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Cambridge IGCSE0625

Thermal properties and temperature

Physics 0625 Chapter Notes

What this chapter covers

Thermal properties and temperature - Thermal expansion of solids, liquids and gasesThermal properties and temperature - Specific heat capacityThermal properties and temperature - Melting, boiling and evaporation
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Thermal properties and temperature notes

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In this chapter

  1. 1Expansion of solids, liquids and gases
  2. 2Everyday applications and consequences
  3. 3Why a heated gas or liquid expands
  4. 4Solids, liquids and gases compared
  5. 5Temperature and internal energy
  6. 6Temperature and particle energy
  7. 7Specific heat capacity
  8. 8Using the equation
  9. 9Measuring specific heat capacity
  10. 10Melting and boiling
  11. 11Condensation and solidification
  12. 12Evaporation and cooling
  13. 13Boiling and evaporation compared
  14. 14What changes the rate of evaporation
  15. 15Cooling an object by evaporation

1. Expansion of solids, liquids and gases

Solids, liquids and gases all expand when they are heated.

When a liquid expands, its volume increases. The same liquid now fills more space, so it is less dense.

For a gas, the expansion is described at constant pressure. The pressure stays the same while the volume increases.

Examiner insight

“A common error was answers referring to increasing pressure, despite the question stating constant pressure.” “Only stronger candidates recognised that mercury, like virtually all substances, will expand in all three states of matter when heated.”

Worked example 11 mark

What happens when the temperature of a liquid increases?

A The mass of the liquid increases, making the liquid less dense. · B The mass of the liquid increases, making the liquid more dense. · C The volume of the liquid increases, making the liquid less dense. · D The volume of the liquid increases, making the liquid more dense.

  1. 1

    C: the volume of the liquid increases, making the liquid less dense. [1]

2. Everyday applications and consequences

Expansion has everyday consequences. Metal rails expand on a hot day, so small gaps are left between them. Without the gaps the track would buckle.

Overhead cables also expand in hot conditions. If they are too long, a lorry might hit them.

A liquid-in-glass thermometer is an application. It relies on the expansion of the liquid. The glass also expands, but significantly less than the liquid.

Examiner insight

“When the example chosen was the thermal expansion of railway tracks it was necessary to state the consequent problem. Candidates who referred to the buckling of the track in some way were awarded credit.” “A common misconception was that materials that expand do not return to their original size or shape.”

Worked example 11 mark

Why are small gaps left between the metal rails of a railway track?

A to allow for expansion of the rails on a hot day · B to allow for contraction of the rails on a hot day · C to allow for expansion of the rails on a cold day · D to allow for contraction of the rails on a cold day

  1. 1

    A: to allow for expansion of the rails on a hot day. [1]

3. Why a heated gas or liquid expands

Heated particles have more kinetic energy and move faster. A heated gas or liquid expands because its particles move apart.

The same particles now fill a bigger volume, so the gas or liquid is less dense. The particles themselves do not expand.

Examiner insight

“Many candidates incorrectly described particles expanding and particles becoming less dense.” “Candidates should be careful not to describe molecules as being less dense or expanding.”

Worked example 12 marks

A flame under the balloon heats the air in the balloon. Hot air inside the balloon rises to the top of the balloon.

Explain why the hot air rises.

  1. 1

    (heated air) expands OR air particles move apart [1]

  2. 2

    (heated air) is less dense [1]

4. Solids, liquids and gases compared

In a solid the particles are closely packed in a regular arrangement. They vibrate about fixed positions. In a gas the particles are further apart, with no force, or less force, between them.

The force of attraction between solid particles is greater than the force between gas particles. It is therefore easier to increase the separation of gas particles.

So for the same rise in temperature, a gas expands the most and a solid the least. A liquid lies between them.

Examiner insight

“The greater force of attraction between solid molecules than between gas molecules was seen in most answers. The fact that it is therefore easier to increase the separation of gas molecules was almost always missed.” “Candidates generally either got both (i) and (ii) correct or incorrect, suggesting that they knew the correct states of matter to select so they could compare the extremes of expansion but were unsure of the order.”

