Thermal physics runs along a single chain of reasoning. Particles are arranged and move in a way that depends on the forces between them; heating makes them move faster, which raises temperature; faster particles collide harder and more often, which creates gas pressure and drives expansion; energy supplied at a change of state pulls particles apart instead of speeding them up, so the temperature holds still; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.

Topic 2 of Cambridge IGCSE Physics 0625 contains 45 printed statements: 24 Core, taught to all candidates, and 21 Supplement, taught in addition to Core for candidates following the Extended route. The Core statements cover the kinetic particle model and states of matter, Brownian motion as evidence for that model, gas pressure and the qualitative gas relationships, the kelvin scale, thermal expansion and its applications, internal energy, changes of state and evaporation, and thermal energy transfer by conduction, convection and infrared radiation with basic everyday applications. The Supplement statements add inter-particle forces, force per unit area in gases, \(pV = \text{constant}\), the particle explanation of expansion, average kinetic energy and specific heat capacity with its two experiments, the boiling–evaporation comparison and evaporative cooling of an object, the mechanisms and limits of conduction, the radiation balance including the Earth's temperature, emitter and absorber experiments, and complex multi-route applications.

In a solid, particles are close together in a regular arrangement and vibrate about fixed positions. In a liquid they are still close but disordered, and they move around one another. In a gas they are far apart and move rapidly and randomly. Raising the temperature makes the particles move faster; at −273 °C — absolute zero — they have their least kinetic energy. Gas particles collide with the walls of their container, and those collisions are what produce gas pressure. The random jiggling of microscopic particles suspended in a fluid, known as Brownian motion, is direct evidence that the fluid is made of particles in constant random motion.

Heating a substance normally increases the average separation of its particles, so the substance expands; Extended candidates also compare how much each state expands. A rise in temperature increases an object's internal energy. When a pure substance melts or boils, the energy supplied separates the particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; condensation and solidification are the reverse, with particles coming closer together and energy being given out. Evaporation is different again: it happens only at the surface, at any temperature below the boiling point, and it cools the liquid left behind.

Thermal energy always travels from a hotter region to a cooler one, by three distinct routes. Conduction passes energy through a material without the material itself moving, and is fastest in metals. Convection occurs only in fluids: the heated fluid expands, its density falls, it rises, cooler denser fluid sinks to replace it, and a convection current carries energy by bulk movement of the fluid itself. Infrared radiation is emitted and absorbed by every object and needs no medium at all; dull black surfaces are the best emitters and absorbers, while shiny light surfaces are the best reflectors.

Key ideas to remember

The one sentence that unlocks the chapter: temperature is about how fast the particles move, internal energy is the total energy stored by all the particles, and thermal energy transfer is what happens because two places are at different temperatures. Confuse those three and half the marks in this topic disappear.
Where the marks concentrate. Ten of the 21 Supplement statements sit in 2.3 alone, and six of those are about infrared radiation. If you are on the Extended route and short of time, the radiation subsection is the highest-yield revision in this chapter.
One habit fixes four of the six. Before writing an explain answer in this topic, sketch the causal chain as four or five arrows in the margin, then write one sentence per arrow. Missing links become visible before they cost you anything.
Section 2.1 in one line: arrangement and separation decide the state; particle speed decides the temperature; collisions decide the pressure; and Brownian motion is the evidence that all of it is really happening. Mastery check: can you explain a pressure rise twice — once for heating at constant volume, once for compression at constant temperature — using different words each time?
Section 2.2 in one line: energy either speeds particles up (the temperature rises) or pulls them apart (the state changes and the temperature holds still) — and evaporation is the special case where only the fastest surface particles leave, so what stays behind is cooler. Mastery check: can you describe condensation and solidification in particle terms without using the word “opposite”?
The habit that protects all four: label both axes with a quantity and a unit before you plot anything. An unlabelled axis costs a mark on every graph question, and it is the one mistake that takes five seconds to prevent.
The one equation Core has to recall: \(T\,(\text{in K}) = \theta\,(\text{in }^{\circ}\text{C}) + 273\). Everything else in Core Topic 2 is explanation, and explanations are marked one link at a time. Mastery check: can you produce all seven links of the convection chain, and all five links of the gas-pressure chain, without looking?
Where the Extended marks concentrate: ten of the twenty-one Supplement statements are in section 2.3, and six of those are about infrared radiation. If you are short of time, revise the radiation subsection and the two experiments first. Mastery check: can you state, for each of the two Supplement equations, every condition under which it may be used?

