Why solids, liquids and gases behave differently
Matter exists mainly as solids, liquids and gases. Solids, like a brick, have a fixed shape and volume because their particles are closely packed and held by strong forces of attraction. Liquids, like water, have a fixed volume but take the shape of their container, because their particles are close but can move around each other. Gases, like air, have no fixed shape or volume and spread to fill any space, because their particles are far apart and move quickly in all directions.
Gases can be compressed easily because of the large spaces between their particles, while solids and liquids hardly compress at all. Heating gives particles more energy, which can change the state of a substance. Melting turns a solid into a liquid, and freezing does the reverse. Boiling and evaporation turn a liquid into a gas, and condensation turns gas back into liquid, as when water droplets form on a cold glass. Some substances, like camphor and naphthalene balls, change straight from solid to gas through sublimation.
Why it matters
Changes of state explain the water cycle, how ice keeps drinks cool and why clothes dry in the sun. Gas cylinders hold large amounts of cooking gas because gases can be compressed. Understanding particles and forces between them prepares you for the topic of matter and its nature in Class 9.
Key terms
- Intermolecular force
- The force of attraction between particles of a substance.
- Melting
- The change of a solid into a liquid on heating.
- Condensation
- The change of a gas or vapour into a liquid on cooling.
- Compressibility
- How much the volume of a substance can be reduced by applying pressure.