CXC CSEC Chemistry

The particulate theory

All matter is made of very small particles (atoms, molecules or ions) that are in constant motion and have spaces between them. The particles attract one another, and the balance between this attraction and the energy of their motion decides whether a substance is a solid, a liquid or a gas at a given temperature.

Evidence from 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 only makes sense if matter is made of moving particles with gaps between them.

In a classic demonstration, a cotton wool plug soaked in concentrated ammonia solution is placed at one end of a long, dry, horizontal glass tube and a plug soaked in concentrated hydrochloric acid at the other end. After a few minutes a ring of white solid ammonium chloride forms inside the tube, nearer the hydrochloric acid end:

The ring forms where the two gases meet. It is nearer the hydrochloric acid end because ammonia molecules (relative molecular mass 17) are lighter than hydrogen chloride molecules (36.5) and so move faster and travel further in the same time. The experiment shows that gas particles move through the air, that they move at different speeds, and that lighter particles diffuse faster.

A crystal of potassium manganate(VII) dropped into a beaker of still water gives another example. A purple colour slowly spreads through the whole beaker without any stirring, which means the particles of the solid leave the crystal and move between the water particles.

Evidence from osmosis

Osmosis is the movement of water through a partially permeable membrane from a dilute solution to a more concentrated one. A strip of raw pawpaw placed in distilled water takes in water and becomes firm and longer, while a strip placed in concentrated salt or sugar solution loses water and becomes limp and shorter. Water particles are small enough to pass through the membrane of the cells, while the larger solute particles are not, and this only makes sense if matter is made of particles of different sizes.

The same process explains why salt and sugar preserve food and kill garden pests such as slugs. Water leaves the cells of bacteria, moulds and pests by osmosis towards the concentrated salt or sugar, so they dry out and die, and the food is protected from decay.

The three states

Liquids and solids cannot be compressed because the particles are already touching, so there is almost no space to squeeze them into. A gas can be compressed because most of its volume is empty space between particles. Gases have very low densities because a given mass of gas is spread over a large volume. Water is an exception to the rule that the solid is denser than the liquid: ice floats because its particles are held in an open structure.

Changes of state

A change of state is a physical change, because the particles themselves are unchanged and only their arrangement and energy change.

Evaporation differs from boiling because it happens only at the surface and at temperatures below the boiling point. The particles at the surface with the most energy escape, so the average energy of those left falls, which is why evaporation cools a liquid.

Iodine is a substance that sublimes. Warmed gently in a fume hood, solid iodine turns directly to a purple vapour, which deposits as grey-black crystals on a cold surface above it. Ice heated gently melts to water, which can then be boiled. Butter has no sharp melting point because it is a mixture, and softens over a range of temperatures.

Explaining the changes by energy and arrangement

When a solid is heated its particles gain kinetic energy and vibrate more strongly. At the melting point they have enough energy to overcome some of the forces holding them in fixed positions, so the regular arrangement breaks down and the particles can slide past one another. On further heating the average kinetic energy of the liquid rises until, at the boiling point, the particles have enough energy to overcome the remaining forces of attraction and escape from each other into the gas state.

On cooling, the process reverses. Gas particles lose energy and come together as a liquid, releasing the energy as they form attractions between them, and liquid particles settle into fixed positions as a solid.

Heating and cooling curves

A graph of temperature against time for a substance heated steadily shows two flat sections.

1. The solid warms, so the line rises. The energy supplied raises the kinetic energy of the particles.
2. At the melting point the temperature stays constant while the solid melts. The energy supplied is used to overcome the forces between particles, not to raise their kinetic energy.
3. The liquid warms and the line rises again.
4. At the boiling point the temperature is constant again while the liquid boils.
5. The gas warms and the line rises.

A cooling curve is the reverse, with flat sections at the condensation and freezing points, where energy released by forming attractions keeps the temperature constant. A pure substance has sharp, constant melting and boiling points, so the flat sections are level; an impure substance melts over a range and its flat section is sloped, and its boiling point is raised.