What you need to be able to do

I can state the distinguishing properties of solids, liquids and gases. Core 2.1.1.1
I can name the changes of state between solids, liquids and gases, and I know that solid–gas transfers in either direction are outside this syllabus. Core 2.1.1.2
I can describe the particle structure of each state in terms of the arrangement, separation and motion of the particles, and draw a simple particle diagram for each. Core 2.1.2.1
I can describe the relationship between particle motion and temperature, including absolute zero at −273 °C where the particles have least kinetic energy. Core 2.1.2.2
I can describe gas pressure, and changes in gas pressure, in terms of the motion of the particles and their collisions with a surface. Core 2.1.2.3
I know that the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter. Core 2.1.2.4
I can describe and explain that motion — Brownian motion — as the result of random collisions between the suspended microscopic particles and the particles of the gas or liquid. Core 2.1.2.5
I can describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of a change of temperature at constant volume, and of a change of volume at constant temperature. Core 2.1.3.1
I can convert between kelvin and degrees Celsius using \(T\,(\text{in K}) = \theta\,(\text{in }^{\circ}\text{C}) + 273\). Core 2.1.3.2
I can describe qualitatively the thermal expansion of solids, liquids and gases at constant pressure. Core 2.2.1.1
I can describe everyday applications and consequences of thermal expansion. Core 2.2.1.2
I know that a rise in an object's temperature increases its internal energy. Core 2.2.2.1
I can describe melting and boiling as energy input without a change in temperature. Core 2.2.3.1
I know the melting and boiling temperatures of water at standard atmospheric pressure. Core 2.2.3.2
I can describe condensation and solidification in terms of particles. Core 2.2.3.3
I can describe evaporation as the escape of more-energetic particles from the surface of a liquid. Core 2.2.3.4
I know that evaporation causes cooling of a liquid. Core 2.2.3.5
I can describe experiments that demonstrate the properties of good thermal conductors and bad thermal conductors (thermal insulators). Core 2.3.1.1
I know that convection is an important method of thermal energy transfer in liquids and gases. Core 2.3.2.1
I can explain convection in liquids and gases in terms of density changes, and describe experiments that illustrate convection. Core 2.3.2.2
I know that thermal radiation is infrared radiation and that all objects emit it. Core 2.3.3.1
I know that thermal energy transfer by thermal radiation does not require a medium. Core 2.3.3.2
I can describe the effect of surface colour and texture on the emission, absorption and reflection of infrared radiation. Core 2.3.3.3
I can explain basic everyday applications and consequences of conduction, convection and radiation, including heating objects such as kitchen pans and heating a room by convection. Core 2.3.4.1
I know that the forces and distances between particles — atoms, molecules, ions and electrons — and the motion of those particles affect the properties of solids, liquids and gases. Supplement 2.1.2.6
I can describe gas pressure and its changes in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area. Supplement 2.1.2.7
I know that microscopic particles may be moved by collisions with light, fast-moving molecules, and I use the terms atoms and molecules correctly, as distinct from microscopic particles. Supplement 2.1.2.8
I can recall and use \(pV = \text{constant}\) for a fixed mass of gas at constant temperature, and sketch the graph of that relationship. Supplement 2.1.3.3
I can explain, in terms of the motion and arrangement of particles, the relative order of magnitude of the expansion of solids, liquids and gases as their temperatures rise. Supplement 2.2.1.3
I can describe a temperature rise as an increase in the average kinetic energies of all the particles in the object. Supplement 2.2.2.2
I can define specific heat capacity as the energy required per unit mass per unit temperature increase, and use \(c = \Delta E / (m\,\Delta\theta)\). Supplement 2.2.2.3
I can describe experiments to measure the specific heat capacity of a solid and of a liquid. Supplement 2.2.2.4
I can describe the differences between boiling and evaporation. Supplement 2.2.3.6
I can describe how temperature, surface area and air movement over a surface affect evaporation. Supplement 2.2.3.7
I can explain the cooling of an object in contact with an evaporating liquid. Supplement 2.2.3.8
I can describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations, and additionally in terms of free (delocalised) electrons in metallic conductors. Supplement 2.3.1.2
I can describe, in terms of particles, why thermal conduction is bad in gases and in most liquids. Supplement 2.3.1.3
I know that many solids conduct thermal energy better than thermal insulators do, but less well than good thermal conductors. Supplement 2.3.1.4
I know that an object stays at a constant temperature only when it transfers energy away at the same rate as it receives energy. Supplement 2.3.3.4
I know what happens to an object when the rate at which it receives energy is less than, or greater than, the rate at which it transfers energy away. Supplement 2.3.3.5
I know how the temperature of the Earth is affected by the factors controlling the balance between incoming radiation and radiation emitted from the Earth's surface. Supplement 2.3.3.6
I can describe experiments to distinguish between good and bad emitters of infrared radiation. Supplement 2.3.3.7
I can describe experiments to distinguish between good and bad absorbers of infrared radiation. Supplement 2.3.3.8
I can describe how the rate of emission of radiation depends on the surface temperature and the surface area of an object. Supplement 2.3.3.9
I can explain complex applications where more than one type of thermal energy transfer is significant, including a fire burning wood or coal and a radiator in a car. Supplement 2.3.4.2

Why Thermal Physics matters

Why it matters: everything later in the chapter is this section reused. Thermal expansion is separation increasing; specific heat capacity is particle energy increasing; changes of state are particles being pulled apart; convection is a density change caused by expansion. Get the particle picture exact here and the rest is bookkeeping.

