Explain what happens when a liquid boils.
If a liquid is heated the particles are given more energy and move faster and faster expanding the liquid. The most energetic particles at the surface escape from the surface of the liquid as a vapour as it gets warmer. Liquids evaporate faster as they heat up and more particles have enough energy to break away.
The particles need energy to overcome the attractions between them. As the liquid gets warmer more particles have sufficient energy to escape from the liquid. Eventually even particles in the middle of the liquid form bubbles of gas in the liquid. At this point the liquid is boiling and turning to gas. The particles in the gas are the same as they were in the liquid they just have more energy. At normal atmospheric pressure all materials have a specific temperature at which boiling occurs. This is called the "boiling point" or boiling temperature. As with the melting point the boiling point of materials vary widely e.g. nitrogen -210oC, alcohol 78 oC, aluminium 459 oC.
Any material with a boiling temperature below 20oC is likely to be a gas at room temperature. When liquids boil the particles must have sufficient energy to break away from the liquid and to diffuse through the surrounding air particles. As these particles cool down and lose energy they will condense and turn back to liquid. When steam is formed by water boiling at 100oC the particles quickly condense as the surrounding air temperature is likely to be much less that 100oC so the particles cool rapidly.
In fact the "steam" coming out of a boiling kettle can only be seen because some of the gas particles have condensed to form small droplets of water.
When a gas turns to a liquid (condenses) or a liquid turns to a solid (solidifies) the particles lose energy to the surroundings.