Solids, liquids and gases

Matter exists in three states. Cambridge expects you to state the distinguishing properties of each:

• Solid — fixed shape and fixed volume; cannot be compressed (squashed).
• Liquid — fixed volume but takes the shape of its container; cannot be compressed; can flow.
• Gas — no fixed shape and no fixed volume; fills its container; can be compressed; can flow.

Watch out: both liquids and gases can flow, so we call them fluids. The big differences are that a gas has no fixed volume and is easily compressed, while a liquid is not.

Particles in each state

Describe each state in terms of particle separation, arrangement and motion:

Common mistake: heating does not make the particles themselves get bigger. The particles stay the same size — they gain energy, move more, and the spacing between them changes.

Read the particle diagram

Which state?

Tap the state that best matches each particle description.

Changing state

Adding or removing energy moves a substance between states. Cambridge names five changes you must describe:

• Melting — solid → liquid (at the melting point).
• Boiling — liquid → gas, throughout the liquid, at the boiling point.
• Evaporating — liquid → gas from the surface only, below the boiling point.
• Freezing — liquid → solid.
• Condensing — gas → liquid.

Conservation of mass: changes of state are physical changes — no new substance forms and mass is conserved. Sealed ice that melts then boils has the same mass throughout; only the arrangement and energy of the particles change.

Extension note: a few solids (e.g. solid carbon dioxide, iodine) turn straight to gas — sublimation. This is useful background, but the named changes assessed in 0620 Topic 1 are the five above.

Name the change

Did the water particles swell up?

Heating curves & the energy story

Extended only. Explain changes of state using kinetic particle theory. As you heat a solid steadily, a heating curve (temperature against time) shows two flat plateaus:

• Rising sections: energy increases the particles' average kinetic energy, so the temperature rises.
• Lower plateau (melting): energy goes into weakening/overcoming forces holding the solid together — temperature stays at the melting point.
• Upper plateau (boiling): energy separates particles completely into a gas — temperature stays at the boiling point.

Cooling curve: run it in reverse (gas → liquid → solid). Plateaus appear at the boiling point (condensing) and the freezing point (freezing): energy is released as forces between particles form, so the temperature stays constant until the change of state is complete.

Reading the heating curve

Why does the white ring form off-centre?

Temperature, pressure & a gas

Cambridge expects you to describe the effects of temperature and pressure on the volume of a gas (qualitatively):

• Higher temperature (pressure fixed): particles gain energy and move faster, so the gas expands — its volume increases.
• Higher pressure (temperature fixed): the gas is squeezed into a smaller space, so its volume decreases.

Supplement explanation SUPPLEMENT: in terms of kinetic particle theory, gas pressure comes from particles colliding with the walls. Heating makes them move faster and hit the walls harder and more often; if the container can expand, the gas takes up more room. Pushing the walls in (more pressure) packs the particles closer, so the volume falls.

What happens to the gas?

Diffusion — evidence for moving particles

Diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration, until they are evenly mixed.

Explain it with kinetic particle theory: particles are in constant random motion. This random motion causes an overall (net) movement from where there are more of them to where there are fewer, mixing them without stirring. Diffusion is fastest in gases (particles far apart, moving fast), slower in liquids, and negligible in solids.

Why it matters: smelling perfume across a room is diffusion — the smell particles move and spread through the air all on their own.

Diffusion rate & molecular mass

Extended only. The rate of diffusion of a gas depends on its relative molecular mass (M_r): lighter molecules diffuse faster than heavier ones at the same temperature.

In the classic experiment, cotton wool soaked in ammonia solution (giving NH₃ gas) and concentrated hydrochloric acid (giving HCl gas) are put at opposite ends of a glass tube. Where the gases meet, a white ring of ammonium chloride (NH₄Cl) forms — and it forms closer to the HCl end.

Reasoning SUPPLEMENT: at the same temperature the gas particles have the same average kinetic energy, so the lighter NH₃ molecules move faster and cover more of the tube before meeting the slower, heavier HCl molecules.

Which gas wins the race?

Why does the white ring form off-centre?

Topic 1 at a glance

You've covered the key ideas in Cambridge IGCSE Chemistry (0620) Topic 1 — States of matter, with the Supplement (extended) items flagged. Press Finish to see your score.