Predicting physical states
Melting, evaporating and boiling
energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. must be transferred, by heating, to a substance for these changes of stateSolid, liquid or gas. Evaporation is a change of state from liquid to gas. to happen. During these changes the particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. gain energy, which is used to break or overcome:
- some of the bondThe chemical link that holds molecules together. between particles during meltingThe process that occurs when a solid turns into a liquid when it is heated.
- all of the remaining bonds between particles during evaporationThe process in which a liquid changes state and turns into a gas. or boiling
Evaporation can take place below the boiling pointThe temperature at which a substance rapidly changes from a liquid to a gas. of a substance. The rate of evaporation is at its maximum at the boiling point.
Condensing and freezing
Energy must be transferred from a substance to the environment for condensationA change of state in which gas becomes liquid by cooling. and freezeA change of state in which liquid becomes solid by cooling. to happen. During these changes of state the particles lose energy as bonds form between the particles.
Predicting a physical state
The state of a substance at a given temperature can be predicted if its melting pointThe temperature at which a solid changes into a liquid as it is heated. and boiling point are known. The table summarises how to work this out.
| Comparison | Predicted state |
| Given temperature < melting point | Solid |
| Given temperature is between melting and boiling points | Liquid |
| Given temperature > boiling point | Gas |
| Comparison | Given temperature < melting point |
|---|---|
| Predicted state | Solid |
| Comparison | Given temperature is between melting and boiling points |
|---|---|
| Predicted state | Liquid |
| Comparison | Given temperature > boiling point |
|---|---|
| Predicted state | Gas |
Question
The melting point of oxygen is -218°C and its boiling point is -183°C. Predict the state of oxygen at -200°C.
Oxygen will be in the liquid state at -200°C (because this is between its melting and boiling points).