. [2012]

The damage to the railway tracks shown in this image was caused by an environmental factor.

Name the factor and explain how it caused the damage.

1. [2008 OL][2012 OL]

The diagram shows a round-bottomed flask full of coloured water.

1. What would you expect to notice if the flask is heated gently?
2. Give a reason why this should happen.
3. Why is coloured water used during this investigation?
4. A measuring instrument used in this laboratory is based on this behaviour of liquids.

Name this instrument.

1. [2010 OL][2007 OL]

In an investigation to see the effect heating had on gases, a student heated a round-bottomed flask containing air using a hairdryer as shown in the diagram.

1. What would you expect the student to have seen when the flask was heated?
2. What conclusion can you draw from this investigation?

1. [2010]

The apparatus shown in the diagram was used to investigate the expansion and contraction of a gas.

1. What is observed when the flask is heated?
3. What is observed when the flask is allowed to cool?
4. Explain what you observe as the flask cools.
1. [2006]

Describe an experiment to show the expansion of water when it freezes.

You may include a labelled diagram if you wish.

1. [2009]

The boiling point of water can be determined using the apparatus shown in the diagram.

1. Why are boiling (anti-bumping) chips added to the water?
2. At what temperature does water boil, at standard (normal) atmospheric pressure?
3. What effect does the raising of pressure have on the boiling point of water?
4. What effect does the lowering of pressure have on the boiling point of water?

1.  [2011 OL][2007 OL]

Heat is transferred in different ways.

In each case use a word from the list on the right to correctly complete each sentence below.

1. Heat travels through solids by ___________________.
2. Heat travels through liquids and gases by ___________________.
3. Heat travels from the Sun to the Earth by ___________________.

1. [2006 OL]

Choose the method of heat transfer that occurs in each of the following.

1. The boiling of water in a kettle. ___________________________
2. The heating of the Earth by the Sun. ___________________________

1. [2006]

Name the mode of heat transfer from the hot liquid, through the spoon, to the hand.

1. [2009]

Copper, aluminium and iron rods are set-up as shown in the diagram. A metal ball is attached by wax to the end of each rod. Hot water is poured into the beaker. The ball falls from the copper rod first. What conclusion can be drawn from this observation?

1. [2011 OL]

A student set up the investigation shown in the diagram.

The apparatus consisted of a metal box that was filled with boiling water.

A piece of candle wax was placed on the top end of each rod.

The piece of wax on top of the copper rod melted first and the piece of wax on top of the glass rod melted last.

1. What does the result of this investigation tell us about copper, iron and glass?
2. Why was it important to use rods of the same length and thickness and that they dipped into the boiling water to the same depth?

1. [2011]

The experiment shown in the photograph was set up by a student.

1. What changes take place to the water in the beakers A and B as time passes?
2. Explain why these changes occur.
3. What instrument would be used, in this experiment, to monitor the changes?
4. Name a material to replace copper in this experiment that will not allow these changes to occur

1. [2007]
1. What does the experiment shown in the diagram tell us about the transfer of heat energy in water?
2. If you wanted to warm all of the water why would the bottom of the test tube be the best place to heat with the Bunsen flame?

1. [2006]

Heat moves in liquids by convection. Give one difference between convection and conduction.

1. [2009]

The photograph shows a solar panel being installed.

Water passing through the panel is heated by the sun.

1. How does heat from the sun travel, through the vacuum of space, to the earth?

Insulation

1. [2009 OL]

The diagram shows two metal cans equal in size and filled with the same amount of water at 100 °C. Can A is wrapped in cotton wool and can B has no wrapping.

1. After 15 minutes, which can, A or B, would you expect to have the higher temperature?

1. [2012]

An experiment was performed to investigate the effect of pressure on the boiling point of water.

The data from the experiment is given in the table below.

 Pressure (kPa) 100 120 140 160 180 200 Temperature (0C) 100 105 109 14 119 124

1. Draw a graph of pressure against temperature using the grid below.

1. What two pieces of information can be drawn from the graph about the relationship between the boiling point of water and pressure?

1. What effect would reducing the pressure on water below normal atmospheric pressure, about 100kPa, have on its boiling point?

Latent Heat

1. [2006]

The graph is a cooling curve. The substance used in this experiment was naphthalene. Naphthalene has a melting point of 80 0C.

The rate of heat loss was constant throughout the experiment.

