Cambridge IGCSE Chemistry

Solids, liquids and gases

All matter is made of particles, and which state a substance is in depends on how those particles are arranged and how they move. The three states differ in what they do to shape and volume, and each difference follows from the particle picture.

A solid keeps its shape because its particles cannot leave their positions, and a liquid flows because its particles are free to move past each other while still touching. A gas can be squashed into a smaller volume because most of the space between its particles is empty, whereas a solid or liquid has almost no spare space to lose.

Changes of state

Heating a solid gives its particles more energy, so they vibrate harder until they break free of their fixed positions and the solid melts. A liquid boils when it is heated to its boiling point and particles escape from throughout the liquid as bubbles of vapour. Evaporation is different: it happens only at the surface and at any temperature, because the fastest-moving particles near the surface have enough energy to escape. Cooling reverses the changes, so a gas condenses to a liquid and a liquid freezes to a solid.

Heating and cooling curves

Supplement only: changes of state explained with kinetic particle theory, including reading heating and cooling curves.

When a solid is heated steadily, a graph of temperature against time rises, then goes flat at the melting point, rises again, goes flat again at the boiling point and then rises once more. On each flat section the substance is changing state and the temperature does not change, even though energy is still being supplied.

That is because the energy is being used to overcome the forces of attraction between the particles, not to make them move faster. The average kinetic energy of the particles, which is what temperature measures, stays the same until the change is complete. During melting the particles are pulled out of their regular arrangement, and during boiling they are pulled completely apart from each other. On a cooling curve the flat sections appear at the freezing point and at the condensing point, and they are flat because the particles are forming attractions and releasing the same amount of energy to the surroundings.

A pure substance has a sharp melting point and boiling point, so it gives flat sections at fixed temperatures. A mixture melts over a range of temperatures, which is why a melting point is used as a test of purity.

Gases, temperature and pressure

A gas exerts pressure because its particles collide with the walls of the container, and each collision pushes on the wall.

Reducing the volume of a fixed amount of gas at constant temperature raises its pressure. Supplement only: the same particles are now in a smaller space, so they hit the walls more often, and more collisions per second means a greater force on each unit of wall area. Increasing the pressure on a gas therefore decreases its volume.

Raising the temperature of a gas in a container that can expand, such as a syringe or balloon, makes its volume increase. Supplement only: the particles gain kinetic energy, so they move faster and hit the walls harder and more often. To keep the pressure constant the walls must move outwards, and the volume grows. In a sealed rigid container the volume cannot change, so the pressure rises instead.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, until they are evenly spread. It happens because the particles are always moving randomly: there are more particles moving out of the crowded region than into it, so the overall drift is down the concentration gradient. No stirring or flow is needed.

A drop of coloured dye in still water spreads out slowly, and the smell of cooking travels across a room. Diffusion is fast in gases, slower in liquids and almost absent in solids, because particles in a gas move freely and quickly and have far to go before meeting another particle. Raising the temperature speeds it up, because the particles move faster.

Relative molecular mass and the rate of diffusion

Supplement only: gases with a lower relative molecular mass diffuse faster than gases with a higher one, at the same temperature. At a given temperature all gas particles have the same average kinetic energy, so the lighter particles must move at a higher average speed.

This can be shown with cotton wool soaked in concentrated ammonia solution at one end of a long glass tube and cotton wool soaked in concentrated hydrochloric acid at the other end. Each gives off a gas, and where the two gases meet they react to form a ring of white solid, ammonium chloride:

Ammonia ($M_r = 17$) diffuses faster than hydrogen chloride ($M_r = 36.5$), so the ring forms closer to the hydrochloric acid end of the tube. The tube needs to be dry and clean, because moisture would dissolve the gases and slow the movement.