Solids, liquids and gases

The kinetic particle model pictures all matter as tiny particles in constant motion. The three states differ in how those particles are arranged, how far apart they are, and how they move:

The changes of state have names: melting (solid→liquid), boiling/evaporation (liquid→gas), condensation (gas→liquid) and solidification/freezing (liquid→solid).

Extended: the forces and distances between particles — and how fast they move — are what give each state its properties. Strong bonds & tiny gaps make a solid rigid; almost no forces & huge gaps let a gas be squashed.SUPPLEMENT

Which state?

Gas pressure & Brownian motion

Gas particles fly about randomly and collide with the walls of their container. Each collision pushes on the wall; the total of countless collisions per second is what we feel as gas pressure.

The random motion of microscopic particles in a suspension (e.g. smoke specks in air, or pollen grains in water) is direct evidence for the kinetic particle model.

Extended: a heavy smoke speck jiggles because it is bombarded unevenly by light, fast-moving air molecules. The molecules transfer momentum in random collisions; more hits on one side knock the visible particle the other way. Use "molecules" for the gas, "microscopic particles" for the bigger visible specks.SUPPLEMENT

Watch out: gas pressure is not particles "pushing" on each other while still — it comes only from particles colliding with a surface. No collisions, no pressure.

Where does the pressure come from?

Temperature, volume & the Kelvin scale

For a fixed mass of gas, in terms of particles:

• Heat it at constant volume → particles move faster, hit the walls harder and more often → pressure rises.
• Squeeze it at constant temperature → same particles hit a smaller area more often → pressure rises.

Cool any gas down and its particles slow. At −273 °C they have the least kinetic energy possible — this is absolute zero, the start of the Kelvin scale.

Watch out: in any gas-law calculation you must use temperature in kelvin, never °C — at constant volume, pressure is proportional to the absolute temperature, so p ∝ T only works with Kelvin.

Your turn — °C to kelvin

Your turn — squeezing a gas

Why does squashed air push back harder?

Why does iron warm up more than water?

Heating makes things expand

When a solid, liquid or gas is heated at constant pressure, its particles vibrate/move more vigorously and push a little further apart, so the material expands.

The size of the expansion follows the order gases > liquids > solids. Everyday examples and consequences:

• Bimetallic strips bend when heated — used in thermostats and fire alarms.
• Expansion gaps in bridges and railway lines stop them buckling in summer.
• Liquid-in-glass thermometers work because the liquid expands up the tube.

Extended: the order solids < liquids < gases follows from particle spacing & forces. In a solid, strong bonds resist separation, so expansion is tiny; in a gas, the particles are already far apart with almost no forces, so the same heating causes a much larger expansion.SUPPLEMENT

Reading expansion

Specific heat capacity

Raising an object's temperature increases its internal energy, because the average kinetic energy of its particles increases. How much energy that takes depends on the material's specific heat capacity, c:

The specific heat capacity is the energy needed per unit mass per unit temperature rise. Water's is high (4200 J/(kg °C)), so it is slow to heat and slow to cool.SUPPLEMENT

Experiment: heat a known mass of a metal block (or liquid) with an electric immersion heater. Measure the energy supplied and the temperature rise, then c = ΔE ÷ (m Δθ). Lagging the block reduces heat lost to the surroundings.

Your turn — heating a metal block

Why does iron warm up more than water?

Where does the energy go when water boils?

Changes of state & the heating curve

While a substance changes state, the energy you put in still raises its internal energy, but it goes into breaking the bonds between particles, not into making them move faster — so the temperature stays constant on the flat plateaus below.

Extension (not in the 0625 syllabus): the energy absorbed or released during a change of state, with no temperature change, is the latent heat. The specific latent heat L is the energy per kilogram:

Condensation (gas→liquid) and solidification (liquid→solid) are the reverse: particles slow, form bonds and release this latent heat.

Watch out: on the flat parts of the curve the substance is still being heated, but the temperature does not rise — the energy is breaking inter-particle bonds, not raising kinetic energy. "No temperature change" does not mean "no energy in".

Your turn — melting ice

Where does the energy go when water boils?

Evaporation cools a liquid

Evaporation happens at the surface of a liquid at any temperature: the most energetic particles escape into the air. Boiling, in contrast, happens throughout the liquid at one fixed temperature.

Because the fastest particles leave, the average kinetic energy of those remaining drops — so the liquid (and anything touching it) cools down. This is why sweating cools you and why a wet swimmer feels cold in a breeze.

Extended: evaporation is faster with higher temperature, larger surface area, and more air movement over the surface (which sweeps escaped particles away). An object in contact with an evaporating liquid cools because the liquid takes the energy needed to evaporate from that object.SUPPLEMENT

Why does sweat cool you?

Why does only the metal spoon get hot?

Three ways thermal energy moves

Thermal energy always flows from hotter to cooler. There are three transfer methods:

• Conduction: energy passes along solids. SUPPLEMENT in all solids by atomic/molecular lattice vibrations; in metals also by fast-moving free (delocalised) electrons — which is why metals are by far the best conductors. Gases and most liquids conduct poorly (particles far apart).
• Convection: heated fluid expands, becomes less dense, rises; cooler dense fluid sinks to replace it — a convection (density) current. Only in liquids and gases.
• Radiation: all objects emit infra-red radiation; it needs no medium (it reaches us from the Sun through empty space).

Good absorbers are good emitters

A surface's colour and texture control how it handles infra-red radiation:

• Dull black surfaces are the best absorbers and the best emitters.
• Shiny white / silvery surfaces are the best reflectors — and the worst absorbers/emitters.

Watch out: a good absorber is always an equally good emitter — they are not opposites. A matte black radiator radiates heat well and would soak it up well; a shiny kettle keeps its drink warm by emitting little.

Extended: for an object to stay at a constant temperature it must emit energy at the same rate it absorbs it. If it receives more than it emits, it warms up; if it emits more than it receives, it cools. The rate of emission increases with higher surface temperature and larger surface area. This balance of incoming vs outgoing radiation also controls the temperature of the Earth.SUPPLEMENT

Everyday applications draw on more than one method: a kitchen pan conducts heat to the food; a room is heated by convection; SUPPLEMENT a wood fire and a car radiator combine conduction, convection and radiation.

Choosing a surface

Match the situation to the transfer

Tap a situation on the left, then its main transfer method on the right.

Name the transfer

Tap the main way thermal energy is transferred in each case.

Why does only the metal spoon get hot?

The equations to know

You've covered the key ideas across the three parts of Cambridge IGCSE Topic 2 — the kinetic particle model, thermal properties & temperature, and the transfer of thermal energy. Press Finish to see your score.