The particle model, the three states of matter, changes of state, the energy and arrangement of particles in solids, liquids and gases, and the limitations of the simple model.

What this topic is asking

OCR wants you to use the particle model to describe and explain the three states of matter, the changes of state between them, and how the energy and arrangement of the particles differ. You also need to recognise the limitations of the simple model. This is the foundation for the whole of GCSE Chemistry, so the vocabulary here (particles, energy, arrangement) is reused constantly.

The three states of matter

The three states differ in the arrangement, movement and energy of their particles:

• Solid. Particles are packed closely together in a regular arrangement, touching their neighbours. They vibrate about fixed positions but cannot move from place to place. Solids have a fixed shape and fixed volume and cannot be compressed.
• Liquid. Particles are still close together and touching, but arranged randomly. They have enough energy to move past one another, so a liquid flows and takes the shape of its container, but it has a fixed volume and is almost incompressible.
• Gas. Particles are far apart, arranged randomly, and move quickly in all directions. A gas has no fixed shape or volume; it spreads out to fill its container and can be compressed because of the large spaces between particles.

Changes of state

Changes of state are physical changes: no new substance is made and the change can be reversed. The mass stays the same. The six named changes are:

• Melting (solid to liquid) and freezing (liquid to solid).
• Boiling or evaporating (liquid to gas) and condensing (gas to liquid).
• Sublimation (solid directly to gas) and deposition (gas directly to solid).

When a substance is heated, the particles gain energy and move or vibrate faster. At the melting point the particles have enough energy to break out of their fixed positions, and at the boiling point they have enough energy to fully separate. When a substance is cooled, the particles lose energy, move more slowly, and the reverse changes happen.

Predicting the state from melting and boiling points

You can predict the state of a substance at a given temperature by comparing it with the melting point and boiling point:

• Below the melting point the substance is a solid.
• Between the melting and boiling points it is a liquid.
• Above the boiling point it is a gas.

For example, oxygen has a melting point of $-219\ \degree\text{C}$ and a boiling point of $-183\ \degree\text{C}$, so at room temperature ($25\ \degree\text{C}$) it is well above its boiling point and is a gas.

Limitations of the simple particle model

OCR expects you to know that the simple model is useful but not perfect. Its limitations are that it assumes:

• the particles are solid spheres, when real atoms are mostly empty space with a tiny nucleus;
• there are no forces between the particles, when in reality the forces of attraction are exactly what hold solids and liquids together;
• the particles do not have their own internal structure or different shapes.

Because of these assumptions, the simple model cannot fully explain, for example, why some substances expand more than others or how bonding affects properties.