States of matter

Cambridge IGCSE Co-ordinated Sciences 0654 and Combined Science 0653 · Core and Extended

Syllabus points

• Describe the arrangement, separation and motion of particles in solids, liquids and gases.
• Describe changes of state and name each one.
• Explain changes of state, gas pressure and the effect of temperature and volume using the kinetic particle model.
• Describe and explain diffusion, and the effect of relative molecular mass on its rate.

The three states

Read that table one column at a time, because Cambridge builds questions by getting one column right and the other wrong.

Arrangement separates solids from the other two. Only a solid has a regular repeating pattern. The moment a solid melts, that order is lost, so any answer calling a liquid's arrangement regular is wrong on that word alone.

Separation separates gases from the other two. A liquid's particles are almost as close together as a solid's, which is why a liquid keeps a fixed volume and can hardly be compressed at all, while a gas can be squashed into a fraction of its volume because it is mostly empty space.

The properties everyone knows follow from those two facts. A solid has a fixed shape because its particles cannot move past one another. A liquid takes the shape of its container but keeps its volume because the particles can slide past each other while still touching. A gas fills its container completely because nothing holds its particles together at all.

Changes of state

Melting and boiling happen when energy is supplied. The particles gain kinetic energy, move faster, and eventually have enough energy to overcome the forces holding them to their neighbours.

Freezing and condensing happen when energy is removed. The particles slow down until the forces between them can pull them together again.

Evaporation and boiling are not the same. Boiling happens at one fixed temperature, throughout the liquid, with bubbles forming inside it. Evaporation happens at any temperature, only from the surface, and only the fastest-moving particles escape. Because the fastest particles are the ones that leave, the average kinetic energy of those remaining falls, so evaporation cools the liquid left behind. That is why sweating cools you, and why a wet surface feels cold.

Evaporation is faster at higher temperature, over a larger surface area, and in moving air, because moving air sweeps escaped particles away and stops them returning.

Gas pressure

Gas pressure is caused by the particles colliding with the walls of the container. Each collision gives the wall a tiny push, and the pressure is the total effect of billions of them each second.

Everything else about gases follows from that one sentence.

Raise the temperature at constant volume and the pressure rises. The particles gain kinetic energy and move faster, so they hit the walls more often and harder. Both matter, and an answer giving only one of them is usually a partial answer.

Reduce the volume at constant temperature and the pressure rises. The particles have less far to travel between walls, so they hit them more often. Their speed has not changed, because temperature has not changed.

That last point is the one most often got wrong. The speed of gas particles depends on temperature alone, never on the size of the container. Compressing a gas brings the particles closer together, so their separation falls, but it does not make them move faster unless the temperature rises with it. Conversely, letting a gas expand at constant temperature moves the particles further apart and leaves their speed unchanged.

Worked example. The volume of a gas is decreased while its temperature is increased. What happens to the separation of the molecules and to their speed?

Take the two changes separately, because they act on different properties. The smaller volume puts the same number of molecules in less space, so the separation decreases. The higher temperature raises their average kinetic energy, so the speed increases.

Answering "closer together, so slower" is the classic error, and it comes from assuming a smaller space must slow things down.

Diffusion

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion.

It happens because the particles are already moving randomly in all directions. Nothing pushes them; they simply spread out until they are evenly mixed. Diffusion is fastest in gases, slower in liquids, and effectively does not happen in solids, because solid particles cannot change places.

Diffusion is faster at a higher temperature, because the particles have more kinetic energy and move faster.

Lighter particles diffuse faster than heavier ones at the same temperature. At a given temperature all particles have about the same average kinetic energy, and a lighter particle needs to move faster than a heavier one to have the same energy. The classic demonstration puts cotton wool soaked in concentrated ammonia at one end of a long glass tube and concentrated hydrochloric acid at the other. A white ring of ammonium chloride forms where the two gases meet, and it forms nearer the hydrochloric acid end, because ammonia has the lower relative molecular mass and has therefore travelled further in the same time.

Brownian motion is the visible evidence for all this: smoke particles suspended in air are seen to jiggle randomly, because they are being knocked about by air molecules too small to see.

Common mistakes

• Saying the particles in a liquid are arranged regularly.
• Saying the particles in a liquid are much further apart than in a solid. They are only slightly further apart.
• Saying compressing a gas makes its particles move faster.
• Saying heating a gas makes its particles bigger, or that they expand. The particles are unchanged; only their spacing and speed alter.
• Explaining a rise in gas pressure by more frequent collisions only, without saying the collisions are also harder.
• Treating evaporation and boiling as the same process.
• Saying evaporation only happens at the boiling point.
• Saying heavier gases diffuse faster because they have more momentum.
• Saying the white ring forms in the middle of the tube.
• Saying diffusion needs something to push the particles.