IGCSE Physics · Area 2 · Thermal Physics

Kinetic Particle Model of Matter

The kinetic particle model explains the properties of solids, liquids and gases in terms of how their particles are arranged, how far apart they are and how they move. This topic covers the three states of matter, the link between particle motion and temperature including absolute zero and the kelvin scale, Brownian motion, gas pressure, and pV = constant.

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Syllabus coverage

  • Know the distinguishing properties of solids, liquids and gases, and know the terms for the changes in state between solids, liquids and gases (gas to solid and solid to gas transfers are not required)
  • Describe the particle structure of solids, liquids and gases in terms of the arrangement, separation and motion of the particles and represent these states using simple particle diagrams
  • Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (-273 °C), known as absolute zero, where the particles have least kinetic energy
  • Know that the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases
  • Describe the pressure and the changes in pressure of a gas in terms of the motion of its particles and their collisions with a surface, and in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area
  • Know that the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter, and describe and explain this motion (sometimes known as Brownian motion) in terms of random collisions between the microscopic particles in a suspension and the particles of the gas or liquid
  • Know that microscopic particles may be moved by collisions with light fast-moving molecules, and correctly use the terms atoms or molecules as distinct from microscopic particles
  • 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 a change of volume at constant temperature
  • Convert temperatures between kelvin and degrees Celsius; recall and use the equation T (in K) = theta (in °C) + 273
  • Recall and use the equation pV = constant for a fixed mass of gas at constant temperature, including a graphical representation of this relationship

Part of the IGCSE Physics syllabus 0625.

The particle model: solids, liquids and gasesThe state of a substance depends on the arrangement, the separation and the motion of its particles.Every particle is drawn the same size. Only the spacing and the motion change.Solidfixed shape, fixed volumeregular latticeSpacingparticles touchingArrangementregular, repeatingMotionvibrate about fixed positionsLiquidfixed volume, takes the shape of the containerrandom, close togetherSpacingparticles nearly touchingArrangementrandom, no patternMotionslide past one anotherGasno fixed shape, no fixed volumefar apart, fast, randomSpacingfar apart - large empty gapsArrangementrandom, no patternMotionfast, in straight linesOrder of expansion when heated: gases expand most, then liquids, then solids.The particles themselves never change size - the average distance between them increases.
Particle diagrams of a solid, a liquid and a gas, showing how the arrangement, separation and motion of the particles differ between the three states of matter.

The three states of matter and the changes between them

A solid has a fixed shape and a fixed volume. A liquid has a fixed volume but takes the shape of whatever container it is poured into. A gas has neither a fixed shape nor a fixed volume - it spreads out to fill its container, and unlike a solid or a liquid it can easily be squeezed into a smaller space.

The kinetic particle model explains all of this by treating every substance as a collection of tiny particles in constant motion. In a solid the particles are held in fixed positions by strong forces between them, so they can only vibrate about those positions, which is why a solid keeps its shape. In a liquid the forces are strong enough to keep the particles close together but not strong enough to pin them down, so the particles can slide past one another and the liquid flows while keeping a fixed volume. In a gas the forces between the particles are negligible, so they are free to move quickly and randomly and to fill any container.

Heating a substance can change its state. Learn the four terms the syllabus asks for: melting is solid to liquid, boiling or evaporating is liquid to gas, condensing is gas to liquid, and freezing is liquid to solid. During a change of state the temperature does not change even though energy is being supplied, because the energy is used to break the forces holding the particles together rather than to speed the particles up.

