States of Matter and the Particle Model

The particle model describes matter as made of tiny particles in constant motion, and the three common states of matter—solid, liquid and gas—differ in how closely the particles are packed, how strongly they attract each other and how freely they move. In brief: in a solid the particles vibrate about fixed positions, in a liquid they are close together but can slide past each other, and in a gas they are far apart and move randomly at high speed. Changes of state (melting, boiling, condensing, freezing, sublimation) change the arrangement of particles but not the particles themselves.

Ice, water and steam are the same substance in three states. The particle model explains why solids keep their shape, why liquids flow and why gases fill their containers. This guide describes the three states and the changes between them, compares their properties in a table, works through a calculation on changes in volume and corrects common mistakes.

What is the particle model?

According to the particle model, all matter consists of particles (atoms, molecules or ions). The particles are in constant motion, and the average kinetic energy of the particles increases with temperature. There are attractive forces between particles, and between them there is empty space. Evidence includes Brownian motion (the random jiggling of pollen grains or smoke particles, caused by collisions with invisible molecules), diffusion (the spreading of a scent through a room), and the compression of gases.

How do solids, liquids and gases differ?

• Solid: particles are closely packed in a regular pattern (crystalline solids) or irregularly (glass, amorphous solids), held by strong forces. They vibrate but do not change position. Solids have a fixed shape and volume, and are almost incompressible.
• Liquid: particles are close together, but arranged randomly, and can move past each other. Liquids have a fixed volume but take the shape of the container, flow, and are almost incompressible.
• Gas: particles are far apart (on average about ten times the diameter of a molecule at normal conditions), move rapidly in random directions, and the forces between them are very weak. Gases have no fixed shape or volume, fill the whole container, and are easily compressed. Gas pressure is caused by the particles hitting the walls.
• Plasma is a fourth state, a gas of ions and electrons, found in stars and lightning.

What are the changes of state?

• Melting (solid to liquid) and freezing (liquid to solid) at the melting point (water: 0 °C).
• Boiling (liquid to gas throughout the liquid) at the boiling point (water: 100 °C at normal pressure) and evaporation (only at the surface, at any temperature).
• Condensation (gas to liquid).
• Sublimation (solid to gas directly, as dry ice does) and deposition (gas to solid, as frost).

During a change of state, the temperature stays constant while energy is supplied or removed. The energy changes the potential energy of the particles (the bonds), not their kinetic energy. The mass is conserved: the number of particles does not change.

States of matter compared

Worked example: volume changes with the state

1. Freezing of water. Water (1,000 kg/m³ at 4 °C, about 1,000 kg/m³ near 0 °C) freezes to ice with a density of 917 kg/m³. For 1.0 kg of water: V_water = 1.0 ÷ 1,000 = 1.00 × 10⁻³ m³; V_ice = 1.0 ÷ 917 = 1.09 × 10⁻³ m³. The volume increases by about 9 percent, which is why pipes burst and ice floats.
2. Boiling. At 100 °C, liquid water has a density of about 958 kg/m³ and steam at normal pressure about 0.60 kg/m³. The ratio of volumes = 958 ÷ 0.60 ≈ 1,600. So 1 liter of boiling water becomes about 1,600 liters of steam.
3. Interpretation: the large increase shows how widely spaced the particles in a gas are. If the volume is 1,600 times larger, the spacing between the particles increases by a factor of the cube root of 1,600 ≈ 11.7.

How was the particle model developed?

Democritus proposed atoms around 400 BCE. Robert Brown observed the random motion of pollen grains in 1827, and Albert Einstein explained it in 1905 by collisions with molecules, which was confirmed experimentally by Jean Perrin in 1908 and helped to establish the reality of atoms. The kinetic theory of gases was developed by Clausius, Maxwell and Boltzmann in the nineteenth century.

Which mistakes are common?

Mistake 1: Thinking that particles themselves expand or melt

The particles do not change in size. The spacing and motion change.

Mistake 2: Saying there is air between gas particles

There is empty space (vacuum) between particles.

Mistake 3: Believing that the temperature rises during melting or boiling

It stays constant while the substance changes state.

Mistake 4: Confusing boiling with evaporation

Boiling occurs throughout the liquid at the boiling point. Evaporation occurs only at the surface, at any temperature.