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Topic 1 · 1.1–1.2

States of matter

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • State the distinguishing properties of solids, liquids and gases, and describe their particles: separation, arrangement and motion.
  • Describe melting, boiling, evaporating, freezing and condensing.
  • Describe how temperature and pressure affect the volume of a gas.
  • Explain changes of state with kinetic particle theory, including heating and cooling curves EXTENDED.
  • Explain the effects of temperature and pressure on gas volume in terms of particles EXTENDED.
  • Describe and explain diffusion; explain how relative molecular mass affects the rate of diffusion of a gas EXTENDED.

📚The chemistry

Three states, three particle pictures

Solid: fixed shape and volume. Particles close together, in a regular arrangement, vibrating about fixed positions.
Liquid: fixed volume, takes the shape of its container. Particles close together, randomly arranged, moving past each other.
Gas: no fixed shape or volume; fills its container and is easily compressed. Particles far apart, randomly arranged, moving quickly in all directions.
Three boxes of particles. Solid: particles touching in a regular grid. Liquid: particles touching but randomly arranged. Gas: a few particles far apart with arrows showing fast random motion. Arrows between the boxes are labelled melting and freezing, and boiling or evaporating and condensing.
The same particles in three arrangements. Changes of state move particles between them without changing the particles themselves.

Changes of state

Melting (solid → liquid) and boiling (liquid → gas) take energy in; freezing and condensing give it out. Evaporation is also liquid → gas, but it happens only at the surface and at any temperature below the boiling point, whereas boiling happens throughout the liquid at one temperature. A pure substance melts and boils at sharp, fixed temperatures — which is used to test purity (topic 12a).

EXTENDED On a heating curve the temperature rises while one state is heated, then stays constant during a change of state: the energy supplied is used to overcome the forces of attraction between the particles, not to make them move faster. A cooling curve is the reverse, with flat sections where the substance condenses and freezes, giving out energy as the attractions reform.

A heating curve of temperature against time for a pure substance. Temperature rises through the solid, stays flat at the melting point while solid and liquid are present, rises through the liquid, stays flat at the boiling point while liquid and gas are present, then rises again through the gas.
Flat sections are changes of state: the energy breaks attractions between particles instead of raising the temperature. (Schematic.)

Gases: temperature and pressure

At constant pressure, heating a gas makes its volume increase; at constant temperature, increasing the pressure makes its volume decrease. EXTENDED Heated particles have more kinetic energy, move faster and hit the walls harder and more often, so the gas expands until the pressure is back to its original value. Squeezing a gas into a smaller volume means the same particles hit the walls more often, which is why pressure and volume are linked.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It happens in gases and liquids (slowly in liquids, whose particles are closer together and move less freely).

EXTENDED At the same temperature, gas particles with a lower relative molecular mass move faster, so they diffuse faster. In the classic experiment, ammonia (Mr 17) and hydrogen chloride (Mr 36.5) diffuse towards each other along a tube and meet to form a white ring of ammonium chloride — closer to the hydrogen chloride end, because the ammonia travelled further in the same time.

A long horizontal glass tube with cotton wool soaked in concentrated ammonia at the left end and cotton wool soaked in concentrated hydrochloric acid at the right end. A white ring of ammonium chloride forms inside the tube, nearer the hydrochloric acid end.
Ammonia (Mr 17) diffuses faster than hydrogen chloride (Mr 36.5), so the ring forms nearer the HCl end.

✏️Worked example

(a) Describe the arrangement and motion of the particles in a liquid. [2] (b) State two differences between boiling and evaporation. [2] (c) EXTENDED Explain why the temperature of a pure solid stays constant while it melts. [2] (d) EXTENDED In the diffusion tube above, explain why the white ring forms nearer the hydrochloric acid end. [2]

(a) Close together (touching), randomly arranged; the particles move around, sliding past each other.

(b) Boiling happens throughout the liquid, evaporation only at the surface; boiling happens at one fixed temperature (the boiling point), evaporation at any temperature below it.

(c) The energy absorbed is used to overcome the forces of attraction between the particles, so the particles’ average kinetic energy — and therefore the temperature — does not increase until all the solid has melted.

(d) Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5), so its molecules move faster and diffuse further in the same time before the two gases meet.

Check it. Every “explain” answer names the particles and says what they do. “Ammonia is lighter” is not enough on its own: link the lower Mr to faster-moving molecules.
“The particles expand.” Particles never change size. When a substance expands on heating, the spaces between the particles increase.

📝Practise

In the style of the multiple-choice papers (1 and 2) and theory papers (3 and 4). EXTENDED marks Supplement content.

1. (Multiple choice.) Which describes the particles in a liquid? A: close together, regular, vibrating only. B: close together, random, moving past each other. C: far apart, random, moving quickly. D: far apart, regular, moving slowly.
B. A describes a solid; C a gas; D describes nothing real.
2. (Theory.) Steam touches a cold window and forms droplets of water. Name this change of state, and state whether energy is taken in or given out. [2]
Condensing (condensation); energy is given out.
3. (Theory.) A sealed balloon is taken from a cold room into a warm room. State what happens to its volume. EXTENDED Explain your answer in terms of particles. [3]
The volume increases. The particles gain kinetic energy and move faster, so they hit the inside of the balloon harder and more often; the balloon expands until the pressure inside balances the pressure outside.
4. (Multiple choice.) EXTENDED Which gas diffuses fastest at room temperature? A: CH4. B: O2. C: CO2. D: Cl2.
A. Mr values are 16, 32, 44 and 71; the lowest Mr diffuses fastest.
5. (Theory.) A student leaves a beaker of water on a windowsill. After a week the level has fallen, although the water never boiled. Name the process and explain how it happened. [2]
Evaporation: particles at the surface with enough energy escape into the air, even below the boiling point.
6. (Theory.) EXTENDED A cooling curve for a pure substance has a flat section at 80 °C. State what is happening during this section and why the temperature does not fall. [3]
The liquid is freezing (liquid and solid are both present). Energy is given out as forces of attraction form between the particles, and this balances the energy lost to the surroundings, so the temperature stays at 80 °C (the melting/freezing point) until all of it is solid.
7. (Theory.) A drop of purple potassium manganate(VII) solution is placed at the bottom of a beaker of water. After an hour the whole beaker is pale purple. Name the process and explain why it is slower than the same process in a gas. [3]
Diffusion. In a liquid the particles are much closer together, so the manganate(VII) particles collide far more often with water particles and move more slowly through the liquid than gas particles move through a gas.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “States of Matter” — heat and compress particles and watch them change state
  • Royal Society of Chemistry — the ammonia and hydrogen chloride diffusion demonstration