1. What is happening to the naphthalene on the horizontal section of the graph?
2. What is the heat loss on the horizontal section of the curve called?

1. [2010]

A substance that is a solid at room temperature was heated above its melting point and then allowed to cool at a steady rate. The temperature was taken at regular intervals. The data is in the graph.

Why is there no drop in temperature between B and C?

1. [2008]

A pupil heated some lauric acid, which is a solid at room temperature, until it turned into a liquid.

The lauric acid was then allowed to cool at a uniform rate. The temperature of the lauric acid was taken every minute.

The data from this experiment is given in the table.

 Temperature (0C) 75 64 54 43 43 43 43 43 32 22 10 Time (minutes) 0 1 2 3 4 5 6 7 8 9 10

1. Draw a graph, using this data, of temperature against time (x-axis) in the grid provided below.

1. Explain the shape of the graph that you obtain.

1. Use the graph to estimate the melting point of lauric acid.

Exam Solutions

1. Temperature is a measure of the hotness of an object.

The unit of temperature is degrees Celsius (0C).

1. Heat is a form of energy (and temperature isn’t).

Temperature is a measure of the hotness of an object (and heat isn’t).

1.
1. Ball expanded / increased in volume
2. Cool the ball / stated cooling method
3. (Metals) expand when heated / contract when cooled
1. Diagram of ball and ring apparatus.

Ball fits through ring when both are at room temperature.

Heat the ball over a Bunsen burner for one minute.

Result: the ball will no longer fit through the ring.

1. Metals expand when heated
2. Heat: solids expand when heated
3.
1. Water rises up the tube
2. Water (liquid) expands (when heated)
3. Easier to see
4. The thermometer
1.
1. Bubbles of air coming from the mouth of the flask into water trough
2. Air (gas) expands when heated
2.
1. Bubbles of air come out of the bottom of the glass tube.
2. The air in the flask expanded.
3. The bubbles stop and water rises up the glass tube.
4. Air in flask contracted therefore the air pressure is less than atmospheric pressure.
1. Fill a bottle with water

Put the bottle in a freezer

The bottle bursts after a few hours.

1.
1. Chips help to ensure that all the water boils at the same temperature{I know – not on the syllabus so shouldn’t have appeared!}
2. 1000 C
3. It raises the boiling point
4. It lowers the boiling point

1.
1. Conduction
2. Convection
1.
1. Convection
1. Conduction
2. Copper is the best conductor
3.
1. Metals conduct (carry) heat, copper is the best conductor and glass is the worst.
2. So that the experiment would be a fair test / compare like with like

1.
1. The temperature of water in A decreases and temperature of water in B increases.
2. Copper conducts (transfers) heat
3.  Thermometer
4. Wood/ plastic/ named plastic e.g. nylon…

1.
1. Water is a poor conductor of heat
2. Because hot water rises (note that ‘heat rises’ alone gets no marks)
1. Convection: particles of liquid move carrying the heat with them.

Conduction: heat is transferred from one particle to another without any overall movement of the particles themselves.

1.
2. Advantage: to reduce fuel bills, reduce CO2 emissions, it is renewable.

Disadvantage: expensive to set up,  less heat is absorbed in winter or on cloudy days.

1. Can A will have a higher temperature after 15 minutes.
2. Because it is insulated so it loses heat more slowly.

1.
1. See graph
2. Boiling point increases with pressure/

The increase in boiling point is proportional to the increase in pressure.

1. It would reduce the boiling point.

Latent Heat

1.
1. It is changing from a liquid to a solid
2. Latent heat of fusion
1. It is changing state (freezing).
2.
1. See graph
2. Initially the temperature of the liquid falls until it reaches 43 0C.

Here it changes state from a liquid to a solid.

Then the solid cools down.

1. 43 0C

Other test questions

1. Draw a labelled diagram of an apparatus used to demonstrate convection in a liquid.

2. Draw a labelled diagram of the apparatus used to compare the conductivity of different metals. How does it work?

3. Give two uses of a bimetallic strip.

4. Draw a labelled diagram of the apparatus you would use to show that water is a poor conductor of heat.

5. How would you demonstrate that solids expand when heated and contract when cooled? Include a fully labelled diagram.

6. How would you demonstrate that liquids expand when heated and contract when cooled? Include a fully labelled diagram.

7. How would you demonstrate that gas expands when heated and contract when cooled? Include a fully labelled diagram.

8. Why is water not a suitable liquid for use in thermometers?

9. Define conduction.

10. Define convection.

11. A student filled a boiling tube with water and placed an ice-cube at the bottom by keeping metal weight on top of it. The student then heated the top of the boiling tube until the water was boiling at the top.
1. What was the student trying to investigate?
 Object Method of Heat Transfer A spoon A saucepan of water Space
1. Why did he put a weight on the cube of ice?
2. Why was it important that the weight was made of metal?
3. What did the student notice?