StateArrangement of particlesSeparation of particlesMotion of particles
SolidClosely packed in a regular, orderly arrangementParticles almost touching, with no room to move closerVibrate about fixed positions; they cannot move from place to place
LiquidClose together but in a random, irregular arrangementParticles almost touching but with small gaps that keep changingMove at random and slide past one another throughout the liquid
GasFar apart and completely randomLarge empty gaps between the particles, many times their own diameterMove quickly and randomly in all directions, with negligible forces between them
Describe every state with the same three words: arrangement, separation and motion of the particles. Those are the words the mark scheme is looking for.
The particle model: solids, liquids and gasesThe state of a substance depends on the arrangement, the separation and the motion of its particles.Every particle is drawn the same size. Only the spacing and the motion change.Solidfixed shape, fixed volumeregular latticeSpacingparticles touchingArrangementregular, repeatingMotionvibrate about fixed positionsLiquidfixed volume, takes the shape of the containerrandom, close togetherSpacingparticles nearly touchingArrangementrandom, no patternMotionslide past one anotherGasno fixed shape, no fixed volumefar apart, fast, randomSpacingfar apart - large empty gapsArrangementrandom, no patternMotionfast, in straight linesOrder of expansion when heated: gases expand most, then liquids, then solids.The particles themselves never change size - the average distance between them increases.
Particle diagrams of the three states of matter: a solid with particles in regular rows, a liquid with particles close together but randomly arranged, and a gas with particles far apart and randomly scattered.

Forces and distances between particles: why the states behave differently

The properties of a substance depend on two things: how far apart its particles are and how strongly they attract one another. In a solid the particles are very close together and the forces between them are strong, so the structure is rigid and cannot easily be compressed. In a gas the particles are far apart and the forces are negligible, so the gas is easy to compress and expands to fill any container it is put into.

The particles themselves may be atoms, molecules, ions or electrons, depending on the substance. In a metal the particles include free electrons moving between positive ions; in an ionic compound such as sodium chloride the lattice is built from ions; in water the particles are molecules. The model works the same way whatever the particles are: the stronger the forces between them and the closer they are, the more fixed the structure.

Temperature matters because it changes the motion. The higher the temperature, the faster the particles move and the more their motion can overcome the forces holding them together. Heat a solid enough and the vibration becomes strong enough to break the regular arrangement, so it melts. Heat a liquid enough and the fastest particles escape from the surface, so it boils and the particles fly apart into the gas state.

  • Solids: particles close together, strong forces between them, fixed arrangement, vibration only.
  • Liquids: particles close together, forces strong enough to hold them together but not to fix them, so they slide.
  • Gases: particles far apart, forces between them negligible, free random motion in all directions.
When a question asks why a solid keeps its shape, the mark is for the strong forces between the particles holding them in fixed positions - not for the particles being heavy or packed tightly alone.

Temperature, particle motion and the kelvin scale

Temperature is a measure of the average kinetic energy of the particles of a substance. The higher the temperature, the faster the particles move and the greater their average kinetic energy. This is why heating a gas makes its particles move faster, and why cooling a gas slows them down.

There is a limit to how far a substance can be cooled. At absolute zero the particles have the least kinetic energy it is possible for them to have. Absolute zero is a temperature of -273 °C, which is written as 0 K on the kelvin scale. The kelvin is the SI unit of temperature, and its steps are exactly the same size as the steps of the Celsius scale, so a change of 1 K is the same as a change of 1 °C. The scales differ only in where they start.

To convert between the two scales, use T (in K) = theta (in °C) + 273. So 0 °C is 273 K, 100 °C is 373 K and a room at 27 °C is at 300 K. To go the other way, subtract 273: theta (in °C) = T (in K) - 273, so 200 K is -73 °C. Notice that a temperature in kelvin can never be negative, because 0 K is the lowest temperature there is.

Temperature on the Celsius scaleTemperature on the kelvin scale
-273 °C (absolute zero)0 K
0 °C (ice point)273 K
27 °C (room temperature)300 K
100 °C (steam point)373 K
The equation T = theta + 273 only converts degrees Celsius into kelvin. If the question starts in kelvin, subtract 273 instead of adding it.
The absolute zero temperature scaleOne temperature line carries two scales: degrees C above it, kelvin below.The kelvin scale starts at absolute zero, the lowest temperature possible. There is no negative kelvin.-273-200-1000100degrees C073173273373kelvinabsolute zero-273 C = 0 Kice point0 C = 273 Ksteam point100 C = 373 KTo change degrees C into kelvin add 273; to change kelvin into degrees C subtract 273.Absolute zero is -273 degrees C = 0 K, the coldest temperature possible. A kelvin temperature is never negative.T (in K) = temperature in degrees C + 273100 C = 373 K0 C = 273 K
The Celsius and kelvin temperature scales side by side, showing that -273 °C lines up with 0 K as absolute zero, the lowest possible temperature.

