When a substance boils, it is converted from its liquid state into its gaseous state as shown below.
The blue particles could be individual atoms, but more commonly they consist of several atoms joined by covalent bonds (as between C and H in CH4 particles).
When heat energy is supplied, the particles move faster. Boiling occurs when blue particles from both the surface of the liquid and underneath the surface have sufficient energy to overcome the attractive forces BETWEEN them and escape to the gas phase.
In the gas phase of the substance
The temperature at which boiling occurs depends on the strength of the attractive forces between the particles (blue particles above or CH4 molecules in methane). Substances that have boiling temperatures below room temperature (22 °C) are gases at room temperature.
The CH4 and H2O particles are molecules.
The attractive forces between the molecules in molecular substances are referred to as intermolecular (inter = between).
| Liquid | Gas | |
 | vaporises on heating to boiling temperature |  |
| increasing energy of system | |
When heat energy is supplied, the particles move faster. Boiling occurs when blue particles from both the surface of the liquid and underneath the surface have sufficient energy to overcome the attractive forces BETWEEN them and escape to the gas phase.
In the gas phase of the substance
the particles are moving faster than in the liquid phase
the attractive forces BETWEEN particles are weaker than in the liquid phase
the composition of the particles is the same as in the liquid phase.
the attractive forces BETWEEN particles are weaker than in the liquid phase
the composition of the particles is the same as in the liquid phase.
In CH4 gas, the CH4 molecules are the same as in CH4 liquid. The bonds between C and H are not disturbed when boiling takes place.
The temperature at which boiling occurs depends on the strength of the attractive forces between the particles (blue particles above or CH4 molecules in methane). Substances that have boiling temperatures below room temperature (22 °C) are gases at room temperature.
CH4 is a gas because it has a boiling temperature of -161 °C.
H2O is a liquid because it has a boiling temperature of 100 °C.
H2O is a liquid because it has a boiling temperature of 100 °C.
The CH4 and H2O particles are molecules.
The attractive forces between the molecules in molecular substances are referred to as intermolecular (inter = between).