1. Fill in the table with the words convection, conduction, or radiation where appropriate

2. Why is the heating element of a kettle placed near the bottom?

3. What is the main way in which heat is transferred when the water in an electric kettle is heated?

4. What is the main way in which heat is transferred when the Sun heats the Earth?

Teaching Heat

Syllabus

OP22   Understand that heat is a form of energy and that it can be converted into other forms of energy

OP23   Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

OP24   Demonstrate the expansion of water on freezing

OP25   Measure the temperature of various solids and liquids at, above and below room temperature; determine the            melting point of ice and the boiling point of water

OP26   Investigate the effect of pressure on the boiling point of water

OP27   Explain the difference between heat and temperature

OP28   Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

OP29   Plot a cooling curve and explain the shape of the curve in terms of latent heat

OP30   Understand that all hot bodies radiate heat

OP31   Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation;             investigate conduction and convection in water

OP32   Identify good and bad conductors of heat and compare insulating ability of different materials.

This chapter contains over 15 Experiments / Demonstrations!

It could easily take you four week unless you organise it properly.

Adequate organisation will also help the weaker students considerably.

To enable this I have put together multiple sets of experiments into appropriate boxes.

This enables student to carry out up to three experiments in a single class period.

For example the three experiments, expansion of solids, liquids and gas, can be carried out in about 7 minutes each, and the students can rotate from one to the other after a set period of time (I allow them 10 minutes – if they’re finished early they get to write it up in their blue lab-books).

To facilitate this I have included four sets of each of the three experiments in the relevant tray (to accommodate 24 students, working in pairs).

Therefore at any one time there should be four pairs of students carrying out the same experiment (ideally all on the same row).

To ensure smooth running the students should be familiar with the experiment in advance.

A similar approach can be used for the other experiments.

Finally, bear in mind that there are so many Experiments/Demonstrations in this chapter it must be horrendous for any student - particularly the weaker ones – to remember what each one is for.

One way to alleviate their suffering is to summarise all experiments on one A3 page using diagrams.

I have got my students to do this and have kept some of the neatest for photocopying.

The trays for these are available in the resource room.

Please do not mix up the contents of the various trays.

Please stack the trays back as you found them; they are designed to stack on top of each other if arranged appropriately.

OP22: Understand that heat is a form of energy and that it can be converted into other forms of energy

1. Understand that heat is a form of energy
• It’s best to remind students that all matter is made up of atoms, and that heat is best understood in terms of the movement of these atoms.
• Remember to show that something is a form of energy we need to show that it can move something (from our definitions; energy is the ability to do work, and work in turn is force × distance). Now how can we show that heat can move something (think boiling water)?

1. Understand that heat can be converted into other forms of energy

You can heat a wire until it gives off light

OP23: Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled

• There are four sets of each of these experiments which allows a total of 12 experiments (24 students) to be active at any one time. Students rotate after 10 minutes (see introduction above).
• Warn students not to leave hot apparatus lying around their bench; a metal at a temperature of 200 0C looks just like one which is at room temperature!
• For all three expansions remember that you must also observe and record contraction upon cooling down.
• Experiments 2.1, 2.2 and 2.3 are mandatory and must be written up.

2.1 Expansion of solids when heated and contraction when cooled

As described in the text book.

Because the ball comes off its chain regularly (due to expansion of the links when heated) I have opted to have the ball and ring on separate rods.

If possible, encourage the students to figure out for themselves what to do with the equipment, rather than telling them straight off.

It is not necessary to let the ball get red-hot.

Note that the wooden handles sometimes come off the metal rods; they should still be in the relevant box so please don’t lose them.

I have included a set of bimetallic strips in the resource box - not actually on the syllabus still worth investigating.

2.2 Expansion of liquid when heated and contraction when cooled

Here we use boiling tubes with thistle funnels and Bunsen burners.

Warn the students to have the Bunsen on a low flame.

Put a little food colouring or copper-sulphate in the water for visibility.

Don’t fill the tube completely; leave about 1 cm of air at the top – it’s a little more impressive.

