**Ideal gas**: A gas that obeys **Boyle's law** and **Charles's law**, and hence the **Ideal gas equation**.

**Boyle's law**: The pressure, *p*, of a fixed mass of gas is inversely proportional to its volume, *V*, at constant temperature (i.e. *p*** ∝ **1/*V*).

**Charles's law**: The volume, *V*, of a fixed mass of gas is proportional to its absolute temperature, *T*, at constant pressure (i.e. *V*** ∝ ***T*).

It is found experimentally that at constant pressure and temperature the volume of a gas is directly proportional to the amount of gas, *n*, and at constant volume and temperature the pressure is directly proportional to the amount of gas, *n*. Here are two graphs which show these facts clearly.

**Avogadro's hypothesis**: Equal volumes of different gases at the same temperature and pressure contain the same number of molecules (or atoms for monoatomic gases).

All this is expressed by the **ideal gas equation**.

**Ideal gas equation**: *pV* = *nRT*. *R* is called the **universal gas constant**,

and has the value 8.314 J K^{−1} mol^{−1}. This equation is particularly useful in determining the amount of a gas from its volume, pressure and temperature.

**Ideal gas**: a gas for which the individual molecules or atoms occupy negligible volume, and for which there are no attractive or repulsive forces between the molecules (or atoms).

**Dalton's law of partial pressures**: The total pressure of a mixture of gases is the sum of the **partial pressures** of the individual gases in the mixture. All real gases deviate from ideal behaviour, the deviation increasing with increasing *p* and decreasing *T*.

**Partial pressure**: The pressure that a particular gas in a mixture of gases would exert if it alone occupied the container.

The **kinetic theory of gases** makes the following assumptions for an ideal gas:

- Gases are made up of molecules whose sizes are negligible compared with the distance between them.
- There are no forces between the molecules (except in a collision).
- Between collisions the molecules are constantly moving in straight lines and their motion is completely random.
- The molecules are constantly colliding with one another and with the walls of the container, all collisions being elastic (i.e. no loss of kinetic energy).
- The collisions with the walls of the container give rise to the measured gas pressure.

**Vapour**: An alternative term for gas. It is usually used when it is in contact with the liquid form, solid form or solution of the same substance, or is at a temperature at which it could be made to condense by increasing its pressure. The term **evaporation** illustrates this.

In each of the following assume that the ideal gas equation is obeyed.