Brownian motion: evidence for the particle model

You cannot see the molecules of a gas or a liquid directly, but you can see the effect they have. If you look down a microscope at smoke particles floating in air, or at tiny pollen grains suspended in water, they move in a continuous random zigzag path instead of drifting smoothly. This is Brownian motion, and it is strong evidence for the kinetic particle model of matter.

The explanation is that the microscopic particle you can see is much larger than the atoms or molecules around it. The molecules of the gas or liquid are far smaller and move very fast, and they bombard the microscopic particle from every side. Because the collisions are random and uneven - at any instant more molecules happen to hit one side than another - the net force on the particle keeps changing direction, so it jitters about unpredictably.

Brownian motion is evidence for the kinetic particle model because it shows that something invisible, tiny and fast is hitting the particle. If the air or water were a still, continuous fluid, the smoke particle would simply sit there. The constant random jitter can only be explained by collisions with moving particles. Be careful with the words: the microscopic particles you can see (smoke or pollen) are not the same thing as the molecules that hit them. The molecules are the particles of the gas or liquid, and they are far too small to see.

Say the smoke particles are hit unevenly by the much smaller, faster-moving molecules of the gas or liquid. Saying only that they are 'knocked about by the air' loses the mark.
Brownian motion: evidence for the kinetic particle modelSmoke particles in air jitter along a random zigzag path.The molecules of the air are far too small to see, but the smoke particles they hit are large enough to watch.air molecules: tiny and fastsmoke particlesstartendzigzag path of one smoke particlethe arrow marks an uneven hit by the moleculesWhy the path is a zigzagThe smoke particle is far bigger than amolecule of air.The fast air molecules bombard it from everyside.At any instant more hit one side than theother, so the net force keeps changing.The particle therefore sets off in a newdirection after every uneven hit.What Brownian motion provesSomething invisible, extremely small and moving very fast must be hitting the smoke particles.That is exactly what the kinetic particle model says: matter is made of tiny particles in constantrandom motion.Be careful with the words: the smoke particle is the large particle you can see; the air moleculesare the much smaller particles that hit it.
A smoke particle viewed under a microscope: the dotted line shows the random zigzag path it follows as it is hit unevenly by the much smaller, faster-moving air molecules.

Gas pressure: particles colliding with a surface

The particles of a gas are in constant random motion, and they collide with the walls of their container. Each individual collision gives the wall a tiny push. There are so many particles colliding every second that the pushes add up to a steady force spread over the area of the wall, and this is what we call the pressure of the gas.

Pressure is defined as force per unit area, so p = F / A, where p is the pressure in pascals (Pa), F is the force in newtons (N) and A is the area in square metres (m2). One pascal is one newton per square metre, so 1 Pa = 1 N/m2. For example, a force of 30 N acting evenly on an area of 0.02 m2 gives p = 30 / 0.02 = 1500 Pa, which is 1.5 kPa.

Two changes raise the pressure of a fixed mass of gas. Heat it at constant volume: the particles gain kinetic energy and move faster, so they strike the walls more often and each collision is more forceful, and the pressure rises. Squeeze it at constant temperature: the same number of particles is crammed into a smaller volume, so they strike the walls more frequently, and the pressure rises again. In both cases the key idea is the rate and the force of the collisions with the surface.

  • Hotter gas at constant volume: particles move faster, collide with the walls more often and more forcefully, so the pressure rises.
  • Smaller volume at constant temperature: the particles collide with the walls more often, so the pressure rises.
  • p = F / A, measured in pascals; 1 Pa = 1 N/m2.
Every gas-pressure explanation needs two ideas: the particles hit the walls more often, and they hit with more force. One without the other is usually only half the marks.

Boyle's law: pV = constant

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume. In symbols, pV = constant, so for the same gas before and after a change p1 x V1 = p2 x V2. This relationship is known as Boyle's law.

Worked example. A fixed mass of gas has a volume of 200 cm3 at a pressure of 100 kPa. The pressure is increased to 250 kPa at constant temperature. What is the new volume? Use p1 x V1 = p2 x V2: 100 x 200 = 250 x V2, so V2 = (100 x 200) / 250 = 80 cm3. The pressure went up and the volume went down, which is the check you should always make - if your answer has the volume rising when the pressure rises, something has gone wrong.