Worked example 12 marks

Describe, in terms of particles and their motion, two ways in which a gas differs from a liquid.

  1. 1

    (gas particles are) further apart OR (particles) fill the container [1]

  2. 2

    (gas particles have) greater kinetic energy [1]

  3. 3

    Any two points score. The third point credited is: no/less force between (gas) particles

Recap

  • Solids, liquids and gases expand when heated.
  • A liquid that expands has a bigger volume, so it is less dense.
  • Rails have gaps for expansion on a hot day. Without them the track would buckle.
  • Hot cables expand, so cables that are too long might be hit by a lorry.
  • A liquid-in-glass thermometer relies on the liquid expanding more than the glass.
  • Extended: particles move further apart. They do not expand.
  • Extended: a gas expands the most and a solid the least. It is easier to increase the separation of gas particles.

5. Temperature and internal energy

A rise in the temperature of an object increases its internal energy.

If the temperature falls, the internal energy falls.

Examiner insight

“When thinking about thermal energy being stored in molecules, candidates should take care to be specific about the type of energy store, and not just say that energy is gained by molecules.”

Worked example 11 mark

The temperature of a copper block increases. Which energy store in the block must increase?

A internal · B elastic · C gravitational potential · D chemical

  1. 1

    A: the internal energy of the block increases. [1]

6. Temperature and particle energy

A rise in temperature means an increase in the average kinetic energy of all the particles in the object.

The internal energy of an object is linked to the average kinetic energy of its particles. At a lower temperature, the average kinetic energy of the particles is lower.

Examiner insight

“Some candidates referred to kinetic energy without clarifying that this meant kinetic energy of the particles.” “Candidates needed a clearer understanding that the internal energy of a gas is linked to the average kinetic energy of the particles in the gas.”

Worked example 12 marks

Explain, in terms of particles, why the internal energy of the gas decreases when the gas is at a lower temperature.

  1. 1

    kinetic energy of the particles decreases [1]

  2. 2

    average kinetic energy of the particles decreases [1]

7. Specific heat capacity

Specific heat capacity is the energy required per unit mass per unit temperature increase.

For the same mass and the same energy, a large temperature rise means a small specific heat capacity.

c = (Δ E)/mΔθ

Δ E = mcΔθ

Examiner insight

“In the definition required for this answer, it was necessary to refer to unit mass (since the word ‘specific’ refers to 1 kg) and a unit temperature change.” “Candidates still gained credit when they used terms such as unit mass, but they should be encouraged to be precise to show that it is the energy required to raise the temperature not just to change the temperature.”

Worked example 11 mark

900 kJ of energy is transferred to a mass of water. The temperature of the water increases from 20 °C to 45 °C. The specific heat capacity of water is 4200 J / (kg °C). Which mass of water is heated?

A 0.0086 kg · B 5.4 kg · C 8.6 kg · D 11 kg

  1. 1

    Δ E = 900 000 J and Δθ = 45 - 20 = 25 °C

  2. 2

    m = (Δ E)/cΔθ = (900 000)/(4200 × 25) = 8.6 kg, so C [1]

8. Using the equation

Use Δ E = mcΔθ with the temperature change Δθ: final temperature minus initial temperature.

Put the mass in kg when c is in J / (kg °C).

A heater of power P transfers energy E = Pt in time t. Put the time in seconds.

Δ E = mcΔθ

P = E/t

Examiner insight

“Some weaker candidates used time instead of temperature change in their calculation.” “This was a well answered question with relatively few candidates failing to notice that the mass was given in grams whereas the specific heat capacity was given in J / kg°C.”

Worked example 12 marks

The temperature of water as it enters a shower is 15 °C. The water leaves the shower at a temperature of 38 °C. The specific heat capacity of water is 4200 J / (kg °C). Calculate the energy required to increase the temperature of 18 kg of water from 15 °C to 38 °C.