Common mistakes to avoid

1. “Particles expand when heated.” Core 2.2.1.1 Why it is wrong: heating changes how energetically particles move, not their size. An atom is not compressible or inflatable. Corrected model: the particles gain kinetic energy, vibrate or travel over a larger range, so their average separation increases and the object expands. Exam-safe: “The particles gain kinetic energy and vibrate more, so their average separation increases and the object expands; the particles themselves do not change size.”
2. “Temperature is the total energy in an object.” Core 2.2.2.1 Why it is wrong: temperature takes no account of how much substance is present. A spark and a bonfire can be at similar temperatures with vastly different energy. Corrected model: temperature indicates how fast the particles are moving; the total is the internal energy, which also depends on mass and state. Exam-safe: “Temperature indicates how fast the particles move, whereas internal energy is the total kinetic and potential energy of all of them.”
3. “Heat and temperature mean the same thing.” Core 2.2.2.1 Why it is wrong: one is an energy transfer, the other is a measure of particle motion. Adding energy to melting ice raises no temperature at all. Corrected model: thermal energy is transferred because of a temperature difference; temperature is what that transfer may or may not change. Exam-safe: “Thermal energy is transferred from the hotter object to the cooler one because of the temperature difference between them.”
4. “Gas pressure is caused by particles resting against the wall.” Core 2.1.2.3 Why it is wrong: gas particles are in continuous free motion and touch the wall only briefly. Something resting against a surface produces no repeated impacts. Corrected model: pressure arises from an enormous number of brief collisions, each of which pushes on the wall as the particle rebounds. Exam-safe: “The particles collide with the wall; each collision exerts a force on it, and the greater the total force from all the collisions, the greater the pressure.”
5. “The temperature rises continuously while a pure substance melts or boils.” Core 2.2.3.1 Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it separates the particles instead. Corrected model: the temperature stays constant throughout melting and throughout boiling, giving the flat sections of a heating curve. Exam-safe: “The energy supplied separates the particles against the forces between them, so they do not move any faster and the temperature stays constant.”
6. “Energy is needed to make a gas condense.” Core 2.2.3.3 Why it is wrong: it reverses the direction of the energy flow. Condensation and solidification bring particles closer together, which releases energy. Corrected model: moving towards the gas end — melting, boiling, evaporation — takes energy in. Moving the other way — condensation, solidification — gives energy out. Exam-safe: “As the gas condenses, the particles slow down and come close enough for the forces between them to hold them together, and energy is given out to the surroundings.”
7. “Evaporation occurs throughout a liquid.” Core 2.2.3.4 Why it is wrong: a particle deep inside is surrounded on all sides by neighbours attracting it and has no free direction in which to escape. Corrected model: evaporation is a surface process; only surface particles can leave the liquid. Exam-safe: “Only particles at the surface can escape, so evaporation takes place at the surface of the liquid.”
8. “The coldest particles escape during evaporation.” Core 2.2.3.5 Why it is wrong: slow particles do not have enough energy to overcome the attractive forces. If the slowest left, the liquid would warm up, not cool. Corrected model: the most energetic surface particles escape, so the ones remaining are slower. Exam-safe: “The most energetic particles escape from the surface, so the particles remaining are slower and the liquid cools.”
9. “Convection occurs in solids.” Core 2.3.2.1 Why it is wrong: convection requires the material itself to move from place to place, and the particles of a solid are held in fixed positions. Corrected model: convection occurs only in fluids — liquids and gases. In a solid, energy travels by conduction. Exam-safe: “Convection cannot occur in the solid because it cannot flow; energy is transferred through it by conduction.”
10. “Hot air rises” offered as an explanation. Core 2.3.2.2 Why it is wrong: it is an observation, not a mechanism. The syllabus statement asks for convection to be explained in terms of density changes, and the density step is exactly what this answer leaves out. Corrected model: heated → expands → the same mass occupies a greater volume → density decreases → rises. Exam-safe: “The air is heated and expands, so its density decreases and it rises; cooler, denser air sinks to take its place.”
11. “Cold moves through a conductor.” Core 2.3.1.1 Why it is wrong: there is no such thing as cold energy. Only energy is transferred, and it always goes from the hotter region to the cooler one. Corrected model: an object feels cold because thermal energy flows rapidly out of your hand into it. Exam-safe: “Thermal energy is conducted from your hand into the metal, which is at a lower temperature, so your hand cools.”
12. “Black surfaces only absorb” and “shiny surfaces are good emitters”. Core 2.3.3.3 Why they are wrong: emission and absorption depend on the same surface property, so a good absorber is necessarily a good emitter. A shiny surface reflects well, and that is exactly why it emits poorly. Corrected model: dull black = good absorber and good emitter. Shiny light = poor absorber, poor emitter, good reflector. Exam-safe: “The dull black surface is a good emitter as well as a good absorber, so it loses energy by radiation quickly.”
13. “Radiation cannot travel through a vacuum.” Core 2.3.3.2 Why it is wrong: it assumes every transfer route needs particles. Conduction and convection do; radiation does not. Energy from the Sun crosses about 150 million kilometres of near-vacuum to reach us. Corrected model: infrared radiation is the one route that requires no medium at all. Exam-safe: “Thermal energy reaches the Earth from the Sun by infrared radiation, because radiation does not require a medium.”
14. “The smoke molecules jiggle about.” Core 2.1.2.5 Why it is wrong: the visible speck is a microscopic particle made of very many molecules, not a molecule. Calling it a molecule loses the distinction the question is testing. Corrected model: the microscopic smoke particle is the thing you can see; the air molecules colliding with it are far smaller and invisible. Exam-safe: “The air molecules collide randomly with the much larger smoke particle, pushing it first one way and then another.”
15. Converting a temperature difference to kelvin by adding 273. Core 2.1.3.2 Why it is wrong: the two scales have the same size of degree and differ only in where zero is. A difference is unaffected by where zero is. Corrected model: convert a temperature by adding 273; leave a temperature change exactly as it is. Exam-safe: “A rise of 50 °C is a rise of 50 K.”
16. “Pressure is proportional to volume.” Supplement 2.1.3.3 Why it is wrong: doubling the volume halves the pressure. Proportionality would give a rising straight line through the origin, which is not what is observed. Corrected model: \(p \propto 1/V\), so \(pV\) is constant and the \(p\)–\(V\) graph is a falling curve that never touches either axis. Exam-safe: “For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume, so \(pV\) has the same value before and after.”
17. “Insulation prevents all energy transfer” and “an object at a steady temperature has stopped emitting”. Supplement 2.3.3.4 Supplement 2.3.3.5 Why they are wrong: both treat transfer as something that can be switched off. No material stops transfer completely; insulation only slows it. And a steady temperature is a balance, not a stop: the object is still emitting, and absorbing just as much back. Corrected model: think in rates. Insulation reduces the rate of transfer. An object holds a constant temperature while the rate at which it transfers energy away equals the rate at which it receives energy. Exam-safe: “The insulation greatly reduces the rate of energy transfer, so the liquid cools much more slowly; it stops cooling only once it is transferring energy away at the same rate as it receives it.”