The graph of pressure against volume is a curve that climbs more and more steeply as the volume gets smaller. If instead you plot pressure against 1 / volume, the points lie on a straight line through the origin, because p is proportional to 1/V. Either graph may be asked for, so be ready to describe both and to say what the straight-line version shows.

Volume / cm3Pressure / kPap x V / kPa cm3
20010020000
10020020000
8025020000
5040020000
pV = constant only holds when the temperature and the mass of gas are fixed. If either changes, the product is no longer constant and the equation does not apply.

Key terms you must be able to define

Kinetic particle model
The model that describes a substance as tiny particles in constant motion, whose arrangement, separation and motion explain the properties of solids, liquids and gases.
Absolute zero
The lowest possible temperature, -273 °C or 0 K, at which the particles of a substance have the least kinetic energy.
Kelvin
The SI unit of temperature; the kelvin scale starts at absolute zero, and a temperature in kelvin is found from T (in K) = theta (in °C) + 273.
Brownian motion
The random zigzag motion of microscopic particles in a suspension, caused by uneven random collisions with the much smaller, faster-moving molecules of the surrounding gas or liquid.
Pressure
Force per unit area, p = F / A, measured in pascals (Pa) where 1 Pa = 1 N/m2; the pressure of a gas is caused by its particles colliding with a surface.
Boyle's law
For a fixed mass of gas at constant temperature, pV = constant, so p1 x V1 = p2 x V2; pressure is inversely proportional to volume.
Suspension
A mixture in which microscopic particles are spread through a gas or a liquid, for example smoke particles in air or pollen grains in water; the random motion of these particles is Brownian motion.
Change of state
The change of a substance from one state to another by heating or cooling: melting, boiling or evaporating, condensing and freezing; the temperature stays constant while the change takes place.

Applied to the real world

Cambridge sets physics in unfamiliar contexts - a car braking, a power station, a hospital scanner, a satellite. Use these as models for how to apply the physics, then write your own.

In a car

The air in a car tyre is a fixed mass of gas in a fixed volume. As the car is driven, the tyres flex and warm up, so the air particles inside move faster and collide with the tyre walls more often and with more force, and the pressure rises. This is why tyre pressures are always checked when the tyres are cold: a reading taken after a long motorway run is higher than the true cold pressure.

At home

A bicycle pump works by reducing the volume of the air trapped inside it. Pushing the handle down squeezes the same number of air particles into a smaller space, so they collide with the walls of the pump more frequently and the pressure rises, forcing the air through the valve and into the tyre. An aerosol can is the opposite problem: warming a sealed can raises the pressure inside because the particles move faster, which is why cans carry a warning to keep them away from heat.

In a hospital

Oxygen cylinders hold a fixed mass of gas in a fixed volume at very high pressure. If a cylinder is left in direct sunlight it warms up, the oxygen molecules move faster and strike the cylinder walls more often and more forcefully, and the pressure rises. That is why cylinders are stored upright in a cool, well-ventilated place and never beside a radiator. Hospitals also work in kelvin when they handle very cold liquids such as liquid nitrogen, which boils at 77 K, that is -196 °C.

In a power station

Water is heated in a sealed boiler to make steam at high temperature and pressure. Heating gives the water molecules more kinetic energy, so they move faster and hit the walls of the boiler more often and with more force, which raises the pressure. The high-pressure steam is then allowed to expand through a turbine, where its stored pressure energy is transferred to the kinetic energy of the turbine blades and on to electrical energy in the generator.