  1. 1

    Δ E = mcΔθ = 18 × 4200 × (38 - 15) [1]

  2. 2

    = 1.7 × 10⁶ J [1]

9. Measuring specific heat capacity

To find c for a solid or a liquid, heat a known mass of it. Measure the initial temperature, the final temperature and the time.

The energy supplied is E = Pt. Then use the temperature change in c = (Δ E)/mΔθ.

Some energy is transferred to the surroundings, so not all of it reaches the substance.

Examiner insight

“Weaker candidates did not recall that a temperature change requires measurement of the initial and final temperature.” “Few candidates commented on how the energy supplied could be determined.”

Worked example 17 marks

A student investigates the rate at which the temperature of water inside a beaker increases when the water is heated. Plan an experiment to investigate how one variable affects the rate at which the temperature of the water increases. The apparatus available includes a selection of beakers, an electrical water heater and a supply of water.

  1. 1

    variable chosen: the mass of water in the beaker [1]

  2. 2

    other apparatus: thermometer and stop-watch [1]

  3. 3

    measure the initial temperature, the final temperature and the time; repeat for a new mass of water [1]

  4. 4

    keep constant: the same heater and the same starting temperature [1]

  5. 5

    table columns: mass of water / g, initial temperature / °C, final temperature / °C, time / s [1]

  6. 6

    compare the readings to see how the mass changes the rate of temperature increase [1]

  7. 7

    additional point: repeat each measurement and take the average [1]

Recap

  • A rise in temperature increases the internal energy of an object.
  • Extended: a rise in temperature means a rise in the average kinetic energy of all the particles.
  • Extended: specific heat capacity is the energy required per unit mass per unit temperature increase.
  • Extended: Δ E = mcΔθ. Δθ is final minus initial temperature, never time. Mass in kg.
  • Extended: to measure c, measure the mass, the initial and final temperatures and the time.

10. Melting and boiling

Melting and boiling need an energy input, but the temperature does not change while they happen. On a heating curve each one is a flat section. The line rises only where the energy input causes a temperature change.

At standard atmospheric pressure, water melts at 0 °C and boils at 100 °C. In kelvin these are 273 K and 373 K.

Examiner insight

“Many candidates did not interpret the graph correctly and had a misconception that all of the solid substance had melted before the temperature started to increase again.” “Option A was the most popular choice, whereas Syllabus statement 2.2.3.1 states quite clearly that there is no temperature change during melting.”

Worked example 11 mark

A solid substance is heated from room temperature. The graph shows how the temperature of the substance changes with time as it is heated constantly. At which time has the substance just become completely liquid?

  1. 1

    The flat section is the melting, so the temperature is constant. The substance is all liquid when the line starts to rise again at C. [1]

11. Condensation and solidification

In condensation a gas becomes a liquid. The particles move closer together, so their separation decreases. Their average speed does not change. In the liquid they are close together, but they do not vibrate about fixed points.

In solidification a liquid becomes a solid. The particles are in a regular arrangement, closely packed, and they vibrate about fixed positions.

A substance solidifies at the same temperature at which it melts.

Examiner insight

“Many candidates were not aware that there is no change in the average speed of the particles as a gas condenses to form a liquid.” “Some candidates chose the correct option A, but others had the misconception that the particles in a condensed substance vibrate about fixed points.”

Worked example 13 marks

Describe the arrangement, separation and motion of the particles in a solid.

  1. 1

    regular arrangement [1]

  2. 2

    closely packed [1]

  3. 3

    particles vibrate about fixed positions [1]

12. Evaporation and cooling

Evaporation is the escape of more-energetic particles from the surface of a liquid.

The particles left behind have less energy on average. So evaporation causes cooling of the liquid.

Examiner insight

“Two ideas were then required: that the faster or more energetic molecules evaporated or escaped, and that the slower or less energetic molecules remained.” “References to hotter or colder molecules were not acceptable.”

Worked example 11 mark

At the surface of a liquid, the more-energetic molecules can escape from the liquid into the atmosphere. Which name is given to this process?