Examiner tips

About the reference codes in this chapter Every tagged statement carries a code such as Core 2.1.2.3. The first three numbers are Cambridge's own subsection reference — here, subsection 2.1.2 of the 0625 subject content. The fourth number is the position of that statement within the printed table for that subsection, and is Academiq's internal navigation aid rather than official Cambridge notation. Quote the subsection, not the fourth digit, if you ever refer to the syllabus itself.
Reading the codes In Core 2.1.2.3, the first three numbers — 2.1.2 — are Cambridge's own subsection reference. The fourth number is the statement's position in the printed table for that subsection and is Academiq's internal navigation aid, not official Cambridge notation.
What the shape of Topic 2 means for your answers Most of the syllabus statements in this topic begin with describe or explain rather than calculate, and the explanations are causal chains. One sentence per link is the safest structure, because a missing link cannot be credited even when the conclusion is right. The recall-and-use statements are concentrated in the Supplement: \(pV = \text{constant}\) and \(c = \Delta E/(m\,\Delta\theta)\). The only Core equation in this topic is the kelvin conversion. Every investigation in this chapter carries an error analysis, and each one names a direction — "too large" or "too small" — or an unfair element. Practise giving the direction and a specific improvement rather than "be more careful".
You must be able to draw this, not just recognise it Core 2.1.2.1 The syllabus asks you to represent the three states using simple particle diagrams. A full-mark sketch needs only four things, and takes about twenty seconds: All the circles the same size in all three boxes. Drawing bigger circles for a gas is the single most common way to lose the mark. Solid: circles touching, in a regular grid, filling the box. Liquid: circles touching but irregular, with a small gap or two, filling the box. Gas: a few circles, widely spaced and irregular, spread over the whole box. Add short motion marks if you have time — vibration arcs on the solid, curved arrows on the liquid, straight arrows on the gas — but the arrangement and the separation are what is being marked.
Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
The four questions every practical answer must answer What did you change? (independent variable) — exactly one thing. What did you measure? (dependent variable) — with what instrument, to what precision. What did you keep the same? (controlled variables) — this is where fair-test marks live. Why might the answer be wrong, and in which direction? — state whether the error makes the result too large or too small.
Why heat loss makes \(c\) come out too large — say it in one line The calculation assumes all of the electrical energy \(VIt\) entered the sample. In reality some of it left to the surroundings, so the sample received less than \(VIt\) and its temperature rise \(\Delta\theta\) is smaller than that full energy would have produced. Dividing an over-large energy by an under-sized temperature rise, \(c = VIt/(m\Delta\theta)\), gives a value that is larger than the true one.
Five phrases that keep a Topic 2 answer precise “…so the average separation of the particles increases…” (expansion) “…collisions with unit area of the wall become more frequent…” (pressure) “…the fluid expands, so its density decreases, so it rises…” (convection) “…the energy separates the particles, increasing their potential energy rather than their kinetic energy…” (change of state) “…the most energetic particles escape from the surface…” (evaporation) “…it transfers energy away at the same rate as it receives it…” (constant temperature, Extended)
Five phrases to delete from your answers “the particles get bigger” → say the average separation increases “hot air rises” on its own → add the expansion and density steps “the cold comes in” → say energy is transferred out “radiation needs air to travel through” → radiation is the one route that needs no medium “heat is lost” when you mean the temperature falls → say thermal energy is transferred to the surroundings
Three rules that make the schedule work Answer before you reveal, every time. A reveal panel opened before you have tried is a reading exercise, and reading is the weakest form of revision there is. Repair narrowly. When you miss something, reread the one subsection that owns it — not the whole chapter. The chapter is indexed by the coverage map precisely so that you can. Write the chains out by hand. The explain statements in this topic are causal chains, and the only reliable way to know you have all the links is to produce them on paper with the page shut.

