States of matter and the particle model

Ice, liquid water and steam are the same substance, yet one holds its shape, one pours and one spreads through a whole room. The difference lies in the particles: how close together they are, how they move and how strongly they pull on one another. This picture, the kinetic particle model, is the starting point for the whole of thermal physics.

The kinetic particle model

The model describes all matter as made of tiny particles, such as atoms or molecules, that are:

• always moving, in a random way;
• attracted to one another, with forces that are strong when the particles are close and very weak when they are far apart.

Every observable property of a solid, liquid or gas comes from these two facts and from how far apart the particles are.

Comparing solids, liquids and gases

Scroll across the figure to read all labels.

Turn up the heating and watch the arrangement and motion change from solid to liquid to gas.

Reset

States of matter with the heater at 5%. The substance is a solid: particles vibrate in a regular pattern.

State

solid

Average close neighbours

4.9

SolidLiquidGasGas pressure

Model States of matterGas pressureBrownian motionDiffusion

Heating

%

Turn the heater up or down. The particles gain or lose kinetic energy.

More settings

Try this

0 of 4 done

1. Cool the substance until its particles are fixed in a regular pattern. (not done yet)

2. Heat it until the particles spread out through the whole container. (not done yet)

3. Halve the volume of the gas at the same temperature and wait for the pressure to settle. (not done yet)

4. Remove the partition and wait until the two gases have mixed. (not done yet)

“Fixed volume” does not mean a solid or liquid can never expand; most expand a little when warmed. Solids are usually denser than their liquids, but not always: ice floats on water.

Why each state behaves as it does

Solids

The particles are packed closely and held by strong attractive forces, so each one can only vibrate about a fixed position. Because no particle can move to a new place, the solid keeps its shape. Because there is almost no space between the particles, it is very hard to compress.

In a crystalline solid, such as salt, the particles form a regular repeating pattern. Glass is a solid without a regular pattern; it keeps its shape for the same reason, because its particles cannot slide past one another.

Liquids

The particles are still close together, held by attraction, but they have enough energy to move about and slide past one another. Being close together, they have little space to squeeze into, so a liquid keeps its volume and is hard to compress. Being free to move past one another, they let the liquid flow and take the shape of its container.

Gases

The particles are far apart, typically about ten times their own size, and move quickly in all directions. At such distances the attractive forces are very weak, so nothing holds the particles together: a gas spreads out to fill any container. Most of a gas is empty space, so squeezing it simply moves the particles closer together. That is why a gas is easy to compress.

Worked example 1

A fixed volume but no fixed shape

Problem

Which state of matter has a fixed volume but no fixed shape? Explain both properties using the particles.

Worked solution

1. Name the state

   Method

   The state is a liquid.

   Reason

   A liquid keeps its volume but takes the shape of its container.

   Working

   Fixed volume + no fixed shape → liquid

2. Explain each property with a different particle feature

   Method

   Link the volume to closeness and the shape to movement.

   Reason

   Each property needs its own cause; “the particles are close” explains the volume but not the flow.

   Working

   The particles are close together and attract one another, so the volume stays the same. They can slide past one another, so the liquid flows into the shape of its container.

Guided practice 2

Why a gas is easy to compress

About 4 min

Problem

Explain why a gas is easy to compress but a solid or liquid is not.

Write your explanation, then check the key idea

Your explanation

What does compressing a gas reduce?

The gaps between the particlesThe size of the particlesThe number of particles

Check answer

Hints

Hint 1: compare the spacing

How far apart are the particles in each state?

Hint 2: keep the particles the same

Compression changes the spacing, not the particles themselves.

Show solution step by step

1. Compare the spacing

   Method

   Gas particles are far apart; solid and liquid particles are already close together.

   Reason

   Only the gas has a lot of empty space between its particles.

   Working

   Gas: gaps much larger than the particles. Solid and liquid: almost no gaps.

2. Describe the compression

   Method

   Squeezing a gas pushes its particles closer together.

   Reason

   There is plenty of space to remove before the particles meet, so the volume can fall a lot.

   Working

   Same number and size of particles in a smaller space.

Check your understanding 1: Flows but will not squash

A liquid can change shape easily, yet it is very hard to compress. Explain both, using a different particle feature for each.

Show answer

The particles can slide past one another, so the liquid flows and changes shape. They are already close together, so there is almost no empty space to remove when it is squeezed.

Temperature and particle motion

Heating a solid, liquid or gas makes its particles move faster on average, or vibrate more strongly in a solid. A rise in temperature means a rise in the average kinetic energy of the particles. The particles never all have the same speed; temperature tracks their average.

During melting or boiling, the energy supplied separates the particles instead, and the temperature stays the same. The internal energy lesson explains this.

Common mistakes

• Saying the particles in a solid do not move. They vibrate about fixed positions.
• Saying a liquid has no fixed volume. It changes shape, not volume.
• Saying compression makes the particles smaller. It moves them closer together.
• Stopping at “strong forces” or “weak forces”. Say what the forces let the particles do, and which property that explains.
• Saying every solid has a regular arrangement. That is true of crystalline solids only.