A boiling B condensation C evaporation D melting

  1. 1

    C: evaporation [1]

13. Boiling and evaporation compared

Both turn a liquid into a gas. Boiling happens at one fixed temperature. Evaporation happens at any temperature.

Boiling takes place throughout the liquid. Evaporation takes place only at the surface, and no bubbles form.

While a liquid boils its temperature is fixed, so the average kinetic energy of its particles is constant.

Examiner insight

“Weaker candidates often referred to a change of state occurring in evaporation but not in boiling, or they found difficulty in stating that boiling only took place at one temperature.” “Many candidates failed to realise that, as boiling takes place at fixed temperature, the average KE is constant.”

14. What changes the rate of evaporation

Evaporation is faster when the liquid is hotter, when it has a larger surface area, and when air moves over the surface.

Moving air carries away the particles that have already left the surface. Evaporation does not stop in still, cold air. It is only slower.

Examiner insight

“A common mistake was to suggest that the wind gives KE to the water particles so they can escape more easily, or that wind blows off the top layer of water particles.” “No credit was given for saying that no water evaporates from dish C. There will be some evaporation, but the rate of evaporation will be lower.”

15. Cooling an object by evaporation

An object in contact with an evaporating liquid loses energy to the liquid. The more-energetic particles escape from the liquid, so those that remain have less energy.

The temperature of the liquid falls, and the temperature of the object falls with it.

Examiner insight

“A common explanation referred to air molecules that were being blown across the surface transferring coldness to the molecules at the surface of the water.” “Full answers covered molecules of sweat gaining energy from the body, more energetic molecules escaping and leaving behind less energetic molecules thus lowering the temperature of the skin or body.”

