4.3.1 Changes of state and the particle model

Key content

The particle model says that all matter is made of tiny particles. The way the particles are arranged and move explains the properties of solids, liquids and gases.

Density of materials

Density is the mass per unit volume of a material:

\rho = \frac{m}{V}

Density is measured in kg/m³ (or g/cm³).

The particle model explains why states of matter have different densities:

Solids: particles are packed closely in a regular pattern and vibrate about fixed positions. Solids are usually the most dense.
Liquids: particles are still close together but are arranged randomly and can move past each other.
Gases: particles are far apart and move randomly at high speeds. Gases have a much lower density.

You should be able to recognise and draw simple particle diagrams for the three states: circles touching in rows (solid), touching but jumbled (liquid), and a few circles spread far apart (gas).

When a substance changes state, its mass stays the same but its volume changes, so its density changes ( $\rho = \dfrac{m}{V}$ ). For example, when water boils, the same mass of steam takes up a much larger volume, so its density is much lower.

You measure the density of regular and irregular objects and liquids in Required practical 5: find the mass with a balance; for a regular solid, measure its dimensions (with a ruler, Vernier callipers or a micrometer) and calculate the volume; for an irregular solid, find the volume by displacement (a eureka can and a measuring cylinder); for a liquid, read the volume from a measuring cylinder. Then use $\rho = \dfrac{m}{V}$ .

Changes of state

The changes of state are melting, freezing, boiling, evaporating, condensing and sublimating (solid straight to gas). Boiling and evaporating both turn a liquid into a gas; evaporation happens only at the surface, below the boiling point.

When a substance changes state, its mass is conserved – the number of particles stays the same.

Changes of state are physical changes, not chemical changes. The material recovers its original properties if the change is reversed.

Equations

Must recall

density = mass ÷ volume

\rho = \frac{m}{V}

Symbol	Meaning	Unit
ρ	density	kg/m³
m	mass	kg
V	volume	m³

Worked examples

Worked example

A block has a mass of 540 g and a volume of 200 cm³. Calculate its density in kg/m³.

Convert to SI units: $m = 0.540$ kg and $V = 200 \times 10^{-6} = 0.000200$ m³

$\rho = \dfrac{m}{V} = \dfrac{0.540}{0.000200} = 2700$ kg/m³

Common misconceptions

Common misconception

"Particles get bigger when a substance expands or melts." The particles stay the same size – the spacing between them changes.

Common misconception

"Mass is lost when water boils." The mass is conserved – the particles are still there as water vapour.