The flask is heated and the water gets replaced by each new pair of students (wooden tongs supplied).

2.3 Expansion of gas when heated and contraction when cooled

Round-bottom flasks and hair dryers are in the resource boxed.

Bear in mind that the bubbles of air come out of the tube fairly slowly.

The advantage of this is that there is no fear of the flask imploding.

Be sure to ask the students to observe what happens when the hair dryer is turned off and the flask is cooled.

If using a Bunsen burner only allow it to heat the flask (to bubble) for 10 seconds – explain why.

You should use a round-bottomed flask to reduce the risk of implosion; the strain is shared equally by all points.

Incidentally one of the first major improvements in commercial airline safety was when an engineer realised that many previous tragedies were caused by the square windows cracking at the corners. Now all windows in airplanes are curved.

Demo: Attach a balloon to the top of a boiling tube and heat with a bunsen burner. The balloon inflates!

OP24: Demonstrate the expansion of water on freezing

A nice variation on the standard is to get two different types of lucozade bottles; plastic (from the shop) and glass (from a pub). Why is the glass bottle more likely to burst?

Make sure to put them inside a tied plastic bag or container to prevent broken glass becoming a hazard.

Note that there is no reference to why water expands when it goes below four degrees.

Check with your friendly chemistry teacher for an explanation.

It would be nice to think that the students would demand an explanation for this, but don’t hold your breath.

OP25: Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water

• Students should have already used a thermometer to measure room temperature in first year.

• There is a skill to reading the thermometer; it takes time for heat from the boiling water to transfer through the glass in the thermometer and from there to the alcohol inside, so you may well find that the reading on the thermometer continues to go up even after it has been removed from the boiling water.

This in itself is a valuable lesson and should be covered.

• Note that the temperature of boiling water depends on atmospheric pressure, among other things (see the extract at the end, taken from the book The Golem).

OP26: Investigate the effect of pressure on the boiling point of water

• There is a set of large syringes available in the resource-box.
• Suck up water which is at about 80 0C so that the syringe is about one-quarter full.
• Now cover the open end (watch out- it’s hot!) and pull back the handle to create a partial vacuum; the water boils!
• Make sure students are familiar with the text-book version which is a lot clumsier.
• It would be nice if students were expected to know why the water boils at a reduced pressure (or even that they would ask themselves). I explain it in terms of the air acting like a blanket which presses down on the water and makes it difficult for the water molecules to leave the liquid and become part of the air. Higher pressure therefore results in a higher boiling point (the molecules need to have more energy/ move more rapidly to make the transition).

This is the principle behind the pressure cooker (does anyone use these anymore?) Increased pressure increases the boiling point of water in the cooker so water can reach a temperature of 160 0C and therefore the food cooks more quickly.

OP27: Explain the difference between heat and temperature

Heat is a form of energy while temperature is a measure of the hotness or coldness of that object.

An alternative way to think of temperature is to say that “the temperature of an object is a number (on some manmade scale) that indicates the hotness of the object”.

Hotness’ in turn is a measure of the kinetic energy of the molecules of the material.

So why can’t we say that “the temperature of an object is a measure of how hot or cold an object is”?

‘Hot’ is a vague term. Does it refer specifically to temperature, or to the amount of heat in the object?

After all, a litre of water at 1000 Celsius has twice as much heat as half a litre of water at 1000 Celsius.

Hotness’ is the physicists’ way of overcoming this potential confusion.

Damn tricky for a junior cert student to understand this subtle difference.

It may help to note that heat can be transferred but temperature can’t.

OP28: Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

You can use water, butter or acetic acid for the solid to liquid change, while water will suffice for liquid to gas.

OP29: Plot a cooling curve and explain the shape of the curve in terms of latent heat

Best done with a data-logger, but you need to play with these in advance.

I still haven’t come across ‘the best way’ to this, or the best material to use.

OP30: Understand that all hot bodies radiate heat

This is actually misleading in that the word hot is a very subjective term. In actual fact all objects radiate heat, because all objects have heat energy (their molecules vibrate at all temperatures above absolute zero). There are plenty of videos about this on YouTube if you want to check it out.

Look up infra-red images on google-images. Many students find it difficult to accept that ‘cold’ objects radiate heat.

Purchase an infra-red thermometer and use it to measure the temperature of various objects including ice. The troops find it fascinating.