Recap

  • Melting and boiling need energy, but the temperature does not change. Water: 0 °C and 100 °C, or 273 K and 373 K.
  • Condensation: gas to liquid, separation decreases. Solidification: particles regular, closely packed, vibrating about fixed positions.
  • Evaporation: more-energetic particles escape from the surface. The liquid left behind is cooler.
  • Extended: boiling is at one fixed temperature, throughout the liquid. Evaporation is at any temperature, from the surface only.
  • Extended: hotter, larger surface area and moving air all make evaporation faster.
  • Extended: an object touching an evaporating liquid cools, because the liquid takes energy from it.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what happens to the volume of a solid when it is heated.1 mark
  2. A liquid is heated in a beaker. State what happens to its volume and to its density.2 marks
  3. A gas is heated at constant pressure. State what happens to its volume.1 mark
  4. Rails are laid with no gaps between them. State what could happen to the track on a hot day and explain why.2 marks
  5. A lorry driver sees that overhead cables hang lower on a hot afternoon than on a frosty morning. Explain why.2 marks
  6. The gaps between the rails are wider on a cold morning than on a hot afternoon. Explain why.2 marks
  7. A steel tape measure is used to measure the distance between two posts on a cold day and again on a very hot day. The distance between the posts does not change. State and explain how the reading on the hot day compares with the reading on the cold day.3 marks
  8. A student writes: 'When water is heated, the water molecules expand, so the molecules become less dense.' Identify the two errors and correct them.3 marks
  9. A glass flask is completely filled with cold coloured water. A narrow glass tube passes through its stopper, and the water level is part way up the tube. The flask is lowered into a bath of hot water. The level in the tube first falls a little. It then rises well above where it started. (a) Explain why the level first falls. (b) Explain why the level then rises above where it started.4 marks
  10. A syringe contains air. Its piston is free to move. A steel block and some water in a flask each have the same volume as the air at 20 °C. All three are heated to 60 °C. (a) List the steel, the water and the air in order of their increase in volume, smallest first. (b) Explain, in terms of the forces between particles, why the air expands more than the steel. (c) A student says: 'The air has expanded, so the pressure of the air in the syringe has increased.' State and explain whether the student is correct.5 marks
  11. State what happens to the internal energy of a metal block when its temperature rises.1 mark
  12. Block A and block B are identical iron blocks. A is at 80 °C and B is at 20 °C. State which block has the greater internal energy and give the reason.2 marks
  13. Explain, in terms of particles, why the internal energy of a block increases when it is heated.2 marks
  14. Define specific heat capacity.2 marks
  15. Calculate the energy required to raise the temperature of 0.50 kg of water from 15 °C to 85 °C. The specific heat capacity of water is 4200 J / (kg °C).2 marks
  16. A pan holds 250 g of oil. The specific heat capacity of the oil is 2000 J / (kg °C). Calculate the energy needed to raise the temperature of the oil from 20 °C to 50 °C.3 marks
  17. A student finds the specific heat capacity of a liquid by heating it with an electric heater of known power. State the measurements she must take.3 marks
  18. A student heats a 1.0 kg block of metal with a 60 W heater for 5.0 minutes. The temperature of the block rises from 21 °C to 33 °C. Calculate the specific heat capacity of the metal.4 marks
  19. A metal pan of mass 400 g holds 1.2 kg of water. The pan and the water are both at 18 °C. They are heated on a 1.5 kW hotplate until both reach 78 °C. All the energy from the hotplate goes to the pan and the water. The specific heat capacity of water is 4200 J / (kg °C) and of the metal is 900 J / (kg °C). Calculate the time taken. Give your answer in minutes.5 marks
  20. A student measures the specific heat capacity of a metal. She heats a metal block of mass 150 g to 100 °C. She then puts it into 200 g of water at 19.0 °C in an insulated cup. The block and the water reach the same final temperature of 24.0 °C. The specific heat capacity of water is 4200 J / (kg °C). (a) Calculate the energy gained by the water. (b) All the energy lost by the block is gained by the water. Calculate the specific heat capacity of the metal. (c) In practice, some of the energy lost by the block is transferred to the surroundings. State and explain whether the true specific heat capacity is larger or smaller than your answer to (b).6 marks
  21. A beaker of ice at 0 °C is heated and the ice melts. What happens to the temperature while the ice is melting?1 mark
  22. At standard atmospheric pressure, give the melting temperature and the boiling temperature of water (a) in °C, (b) in kelvin.4 marks
  23. A wax is heated steadily from a solid. The table gives its temperature. (a) State the temperature at which the wax melts. (b) State one time in the table at which all of the wax is liquid. | time / min | 0 | 2 | 4 | 6 | 8 | 10 | 12 | | --- | --- | --- | --- | --- | --- | --- | --- | | temperature / °C | 20 | 45 | 70 | 80 | 80 | 80 | 95 |2 marks
  24. A gas condenses to form a liquid. State what happens to the separation of its particles and to their average speed.2 marks
  25. Liquid metal cools in a mould until it is solid. (a) Name this process. (b) Describe the arrangement and motion of the particles in the solid.3 marks
  26. Name the process in which more-energetic particles escape from a liquid. State where on the liquid they escape from.2 marks
  27. Water in a shallow dish cools as it evaporates. Explain why, in terms of particles.3 marks
  28. A pan of water boils steadily on a hotplate for five minutes. (a) State what happens to the temperature of the water during this time. (b) State what happens to the average kinetic energy of the water particles. (c) State one way in which evaporation differs from boiling.3 marks
  29. Equal volumes of water at room temperature are left in the same room in three containers. X is a wide, shallow tray. Y is a tall, narrow glass. Z is a tray identical to X, with a fan blowing air across it. One hour later none of the water has boiled. (a) List the containers in order of the volume of water left, largest first. (b) Explain, in terms of particles, why Z loses water faster than X. (c) A student says no water has left Y because Y was not heated. Explain why the student is wrong.5 marks
  30. Two identical thermometers hang side by side in a room. The bulb of one is wrapped in cloth soaked in water at room temperature. After a few minutes this thermometer gives the lower reading. (a) Explain, in terms of particles, why its reading falls. (b) A fan now blows air from the room over both thermometers. State what happens to each reading, and explain your answers.6 marks

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