States of matter and changes of state

Water from a borehole, the ice in a cool box at a wedding, and the steam rising from a saucepan of matooke are all the same substance. What changes is how its tiny particles are arranged and how much energy they have.

By the end of these notes you should be able to:

• define matter and name its states;
• compare the properties of solids, liquids and gases;
• use the kinetic theory of matter to explain those properties;
• explain that changes of state involve heat gain or heat loss.

What is matter?

Matter is anything that has mass and occupies space (has volume). Air, water, stones, your body and the food you eat are all matter. Light, sound and heat are not matter: they are forms of energy.

Matter exists in four states:

1. Solid: for example stone, wood, ice, salt.
2. Liquid: for example water, paraffin, cooking oil, milk.
3. Gas: for example air, steam, carbon dioxide, cooking gas (LPG).
4. Plasma: a very hot state in which gas particles have lost electrons and become charged. It is found in the sun and stars, lightning and the glow inside fluorescent tubes.

Properties of solids, liquids and gases

The kinetic theory of matter

The word kinetic means "to do with movement". The kinetic theory of matter says:

1. All matter is made of very tiny particles (atoms, molecules or ions).
2. These particles are always moving.
3. There are forces of attraction between the particles.
4. The higher the temperature, the faster the particles move, because they have more kinetic energy.

Evidence that particles exist and move comes from diffusion (the smell of roasting gonja spreading through a room) and Brownian motion (the random, jerky movement of smoke or pollen grains seen under a microscope, caused by invisible particles hitting them).

Particles in each state

Using the theory to explain properties

• Solids have a fixed shape because strong forces hold the particles in fixed positions; they can only vibrate.
• Liquids flow and take the shape of the container because the particles can slide past each other, but the forces are still strong enough to keep them close together, so the volume stays fixed.
• Gases are easy to compress because there are large spaces between the particles. Pushing simply moves them closer together.
• Gases fill any container because the particles move fast in all directions with almost no attraction to hold them back.

Changes of state

A substance changes state when it gains or loses heat energy. The particles themselves do not change; only their energy, arrangement and movement change.

What happens to the particles

• Melting: heat makes the particles vibrate more and more until they have enough energy to overcome some of the forces holding them in place. They begin to slide past each other.
• Boiling: heat gives particles enough energy to overcome almost all the forces between them, so they escape as a gas.
• Condensing and freezing are the reverse: particles lose energy, slow down, and the forces of attraction pull them closer together.

Evaporation and boiling are different

Why does sweating cool you down? The fastest-moving particles escape from the sweat on your skin. The particles left behind have less energy on average, so the skin cools. This is also why water stays cool in an earthenware pot (ensuwa): water seeps through the clay and evaporates from its outer surface.

Melting point and boiling point

• Melting point is the temperature at which a solid turns into a liquid. Pure ice melts at 0 °C.
• Boiling point is the temperature at which a liquid boils. Pure water boils at 100 °C at sea level.

A pure substance has a sharp, fixed melting point and boiling point. Impurities change them: salt lowers the freezing point of water and raises its boiling point. Chemists use melting and boiling points to test purity.

A heating curve

If you heat ice steadily from −10 °C and record its temperature every minute, you get a heating curve with flat parts:

1. The temperature of the ice rises to 0 °C.
2. It stays at 0 °C while the ice melts, even though heating continues. The heat is used to overcome the forces between particles, not to raise the temperature.
3. The temperature of the water rises from 0 °C to 100 °C.
4. It stays at 100 °C while the water boils.
5. The temperature of the steam rises above 100 °C.

Summary

• Matter has mass and occupies space. It exists as solid, liquid, gas and plasma.
• The kinetic theory says matter is made of moving particles with forces between them; higher temperature means faster particles.
• Solids: close, regular, vibrating, strong forces. Liquids: close, irregular, sliding, weaker forces. Gases: far apart, fast, random, very weak forces.
• Melting, boiling, evaporation and sublimation take in heat. Condensation, freezing and deposition give out heat.
• Temperature stays constant during a change of state. Pure substances have fixed melting and boiling points.

Practice questions

1. Define matter and give two examples of each of the three main states.
2. Use the kinetic theory to explain why (a) a gas can be compressed but a solid cannot, (b) a liquid takes the shape of its container.
3. Name the change of state in each case and say whether heat is gained or lost: (a) a puddle drying in the sun (b) dew forming on grass in the morning (c) candle wax hardening.
4. Give three differences between evaporation and boiling.
5. Explain why the temperature of boiling water does not rise above 100 °C even though it is still being heated.
6. Activity of integration. A family in Gulu wants to keep drinking water cool without a fridge. Using what you have learnt about evaporation, design a simple cooler from local materials, draw it, and explain how it works.