Verified Curriculum Content

Every chapter note, MCQ explanation, and structured mark scheme is rigorously vetted by Cambridge curriculum specialists.

Chapter overview

A summary of this Physics chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.

What is Thermal Physics about?

Key ideas to remember

The one sentence that unlocks the chapter: temperature is about how fast the particles move, internal energy is the total energy stored by all the particles, and thermal energy transfer is what happens because two places are at different temperatures. Confuse those three and half the marks in this topic disappear.
Where the marks concentrate. Ten of the 21 Supplement statements sit in 2.3 alone, and six of those are about infrared radiation. If you are on the Extended route and short of time, the radiation subsection is the highest-yield revision in this chapter.
One habit fixes four of the six. Before writing an explain answer in this topic, sketch the causal chain as four or five arrows in the margin, then write one sentence per arrow. Missing links become visible before they cost you anything.
Section 2.1 in one line: arrangement and separation decide the state; particle speed decides the temperature; collisions decide the pressure; and Brownian motion is the evidence that all of it is really happening. Mastery check: can you explain a pressure rise twice — once for heating at constant volume, once for compression at constant temperature — using different words each time?
Section 2.2 in one line: energy either speeds particles up (the temperature rises) or pulls them apart (the state changes and the temperature holds still) — and evaporation is the special case where only the fastest surface particles leave, so what stays behind is cooler. Mastery check: can you describe condensation and solidification in particle terms without using the word “opposite”?
The habit that protects all four: label both axes with a quantity and a unit before you plot anything. An unlabelled axis costs a mark on every graph question, and it is the one mistake that takes five seconds to prevent.
The one equation Core has to recall: \(T\,(\text{in K}) = \theta\,(\text{in }^{\circ}\text{C}) + 273\). Everything else in Core Topic 2 is explanation, and explanations are marked one link at a time. Mastery check: can you produce all seven links of the convection chain, and all five links of the gas-pressure chain, without looking?
Where the Extended marks concentrate: ten of the twenty-one Supplement statements are in section 2.3, and six of those are about infrared radiation. If you are short of time, revise the radiation subsection and the two experiments first. Mastery check: can you state, for each of the two Supplement equations, every condition under which it may be used?

