Change of state
The diagram summarises the common changes of state.
Some substances can change directly from solid to gas without becoming a liquid in between. This is called When a solid turns straight into a gas on heating, without becoming a liquid first - or when a gas turns straight into a solid, without becoming a liquid.. Solid carbon dioxide ('dry ice') can Able to change from a solid to a gas, or from a gas to a solid, without becoming a liquid..
Changes of state are A change during which no new substances are formed. For example changes in state.. Unlike chemical reactions, no new substances are formed during changes of state.
Explaining change of state
Melting, evaporating and boiling
Energy must be transferred, by heating, to a substance for these changes of Solid, liquid or gas. Evaporation is a change of state from liquid to gas. to happen. During these changes the A 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 The chemical link that holds molecules together. between particles during The process that occurs when a solid turns into a liquid when it is heated.
- all the remaining bonds between particles during The process in which a liquid changes state and turns into a gas. or Changing from the liquid to the gas state, in which bubbles of gas form throughout the liquid.
Evaporation can take place below the The temperature at which a substance rapidly changes from a liquid to a gas. of a substance. This is why damp clothes dry when they are hung from a washing line. Boiling happens at the boiling point, when the rate of evaporation is at its maximum.
Condensing and freezing
Energy must be transferred from a substance to the environment for A change of state in which gas becomes liquid by cooling. and A 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 The 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).