On a building site

A compressed-air tool such as a nail gun is powered by air that has been squeezed into a small volume. Compressing the air packs the same number of particles into a much smaller space, so the pressure rises sharply and the stored pressure can do useful work when it is released. Gas cylinders on site are kept out of direct sunlight for the same reason that hospital cylinders are: heating a fixed volume of gas raises its pressure.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1What are the distinguishing properties of a solid?
A solid has a fixed shape and a fixed volume; its particles are closely packed in a regular arrangement and can only vibrate about fixed positions.
Q2What are the distinguishing properties of a liquid?
A liquid has a fixed volume but takes the shape of its container; its particles are close together but randomly arranged, and they can slide past one another.
Q3What are the distinguishing properties of a gas?
A gas has no fixed shape and no fixed volume and fills its container; its particles are far apart and move quickly and randomly with negligible forces between them.
Q4Name the four changes of state between solids, liquids and gases.
Melting (solid to liquid), boiling or evaporating (liquid to gas), condensing (gas to liquid) and freezing (liquid to solid).
Q5What does the temperature of a substance tell you about its particles?
The higher the temperature, the greater the average kinetic energy of the particles and the faster their random motion.
Q6What is absolute zero?
The lowest possible temperature, -273 °C or 0 K, at which the particles of a substance have the least kinetic energy.
Q7Convert 27 °C into kelvin.
T (in K) = theta (in °C) + 273 = 27 + 273 = 300 K.
Q8Convert 350 K into degrees Celsius.
theta (in °C) = T (in K) - 273 = 350 - 273 = 77 °C.
Q9What causes the pressure of a gas on the walls of its container?
The gas particles collide with the surface; each collision exerts a tiny force, and the total force per unit area on the wall is the pressure.
Q10Why does the pressure of a fixed mass of gas rise when it is heated at constant volume?
The particles gain kinetic energy and move faster, so they collide with the walls more often and with more force, increasing the force per unit area.
Q11Why does the pressure of a gas rise when its volume is reduced at constant temperature?
The same number of particles is squeezed into a smaller volume, so they collide with the walls more frequently and the force per unit area increases.
Q12What is Brownian motion?
The random zigzag motion of microscopic particles in a suspension, caused by uneven random collisions with the much smaller, faster-moving molecules of the surrounding gas or liquid.
Q13State Boyle's law.
For a fixed mass of gas at constant temperature, pV = constant, so p1 x V1 = p2 x V2.
Q14What does a graph of pressure against 1 / volume look like for a fixed mass of gas at constant temperature?
A straight line through the origin, because the pressure is proportional to 1 / volume.
0 of 14 marked as known

Exam-style questions

8 questions, 24 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State what happens to the arrangement of the particles of a substance when it melts.

Mark scheme — tick each point you actually made

Examiner tip: Use the word regular for the solid and irregular for the liquid. The mark is for the change in arrangement, not for the temperature staying constant.
CompareAO12 marks
Compare the arrangement and the motion of the particles in a liquid with those in a solid.

Mark scheme — tick each point you actually made

Examiner tip: A compare question wants both substances described. Give one clear point about the liquid and one about the solid, and use the words arrangement and motion.
ExplainAO23 marks
A sealed metal container holds a fixed mass of gas. The container is heated and the pressure of the gas increases. Explain this increase in pressure in terms of the particles of the gas.

Mark scheme — tick each point you actually made

Examiner tip: Both effects of the faster particles are needed: more frequent collisions and more forceful collisions. The phrase 'force per unit area' shows the examiner you know what pressure is.
ExplainAO23 marks
The handle of a bicycle pump is pushed in, reducing the volume of the trapped air at constant temperature. Explain, in terms of the particles, why the pressure of the air rises.

Mark scheme — tick each point you actually made

Examiner tip: Because the temperature is constant, the particles do not move faster. The mark is for the increase in the frequency of collisions caused by the smaller volume.
DescribeAO24 marks
A smoke cell contains smoke particles in air and is viewed under a microscope. Describe and explain the motion of the smoke particles.

Mark scheme — tick each point you actually made

Examiner tip: Say the collisions are uneven and random, and that the air molecules are much smaller and faster than the smoke particles. Do not say the smoke particles collide with each other.
CalculateAO22 marks
Use the equation T (in K) = theta (in °C) + 273 to convert 35 °C into kelvin and 200 K into degrees Celsius.

Mark scheme — tick each point you actually made

Examiner tip: Add 273 to go from degrees Celsius to kelvin, and subtract 273 to go the other way. Give the unit K or °C with each answer.
CalculateAO24 marks
A fixed mass of gas at constant temperature has a volume of 200 cm3 at a pressure of 100 kPa. The pressure is increased to 250 kPa. Use p1 x V1 = p2 x V2 to calculate the new volume of the gas.

