What Is Matter?
Matter is anything that has mass and occupies space — stones, trees, chairs, dust and chalk are all examples. Matter exists in three states: solid, liquid and gas, and these states can change from one to another.
Changes of State
- Evaporation: the change of state from liquid to gas.
- Boiling point: the temperature at which a liquid boils — water boils at 100°C.
- Sublimation: the change of state from solid directly to gas, without passing through the liquid state — camphor and ammonium chloride are examples of substances that sublime.
- Freezing (solidification): the change of state from liquid to solid, at the substance’s freezing point — water becomes ice.
- Condensation (liquefaction): the change of state from gas to liquid.
The Kinetic Theory of Matter
The kinetic theory states that all matter is made up of tiny particles that are in constant, random motion because they possess kinetic energy. When a substance changes state, its particles simply rearrange themselves — the number of particles doesn’t change.
- Solids: particles are packed closely together in a regular arrangement and can only vibrate in place. Heating makes them vibrate more until they break free into a liquid (melting/fusion) or, in some cases, straight into a gas (sublimation).
- Liquids: particles are close together but not fixed in place, and can move past one another over short distances. Heating increases their kinetic energy until some escape as a gas (evaporation).
- Gases: particles are held together only by weak forces of attraction and move freely. Compressing or cooling a gas can turn it back into a liquid (condensation) or, in some cases, straight into a solid.
The kinetic theory explains the nature of matter, why matter exists in three states, why the strength of the forces between particles differs between solids, liquids and gases, and how and why matter changes from one state to another.
Physical and Chemical Changes
| Physical change | Chemical change | |
|---|---|---|
| Reversibility | Easily reversible | Not easily reversible |
| New substance formed? | No | Yes |
| Examples | Melting ice, evaporating water, any change of state | Burning, decomposition, neutralisation reactions |