What you need to be able to do

I can state the distinguishing properties of solids, liquids and gases. Core 2.1.1.1
I can name the changes of state between solids, liquids and gases, and I know that solid–gas transfers in either direction are outside this syllabus. Core 2.1.1.2
I can describe the particle structure of each state in terms of the arrangement, separation and motion of the particles, and draw a simple particle diagram for each. Core 2.1.2.1
I can describe the relationship between particle motion and temperature, including absolute zero at −273 °C where the particles have least kinetic energy. Core 2.1.2.2
I can describe gas pressure, and changes in gas pressure, in terms of the motion of the particles and their collisions with a surface. Core 2.1.2.3
I know that the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter. Core 2.1.2.4
I can describe and explain that motion — Brownian motion — as the result of random collisions between the suspended microscopic particles and the particles of the gas or liquid. Core 2.1.2.5
I can describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of a change of temperature at constant volume, and of a change of volume at constant temperature. Core 2.1.3.1
I can convert between kelvin and degrees Celsius using \(T\,(\text{in K}) = \theta\,(\text{in }^{\circ}\text{C}) + 273\). Core 2.1.3.2
I can describe qualitatively the thermal expansion of solids, liquids and gases at constant pressure. Core 2.2.1.1
I can describe everyday applications and consequences of thermal expansion. Core 2.2.1.2
I know that a rise in an object's temperature increases its internal energy. Core 2.2.2.1
I can describe melting and boiling as energy input without a change in temperature. Core 2.2.3.1
I know the melting and boiling temperatures of water at standard atmospheric pressure. Core 2.2.3.2
I can describe condensation and solidification in terms of particles. Core 2.2.3.3
I can describe evaporation as the escape of more-energetic particles from the surface of a liquid. Core 2.2.3.4
I know that evaporation causes cooling of a liquid. Core 2.2.3.5
I can describe experiments that demonstrate the properties of good thermal conductors and bad thermal conductors (thermal insulators). Core 2.3.1.1
I know that convection is an important method of thermal energy transfer in liquids and gases. Core 2.3.2.1
I can explain convection in liquids and gases in terms of density changes, and describe experiments that illustrate convection. Core 2.3.2.2
I know that thermal radiation is infrared radiation and that all objects emit it. Core 2.3.3.1
I know that thermal energy transfer by thermal radiation does not require a medium. Core 2.3.3.2
I can describe the effect of surface colour and texture on the emission, absorption and reflection of infrared radiation. Core 2.3.3.3
I can explain basic everyday applications and consequences of conduction, convection and radiation, including heating objects such as kitchen pans and heating a room by convection. Core 2.3.4.1
I know that the forces and distances between particles — atoms, molecules, ions and electrons — and the motion of those particles affect the properties of solids, liquids and gases. Supplement 2.1.2.6
I can describe gas pressure and its changes in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area. Supplement 2.1.2.7
I know that microscopic particles may be moved by collisions with light, fast-moving molecules, and I use the terms atoms and molecules correctly, as distinct from microscopic particles. Supplement 2.1.2.8
I can recall and use \(pV = \text{constant}\) for a fixed mass of gas at constant temperature, and sketch the graph of that relationship. Supplement 2.1.3.3
I can explain, in terms of the motion and arrangement of particles, the relative order of magnitude of the expansion of solids, liquids and gases as their temperatures rise. Supplement 2.2.1.3
I can describe a temperature rise as an increase in the average kinetic energies of all the particles in the object. Supplement 2.2.2.2
I can define specific heat capacity as the energy required per unit mass per unit temperature increase, and use \(c = \Delta E / (m\,\Delta\theta)\). Supplement 2.2.2.3
I can describe experiments to measure the specific heat capacity of a solid and of a liquid. Supplement 2.2.2.4
I can describe the differences between boiling and evaporation. Supplement 2.2.3.6
I can describe how temperature, surface area and air movement over a surface affect evaporation. Supplement 2.2.3.7
I can explain the cooling of an object in contact with an evaporating liquid. Supplement 2.2.3.8
I can describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations, and additionally in terms of free (delocalised) electrons in metallic conductors. Supplement 2.3.1.2
I can describe, in terms of particles, why thermal conduction is bad in gases and in most liquids. Supplement 2.3.1.3
I know that many solids conduct thermal energy better than thermal insulators do, but less well than good thermal conductors. Supplement 2.3.1.4
I know that an object stays at a constant temperature only when it transfers energy away at the same rate as it receives energy. Supplement 2.3.3.4
I know what happens to an object when the rate at which it receives energy is less than, or greater than, the rate at which it transfers energy away. Supplement 2.3.3.5
I know how the temperature of the Earth is affected by the factors controlling the balance between incoming radiation and radiation emitted from the Earth's surface. Supplement 2.3.3.6
I can describe experiments to distinguish between good and bad emitters of infrared radiation. Supplement 2.3.3.7
I can describe experiments to distinguish between good and bad absorbers of infrared radiation. Supplement 2.3.3.8
I can describe how the rate of emission of radiation depends on the surface temperature and the surface area of an object. Supplement 2.3.3.9
I can explain complex applications where more than one type of thermal energy transfer is significant, including a fire burning wood or coal and a radiator in a car. Supplement 2.3.4.2

