The first three statements are descriptive; the fourth is the one that explains things. A substance is a solid when the forces of attraction dominate and the particles cannot escape their positions. It is a gas when the particles have so much kinetic energy that the forces of attraction are effectively overcome.

Heating a substance adds energy to the particles. Every change of state in this chapter is that competition tipping one way or the other, and framing your answers in those terms — energy versus forces of attraction — is what turns a description into an explanation.

Compressibility. A gas can be compressed because there is a great deal of empty space between the particles, which can be reduced. In a solid or liquid the particles are already touching, so there is almost no space to remove — which is why they are nearly incompressible.

Density. A gas has a very low density because the same number of particles occupies a far larger volume. This is why the same substance is always least dense as a gas.

Note that a liquid has a fixed volume but no fixed shape. This trips candidates up: the particles are still touching, so the volume cannot change, but they can slide past one another, so the shape can.

Evaporation also explains why a liquid cools as it evaporates. The most energetic particles are the ones that leave, so the average kinetic energy of those remaining falls — and average kinetic energy is what temperature measures. This is exactly why sweating cools the body.

The question 'energy is being supplied, so why does the temperature not rise?' is the point of the whole graph. The answer is that temperature measures the average kinetic energy of the particles. During a change of state, the energy supplied is used to overcome the forces of attraction between particles rather than to make them move faster — so kinetic energy, and therefore temperature, stays constant.

A cooling curve is the same graph in reverse: sloping sections where the temperature falls, and flat sections at the freezing point and condensation point where energy is released as forces of attraction form. Note that the flat section for a pure substance is perfectly horizontal and occurs at a sharp temperature — an impure substance melts over a range, which is the basis of the purity test later in the course.

Gas pressure is caused by particles colliding with the container walls. Anything that makes those collisions more frequent or more forceful increases the pressure.

Be careful with two frequent errors. First, the particles themselves do not expand when heated — they move faster and spread further apart, but each particle is unchanged. Second, a gas is compressible because of the space between particles, not because the particles are squashed.

No particle is being pushed or directed. The word net is essential: diffusion is a statistical consequence of random motion, not a force acting on the particles.

Diffusion occurs in gases and liquids but effectively not in solids, and this follows directly from the table in Section 3. In a gas the particles are far apart and move quickly, so they spread rapidly. In a liquid they are close together and collide constantly, so diffusion is much slower. In a solid the particles only vibrate about fixed positions and cannot move past one another at all.

Raising the temperature speeds diffusion up in both gases and liquids, because the particles gain kinetic energy and move faster.

Bromine and air. A gas jar of orange-brown bromine vapour is placed below a gas jar of air, with a glass cover slid out from between them. Bromine is denser than air, yet the colour is seen to spread upwards into the top jar until both jars are a uniform pale orange. This shows that diffusion is a result of the random motion of particles, not of density or gravity — a denser gas still diffuses against a lighter one because each particle moves independently and randomly.

Potassium manganate(VII) and water. A small crystal of purple potassium manganate(VII) is dropped into a beaker of still water. Without any stirring, purple colour spreads out from the crystal, and over time the whole beaker becomes a uniform pale purple (dilute) solution. This is diffusion in a liquid: particles of manganate(VII) ion move randomly from the region of high concentration around the dissolving crystal to regions of lower concentration, gradually diluting the colour throughout the water. The same particle-level explanation — net movement down a concentration gradient — accounts for how a concentrated coloured solution becomes dilute even with no mixing.

The lower the relative molecular mass, the faster a gas diffuses.

Cotton wool soaked in concentrated ammonia solution is placed at one end of a long glass tube, and cotton wool soaked in concentrated hydrochloric acid at the other. Both give off gases, which diffuse along the tube and react to form a white ring of ammonium chloride:

Predicting where the ring forms, and justifying it with the two $M_r$ values, is the standard exam question. The ring is never in the middle.

Question. A solid is heated steadily. The temperature rises, then stays constant for four minutes, then rises again. Explain, in terms of particles, what is happening during the flat section.

Marking. 1 mark per point. The final point is the crux — without linking temperature to average kinetic energy, the answer describes the graph rather than explaining it.

Question. In a $60\text{cm}$ glass tube, cotton wool soaked in ammonia is at one end and cotton wool soaked in hydrochloric acid at the other. A white ring forms $36\text{cm}$ from the ammonia end. (a) Explain why the ring is not in the middle. (b) Predict what would happen if the hydrogen chloride were replaced by a gas of $M_r=100$.

Marking. 1 mark per point. Quoting both $M_r$ values and linking them through equal kinetic energy is what earns the explanation marks — 'ammonia is lighter so it is faster' alone scores 1.

Question. Explain, in terms of particles, why a gas can be compressed easily but a liquid cannot, and why a liquid has a fixed volume but no fixed shape.

Marking. 1 mark per point. The fixed-volume-but-not-shape distinction depends on separating can the particles be pushed closer from can they move past one another.

Question. A sealed rigid container of gas is heated. Explain what happens to the pressure inside it.

Marking. 1 mark per point. Both more frequently and with greater force are needed; and note the particles themselves do not expand.