OP31: Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water

There are four sets of each of experiments 3.1, 3.2 and 3.3

1. To show that some metals are better conductors than others
2. To demonstrate that water is a bad conductor
3. To demonstrate convection in water

These allow for a total of 12 experiments (24 students) to be active at any one time. Students rotate after 10 minutes (see introduction above).

3.4 and 3.5 are quick demonstrations while 3.6 can take the form of a full class investigation.

1. To demonstrate convection in air
2. To demonstrate the transfer of heat by radiation
3. Identify good and bad conductors of heat and compare insulating ability of different materials.

3.1 To demonstrate transfer of heat by conduction, and to show that some metals are better conductors than others

Here we use a circular piece of timber with four different metals set on top, meeting in the middle. Candle wax is placed at the end of each.

A bunsen burner causes the wax to melt too quickly; a night-light will do fine.

It’s actually quite impressive: students can follow the progress of the heat as it travels along the metal, melting the old wax along the way. Bear in mind that it takes five minutes for the apparatus to cool down sufficiently for the wax to solidify.

Students should be encouraged to write up their experiments or answer other questions while waiting. Alternatively put a few drops of water in the indentations instead of wax, and wait for them to boil. The correct order should be Copper, Aluminium, Brass, Iron.

It’s worth pointing out that this is a different apparatus to that shown in the text-books but that they should be familiar with both.

3.2 To demonstrate that water is a bad conductor

Half fill a boiling tube with water and use piece of  metal gauze to hold down the ice.

Holding the boiling tube at an angle with a tongs, heat it at the top using the Bunsen burner for a short period of time.

Ensure boiling tube is facing away from the student.

A nice tip for obtaining crushed ice is to use a plastic bottle to freeze the water and then hammer the bejaysus out of the bottle when you remove it from the freezer. Then cut the bottle in half to access the crushed ice.

3.3 To demonstrate convection in water

Relatively straight-forward.

Use copper sulphate or potassium permanganate and drop it in via a small funnel.

Make sure the Bunsen is under one of the corners and not under the middle.

3.4 To demonstrate convection in air

Cut tissue paper into narrow strips; tie the strips together at one end using the piece of thread. Then hang them from a retort stand over a hot-plate.

3.5 To demonstrate the transfer of heat by radiation

Strictly speaking the syllabus doesn’t specify that the student must know that bright surfaces are poor absorbers and emitters of radiation compared to dark surfaces, only that we need to demonstrate the transfer of heat by radiation.

With the foil flat on the desk, hold the thermometer 50cm. away from the flame and note the temperature

Now hold the foil between the flame and the thermometer and again note the temperature.

OP32: Identify good and bad conductors of heat and compare insulating ability of different materials.

• Straightforward experiment in theory but very problematic in practice.
• It may well be that the class are getting fed up with the chapter at this stage and there isn’t a heck of a lot happening in this one to excite them.
• It’s also quite likely to produce contradictory results if the initial conditions are not exactly the same, e.g. same mass of water and same initial temperature.
• It may be that it works better as a demo.
• It’s also one which is ideal for data-loggers.
• There is plenty of scope here for discussing the concept of a fair experiment; same quantity of water, same initial temperature, same mass of material etc.

The following table may help the troops to revise this section:

 Conduction Convection Radiation Solid Liquid Gas

When discussing each experiment ask the class if it is reasonable to assume that only one form of heat transfer is occurring in each case.

Why can we ignore the other forms?

Technically conduction can and does occur in liquids and gases, but its effect is negligible.

Provisional approach for teaching this chapter

If you break up the chapter into the following topics it should in theory take a total of 8 class periods (5 singles and  2 doubles), or 2 weeks.

This may be a bit ambitious and doesn’t take into account time spent correcting homework.

It certainly will not work if the teacher is unfamiliar with all the apparatus in advance.

Single period

• Understand that heat is a form of energy and that it can be converted into other forms of energy
• Explain the difference between heat and temperature

Double period

• Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled
• Demonstrate the expansion of water on freezing

Single period

• Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water
• Investigate the effect of pressure on the boiling point of water

Single period

Carry out experiments that involve changes of state from

1. solid to liquid and liquid to solid
2. liquid to gas and gas to liquid

Single period

Plot a cooling curve and explain the shape of the curve in terms of latent heat

Double period

• Understand that all hot bodies radiate heat
• Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water
 Conduction Convection Radiation Solid Liquid Gas

Single period

Identify good and bad conductors of heat and compare insulating ability of different materials.