Why Thermal Physics matters

Common mistakes to avoid

1. “Particles expand when heated.” Core 2.2.1.1 Why it is wrong: heating changes how energetically particles move, not their size. An atom is not compressible or inflatable. Corrected model: the particles gain kinetic energy, vibrate or travel over a larger range, so their average separation increases and the object expands. Exam-safe: “The particles gain kinetic energy and vibrate more, so their average separation increases and the object expands; the particles themselves do not change size.”
2. “Temperature is the total energy in an object.” Core 2.2.2.1 Why it is wrong: temperature takes no account of how much substance is present. A spark and a bonfire can be at similar temperatures with vastly different energy. Corrected model: temperature indicates how fast the particles are moving; the total is the internal energy, which also depends on mass and state. Exam-safe: “Temperature indicates how fast the particles move, whereas internal energy is the total kinetic and potential energy of all of them.”
3. “Heat and temperature mean the same thing.” Core 2.2.2.1 Why it is wrong: one is an energy transfer, the other is a measure of particle motion. Adding energy to melting ice raises no temperature at all. Corrected model: thermal energy is transferred because of a temperature difference; temperature is what that transfer may or may not change. Exam-safe: “Thermal energy is transferred from the hotter object to the cooler one because of the temperature difference between them.”
4. “Gas pressure is caused by particles resting against the wall.” Core 2.1.2.3 Why it is wrong: gas particles are in continuous free motion and touch the wall only briefly. Something resting against a surface produces no repeated impacts. Corrected model: pressure arises from an enormous number of brief collisions, each of which pushes on the wall as the particle rebounds. Exam-safe: “The particles collide with the wall; each collision exerts a force on it, and the greater the total force from all the collisions, the greater the pressure.”
5. “The temperature rises continuously while a pure substance melts or boils.” Core 2.2.3.1 Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it separates the particles instead. Corrected model: the temperature stays constant throughout melting and throughout boiling, giving the flat sections of a heating curve. Exam-safe: “The energy supplied separates the particles against the forces between them, so they do not move any faster and the temperature stays constant.”
6. “Energy is needed to make a gas condense.” Core 2.2.3.3 Why it is wrong: it reverses the direction of the energy flow. Condensation and solidification bring particles closer together, which releases energy. Corrected model: moving towards the gas end — melting, boiling, evaporation — takes energy in. Moving the other way — condensation, solidification — gives energy out. Exam-safe: “As the gas condenses, the particles slow down and come close enough for the forces between them to hold them together, and energy is given out to the surroundings.”
7. “Evaporation occurs throughout a liquid.” Core 2.2.3.4 Why it is wrong: a particle deep inside is surrounded on all sides by neighbours attracting it and has no free direction in which to escape. Corrected model: evaporation is a surface process; only surface particles can leave the liquid. Exam-safe: “Only particles at the surface can escape, so evaporation takes place at the surface of the liquid.”
8. “The coldest particles escape during evaporation.” Core 2.2.3.5 Why it is wrong: slow particles do not have enough energy to overcome the attractive forces. If the slowest left, the liquid would warm up, not cool. Corrected model: the most energetic surface particles escape, so the ones remaining are slower. Exam-safe: “The most energetic particles escape from the surface, so the particles remaining are slower and the liquid cools.”
9. “Convection occurs in solids.” Core 2.3.2.1 Why it is wrong: convection requires the material itself to move from place to place, and the particles of a solid are held in fixed positions. Corrected model: convection occurs only in fluids — liquids and gases. In a solid, energy travels by conduction. Exam-safe: “Convection cannot occur in the solid because it cannot flow; energy is transferred through it by conduction.”
10. “Hot air rises” offered as an explanation. Core 2.3.2.2 Why it is wrong: it is an observation, not a mechanism. The syllabus statement asks for convection to be explained in terms of density changes, and the density step is exactly what this answer leaves out. Corrected model: heated → expands → the same mass occupies a greater volume → density decreases → rises. Exam-safe: “The air is heated and expands, so its density decreases and it rises; cooler, denser air sinks to take its place.”
11. “Cold moves through a conductor.” Core 2.3.1.1 Why it is wrong: there is no such thing as cold energy. Only energy is transferred, and it always goes from the hotter region to the cooler one. Corrected model: an object feels cold because thermal energy flows rapidly out of your hand into it. Exam-safe: “Thermal energy is conducted from your hand into the metal, which is at a lower temperature, so your hand cools.”
12. “Black surfaces only absorb” and “shiny surfaces are good emitters”. Core 2.3.3.3 Why they are wrong: emission and absorption depend on the same surface property, so a good absorber is necessarily a good emitter. A shiny surface reflects well, and that is exactly why it emits poorly. Corrected model: dull black = good absorber and good emitter. Shiny light = poor absorber, poor emitter, good reflector. Exam-safe: “The dull black surface is a good emitter as well as a good absorber, so it loses energy by radiation quickly.”
13. “Radiation cannot travel through a vacuum.” Core 2.3.3.2 Why it is wrong: it assumes every transfer route needs particles. Conduction and convection do; radiation does not. Energy from the Sun crosses about 150 million kilometres of near-vacuum to reach us. Corrected model: infrared radiation is the one route that requires no medium at all. Exam-safe: “Thermal energy reaches the Earth from the Sun by infrared radiation, because radiation does not require a medium.”
14. “The smoke molecules jiggle about.” Core 2.1.2.5 Why it is wrong: the visible speck is a microscopic particle made of very many molecules, not a molecule. Calling it a molecule loses the distinction the question is testing. Corrected model: the microscopic smoke particle is the thing you can see; the air molecules colliding with it are far smaller and invisible. Exam-safe: “The air molecules collide randomly with the much larger smoke particle, pushing it first one way and then another.”
15. Converting a temperature difference to kelvin by adding 273. Core 2.1.3.2 Why it is wrong: the two scales have the same size of degree and differ only in where zero is. A difference is unaffected by where zero is. Corrected model: convert a temperature by adding 273; leave a temperature change exactly as it is. Exam-safe: “A rise of 50 °C is a rise of 50 K.”
16. “Pressure is proportional to volume.” Supplement 2.1.3.3 Why it is wrong: doubling the volume halves the pressure. Proportionality would give a rising straight line through the origin, which is not what is observed. Corrected model: \(p \propto 1/V\), so \(pV\) is constant and the \(p\)–\(V\) graph is a falling curve that never touches either axis. Exam-safe: “For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume, so \(pV\) has the same value before and after.”
17. “Insulation prevents all energy transfer” and “an object at a steady temperature has stopped emitting”. Supplement 2.3.3.4 Supplement 2.3.3.5 Why they are wrong: both treat transfer as something that can be switched off. No material stops transfer completely; insulation only slows it. And a steady temperature is a balance, not a stop: the object is still emitting, and absorbing just as much back. Corrected model: think in rates. Insulation reduces the rate of transfer. An object holds a constant temperature while the rate at which it transfers energy away equals the rate at which it receives energy. Exam-safe: “The insulation greatly reduces the rate of energy transfer, so the liquid cools much more slowly; it stops cooling only once it is transferring energy away at the same rate as it receives it.”

