The three states of matter — AQA GCSE Chemistry

Matter can exist in three physical states: solid, liquid and gas. The state depends on the arrangement, spacing and movement of the particles, and on the strength of the forces between them. In a solid, particles are close together in a regular arrangement and vibrate about fixed positions, so a solid has a fixed shape and volume. In a liquid, particles are close together but randomly arranged and can slide past one another, so a liquid has a fixed volume but takes the shape of its container. In a gas, particles are far apart, randomly arranged and move quickly in all directions, so a gas fills its container and is easily compressed. For example, ice, water and steam are the solid, liquid and gas states of the same substance. Changes of state are physical changes and do not alter the chemical identity of the substance.

Each pure substance changes state at fixed temperatures. Melting is the change from solid to liquid and freezing is the reverse, liquid to solid; both occur at the melting point. Boiling is the change from liquid to gas and condensing is the reverse, gas to liquid; both occur at the boiling point. For a pure substance, the melting point and freezing point are the same temperature, and the boiling point and condensation point are the same temperature. For example, pure water melts and freezes at 0 °C and boils and condenses at 100 °C at normal atmospheric pressure. During a change of state, energy is transferred but the temperature stays constant until the change is complete. Impurities usually lower the melting point and raise the boiling point, which is why melting and boiling points can be used to check purity.

The simple particle model pictures matter as tiny solid spheres. In a solid, the spheres are packed closely in a regular arrangement and only vibrate about fixed positions, so the shape and volume stay fixed. In a liquid, the spheres are still close but randomly arranged and can slide past one another, so the volume is fixed but the shape flows to fit the container. In a gas, the spheres are far apart, randomly arranged and move quickly in all directions, so both shape and volume change to fill the container. For example, ice, water and steam all contain H₂O particles, but the spacing and motion of the spheres differ. The model is useful but limited: it ignores forces, particle size and the internal structure of atoms.

Particle theory explains changes of state by tracking energy, spacing and motion. Melting: a solid is heated, particles vibrate more until the forces holding the regular arrangement are overcome, and the solid becomes a liquid. Boiling: a liquid is heated, particles gain enough energy to overcome the forces between them and escape as a gas throughout the liquid. Freezing: a liquid cools, particles lose energy and move less, so forces pull them into a regular solid arrangement. Condensing: a gas cools, particles lose energy and move closer together, forming a liquid. In each change the particles themselves are unchanged; only their energy, spacing and arrangement alter. For example, steam condensing on a cold window forms liquid water droplets.

Changing state requires energy to overcome the forces between particles. Melting needs energy to break down the regular solid arrangement; boiling needs energy to separate liquid particles into a gas. If the forces between particles are strong, more energy is needed, so the melting point and boiling point are higher. If the forces are weak, less energy is needed, so the melting point and boiling point are lower. For example, water has stronger forces between its particles than methane, so water boils at 100 °C while methane boils at about −162 °C. The energy supplied during melting or boiling increases the potential energy of the particles rather than their average kinetic energy, so the temperature stays constant while the state changes.

The particles that make up a substance are not always the same kind. In a simple molecular substance such as water, the particles are molecules held together by weak intermolecular forces. In a giant ionic structure such as sodium chloride, the particles are ions held in a lattice by strong electrostatic forces. In a giant covalent structure such as diamond, the particles are atoms joined by strong covalent bonds throughout the structure. In a metal such as copper, the particles are positive ions in a sea of delocalised electrons. So the type of bonding and the structure determine what the particles are and how strongly they interact, which in turn affects melting and boiling points and other physical properties.

Melting and boiling happen when energy transferred by heating overcomes the forces holding particles together. Stronger forces need more energy to weaken or break, so the substance melts and boils at higher temperatures. For example, magnesium oxide has strong electrostatic forces between Mg²⁺ and O²⁻ ions, giving a melting point around 2852 °C, whereas iodine has weak intermolecular forces between I₂ molecules and melts near 114 °C. The forces overcome are between particles, not inside molecules: covalent bonds within molecules stay intact during melting and boiling. Comparing substances therefore means comparing the type and strength of forces between their particles, and linking this to the energy needed for the change of state.

The simple particle model draws particles as small solid spheres with empty space between them. This is useful for explaining diffusion, gas pressure and changes of state, but it is a simplification. Real particles exert forces on one another, and those forces explain why substances melt and boil at particular temperatures. Real particles are not solid spheres: atoms and molecules have internal structure, and their shapes affect how they pack and how they interact. The model also omits the range and direction of forces and any vibration or movement within particles. A strong answer identifies a specific limitation and explains how it affects a prediction, for example that a model with no forces cannot explain why energy is needed to melt a solid.

This outcome combines four skills. First, use melting and boiling point data to predict whether a substance is solid, liquid or gas at a stated temperature: below the melting point it is solid, between melting and boiling points it is liquid, and above the boiling point it is gas. Second, explain why changes of state occur at particular temperatures by linking energy transferred by heating to the strength and type of bonding or forces between particles. Third, recognise that a single atom does not have the bulk properties of the material it makes up, such as melting point or electrical conductivity. Fourth, at Higher tier, explain that a model of solid inelastic spheres with no forces between them cannot account for energy transfers during melting and boiling.