Mark scheme — tick each point you actually made

Examiner tip: Write the equation, then the substitution, then the answer with its unit. Check the direction: raising the pressure must reduce the volume.
PlanAO35 marks
Describe an experiment to investigate how the pressure of a fixed mass of gas depends on its volume at constant temperature. Include the measurements you would take and how you would keep the temperature constant.

Mark scheme — tick each point you actually made

Examiner tip: The mark is for controlling the temperature as well as for taking the readings. The strongest finish is to say what graph you would plot and what the straight line would show.

Exam technique for this topic

  • When a temperature has to be converted, write the equation first: T (in K) = theta (in °C) + 273. To go from kelvin to degrees Celsius you subtract 273, not add it, and the commonest error is applying the conversion the wrong way round.
  • In every gas-pressure explanation, give both effects of the faster particles: they collide with the walls more often and each collision exerts more force. One without the other is usually only half the marks.
  • Pressure is force per unit area, so always quote the unit: the pascal (Pa), which is the same as 1 N/m2. A pressure answer with no unit, or with the unit 'N', loses the mark.
  • For Brownian motion, say the microscopic particles are hit unevenly by the much smaller, faster-moving molecules. Do not say the smoke particles collide with each other, and do not confuse the visible microscopic particle with the molecule that hits it.
  • For pV = constant, write down p1 x V1 = p2 x V2, list the values with their units, and check the answer makes sense: if the pressure goes up, the volume must come down.
  • In a particle-diagram question, show arrangement, separation and motion. A solid is regular rows of touching particles, a liquid is random but still close together, and a gas has widely spaced particles with lots of empty space between them.

Why students lose marks here

A student wroteA student wrote: 'When a gas is heated, its particles expand, so the gas takes up more space.'
Why it loses marksThe particles themselves do not change size when they are heated. This answer misses the mark, which is for what happens to their energy and motion and for the increase in the spacing between them.
The answer that scoresHeating transfers energy to the gas particles, so their average kinetic energy increases and they move faster. The particles themselves stay the same size; it is the average distance between them that increases, so the gas expands. If the gas is trapped in a container of fixed volume, the faster particles collide with the walls more often and with more force, and the pressure rises instead.
A hot air balloon: the air inside is heated so its particles move faster and the air expands and becomes less dense than the cooler air outside.
A student wroteA student wrote: 'The pressure of a gas rises when it is heated because the particles get bigger and push harder on the walls.'
Why it loses marksParticles do not get bigger when they are heated. The answer also misses the key idea that the faster particles hit the walls more frequently as well as with more force.
The answer that scoresHeating gives the particles more kinetic energy, so they move faster. They therefore collide with the walls of the container more frequently, and each collision exerts a larger force on the wall. The total force per unit area on the walls increases, so the pressure of the gas rises.
A student wroteA student wrote: 'Smoke particles move about randomly because they collide with each other.'
Why it loses marksCollisions between the smoke particles cannot explain the motion: there are far too few smoke particles, and they are far too large, for this to produce a continuous jitter. The motion is evidence for something much smaller and faster that cannot be seen.
The answer that scoresThe smoke particles are bombarded by the much smaller, much faster-moving molecules of the air. The collisions are random and uneven, so at any instant the pushes on one side do not balance the pushes on the other, and the net force changes direction from moment to moment. The smoke particle therefore follows a random zigzag path. This is Brownian motion, and it is evidence that the air molecules are in constant random motion.
A student wroteA student converted a temperature of 25 K into degrees Celsius by writing '25 + 273 = 298 °C'.
Why it loses marksThe conversion was applied in the wrong direction. Adding 273 changes degrees Celsius into kelvin; to go from kelvin to degrees Celsius you must subtract 273.
The answer that scoresT (in K) = theta (in °C) + 273, so rearranging gives theta (in °C) = T (in K) - 273. For 25 K: theta = 25 - 273 = -248 °C. A negative answer is possible in degrees Celsius but never in kelvin, because 0 K is absolute zero, the lowest temperature there is.

Where this fits in IGCSE Physics

This page is one topic of Cambridge IGCSE Physics 0625. IGCSE Physics is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Physics syllabus page.