Changes 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..
Explaining change of state
Melting, evaporating and boiling
The 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 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 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.
- overcome the remaining forces of attraction 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.
In evaporation, particles leave a liquid from its surface only. In boiling, bubbles of gas form throughout the liquid. They rise to the surface and escape to the surroundings, forming a gas.
The amount of energy needed to change state from solid to liquid, and from liquid to gas, depends on the strength of the forces between the particles of a substance. The stronger the forces of attraction, the more energy is required.
Every substance has its own The temperature at which a solid changes into a liquid as it is heated. and The temperature at which a substance rapidly changes from a liquid to a gas.. The stronger the forces between particles, the higher its melting and boiling points.
The strength of the forces between particles depends on the particles involved. For example, the forces between ions in an ionic solid are stronger than those between molecules in water or hydrogen. This explains the melting and boiling point data in the table.
| Substance | Bonding type | Melting point | Boiling point |
| Sodium chloride | Ionic | 801掳颁 | 1413掳颁 |
| Water | Small molecules | 0掳颁 | 100掳颁 |
| Hydrogen | Small molecules | -259掳颁 | -252掳颁 |
| Substance | Sodium chloride |
|---|---|
| Bonding type | Ionic |
| Melting point | 801掳颁 |
| Boiling point | 1413掳颁 |
| Substance | Water |
|---|---|
| Bonding type | Small molecules |
| Melting point | 0掳颁 |
| Boiling point | 100掳颁 |
| Substance | Hydrogen |
|---|---|
| Bonding type | Small molecules |
| Melting point | -259掳颁 |
| Boiling point | -252掳颁 |
Evaporation can take place below the boiling point of a substance.
Condensing and freezing
Energy is transferred from a substance to the surroundings when a substance condenses or freezes. This is because the forces of attraction between the particles get stronger.
Predicting a physical state
The state of a substance at a given temperature can be predicted if its melting point and boiling point are known. The table summarises how to work this out.
| Temperature | Predicted state |
| Given temperature < melting point | Solid |
| Given temperature is between melting and boiling points | Liquid |
| Given temperature > boiling point | Gas |
| Temperature | Given temperature < melting point |
|---|---|
| Predicted state | Solid |
| Temperature | Given temperature is between melting and boiling points |
|---|---|
| Predicted state | Liquid |
| Temperature | Given temperature > boiling point |
|---|---|
| Predicted state | Gas |
Question
The melting point of oxygen is -218掳C and its boiling point is -183掳C. Predict the state of oxygen at -200掳颁.
Oxygen will be in the liquid state at -200掳颁 (because this is between its melting and boiling points).
Limitations of the particle model
The particle model assumes that particles are solid spheres with no forces between them. However:
- particles are not solid, since atoms are mostly empty space
- many particles are not spherical