States of matter: solid, liquid and gas

Everything around you that has weight and takes up space is called matter. Your book, the water in your bottle and the air in a balloon are all matter. Matter is found mainly in three states: solid, liquid and gas.

Made of tiny particles

All matter is made of extremely small particles, far too small to see. The state of a substance depends on how these particles are packed and how they move.

Solids

In a solid, the particles are packed very closely in a fixed pattern. They cannot move from their places; they only vibrate. That is why a solid has a fixed shape and a fixed volume. A stone stays the same shape wherever you keep it. Examples: wood, iron, ice, a pencil.

Liquids

In a liquid, the particles are close together but not fixed. They can slide past one another. So a liquid has a fixed volume but no fixed shape: it takes the shape of its container, and it can flow. One glass of water poured into a bowl is still the same amount of water. Examples: water, milk, oil.

Gases

In a gas, the particles are far apart and move freely and fast in all directions. A gas has no fixed shape and no fixed volume. It spreads out to fill any container, and it can be squeezed into a smaller space, as when air is pumped into a tyre. Examples: air, oxygen, steam, cooking gas.

Changing from one state to another

Heating gives particles more energy, so they move faster and break free. Cooling does the opposite.

• Melting: solid to liquid. Ice melts into water at 0°C.
• Evaporation and boiling: liquid to gas. Water boils into steam at 100°C.
• Condensation: gas to liquid. Steam touching a cold lid turns into drops of water.
• Freezing: liquid to solid. Water kept in the freezer becomes ice.

Water is special because we meet it in all three states in daily life: ice, water and steam. In each state it is still the same substance. Only the arrangement and movement of its particles have changed. Such a change, which can be reversed, is called a physical change.