Examiner tips

About the reference codes in this chapter Every tagged statement carries a code such as Core 2.1.2.3. The first three numbers are Cambridge's own subsection reference — here, subsection 2.1.2 of the 0625 subject content. The fourth number is the position of that statement within the printed table for that subsection, and is Academiq's internal navigation aid rather than official Cambridge notation. Quote the subsection, not the fourth digit, if you ever refer to the syllabus itself.
Reading the codes In Core 2.1.2.3, the first three numbers — 2.1.2 — are Cambridge's own subsection reference. The fourth number is the statement's position in the printed table for that subsection and is Academiq's internal navigation aid, not official Cambridge notation.
What the shape of Topic 2 means for your answers Most of the syllabus statements in this topic begin with describe or explain rather than calculate, and the explanations are causal chains. One sentence per link is the safest structure, because a missing link cannot be credited even when the conclusion is right. The recall-and-use statements are concentrated in the Supplement: \(pV = \text{constant}\) and \(c = \Delta E/(m\,\Delta\theta)\). The only Core equation in this topic is the kelvin conversion. Every investigation in this chapter carries an error analysis, and each one names a direction — "too large" or "too small" — or an unfair element. Practise giving the direction and a specific improvement rather than "be more careful".
You must be able to draw this, not just recognise it Core 2.1.2.1 The syllabus asks you to represent the three states using simple particle diagrams. A full-mark sketch needs only four things, and takes about twenty seconds: All the circles the same size in all three boxes. Drawing bigger circles for a gas is the single most common way to lose the mark. Solid: circles touching, in a regular grid, filling the box. Liquid: circles touching but irregular, with a small gap or two, filling the box. Gas: a few circles, widely spaced and irregular, spread over the whole box. Add short motion marks if you have time — vibration arcs on the solid, curved arrows on the liquid, straight arrows on the gas — but the arrangement and the separation are what is being marked.
Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
The four questions every practical answer must answer What did you change? (independent variable) — exactly one thing. What did you measure? (dependent variable) — with what instrument, to what precision. What did you keep the same? (controlled variables) — this is where fair-test marks live. Why might the answer be wrong, and in which direction? — state whether the error makes the result too large or too small.
Why heat loss makes \(c\) come out too large — say it in one line The calculation assumes all of the electrical energy \(VIt\) entered the sample. In reality some of it left to the surroundings, so the sample received less than \(VIt\) and its temperature rise \(\Delta\theta\) is smaller than that full energy would have produced. Dividing an over-large energy by an under-sized temperature rise, \(c = VIt/(m\Delta\theta)\), gives a value that is larger than the true one.
Five phrases that keep a Topic 2 answer precise “…so the average separation of the particles increases…” (expansion) “…collisions with unit area of the wall become more frequent…” (pressure) “…the fluid expands, so its density decreases, so it rises…” (convection) “…the energy separates the particles, increasing their potential energy rather than their kinetic energy…” (change of state) “…the most energetic particles escape from the surface…” (evaporation) “…it transfers energy away at the same rate as it receives it…” (constant temperature, Extended)
Five phrases to delete from your answers “the particles get bigger” → say the average separation increases “hot air rises” on its own → add the expansion and density steps “the cold comes in” → say energy is transferred out “radiation needs air to travel through” → radiation is the one route that needs no medium “heat is lost” when you mean the temperature falls → say thermal energy is transferred to the surroundings
Three rules that make the schedule work Answer before you reveal, every time. A reveal panel opened before you have tried is a reading exercise, and reading is the weakest form of revision there is. Repair narrowly. When you miss something, reread the one subsection that owns it — not the whole chapter. The chapter is indexed by the coverage map precisely so that you can. Write the chains out by hand. The explain statements in this topic are causal chains, and the only reliable way to know you have all the links is to produce them on paper with the page shut.

Thermal Physics

Core Revision Module

Revision & Practice Book

Practice & Resources

Chapter overview

What is Thermal Physics about?

Key ideas to remember

What you need to be able to do

Why Thermal Physics matters

Common mistakes to avoid

Examiner tips

Academiq Edu Review Team

Thermal Physics

Core Revision Module

Revision & Practice Book

Practice & Resources

Chapter overview

What is Thermal Physics about?

Key ideas to remember

What you need to be able to do

Why Thermal Physics matters

Common mistakes to avoid

Examiner tips

Academiq Edu Review Team