Topic 1: States of matter

Everything around you is made of tiny particles that are always moving. The way these particles are arranged and how fast they move decides whether a substance is a solid, a liquid or a gas. This topic gives you the model you will use to explain melting, boiling, evaporation and diffusion throughout the whole course.

Kinetic particle theory

The kinetic particle theory states that all matter is made of small particles that are in constant motion. The word kinetic means movement. According to this model, particles attract one another, and the strength of this attraction together with the amount of energy the particles have determines the state of the substance. When particles gain energy they move faster and move further apart; when they lose energy they slow down and move closer together. This simple idea lets us explain almost every physical change you will meet in chemistry, from a melting ice cube to the spread of a smell across a room.

The three states and their properties

In a solid the particles are packed closely together in a fixed, regular pattern. They cannot move from place to place but vibrate about fixed positions, so a solid has a fixed shape and a fixed volume and cannot be compressed. In a liquid the particles are still close together but are arranged randomly and can slide past one another. This is why a liquid has a fixed volume but takes the shape of its container and also cannot be compressed by much. In a gas the particles are far apart, arranged randomly, and move quickly in all directions. A gas has no fixed shape and no fixed volume; it spreads to fill its container and can be compressed easily because of the large spaces between particles. The forces of attraction between particles are strongest in solids, weaker in liquids and weakest in gases.

Changes of state and their names

A substance can change from one state to another when it is heated or cooled. Heating supplies energy that lets particles overcome the forces holding them together; cooling removes energy so the forces pull the particles closer. Melting is the change from solid to liquid, and freezing (solidifying) is the reverse, from liquid to solid. Boiling and evaporation both turn a liquid into a gas, while condensation turns a gas back into a liquid. Sublimation is the direct change from solid to gas without passing through the liquid state, and the reverse direct change from gas to solid is called deposition. Each pure substance changes state at a definite temperature, for example its melting point and its boiling point.

Evaporation versus boiling

Evaporation and boiling both produce a gas from a liquid, but they are not the same process. Evaporation happens only at the surface of a liquid and can occur at any temperature below the boiling point, because the fastest particles near the surface escape into the air. Because the most energetic particles leave, the average energy of those remaining falls, so evaporation causes cooling. Boiling, in contrast, happens throughout the whole liquid, produces bubbles of gas inside it, and occurs only at a fixed temperature called the boiling point. Evaporation speeds up when the temperature is higher, the surface area is larger, or air moves across the surface.

Heating and cooling curves

If you heat a solid steadily and record its temperature, you can draw a heating curve. At first the temperature rises as the solid warms. When melting begins, the temperature stays constant even though heat is still being added, because the energy is used to break the forces holding the particles in place rather than to raise the temperature. Once melting is complete the temperature rises again until boiling starts, where it once more holds steady during the change of state. A cooling curve is the mirror image: the temperature falls, then flattens at the boiling point during condensation and again at the freezing point during solidifying, because energy is released as the forces between particles re-form. These flat sections, called plateaus, mark the melting and boiling points of the substance.

Diffusion

Diffusion is the movement of particles from a region where they are more concentrated to a region where they are less concentrated, until they are spread evenly. It happens because particles are in constant random motion and naturally mix together. Diffusion is fastest in gases, slower in liquids, and extremely slow in solids, because gas particles move quickly and have large spaces to move through. A common example is smelling perfume from across a room, or a coloured dye spreading through still water. Diffusion does not need any stirring; the random motion of the particles does all the work.

Effect of mass and temperature on diffusion

The rate at which a gas diffuses depends on the mass of its particles. Lighter particles, which have a smaller relative molecular mass, move faster at a given temperature and therefore diffuse more quickly than heavier particles. A classic demonstration uses cotton wool soaked in ammonia solution at one end of a glass tube and cotton wool soaked in concentrated hydrochloric acid at the other; a white ring of ammonium chloride forms nearer the acid end because the lighter ammonia molecules travel faster than the heavier hydrogen chloride molecules. Raising the temperature also speeds up diffusion, because the extra energy makes all the particles move faster.