The boiling points of liquids are usually recorded at sea level, where atmospheric pressure equals 14.7 psia. It is at its boiling point that a liquid will liberate the most vapors: that is, the liquid evaporates at its highest rate at its boiling point.
Molecules in the liquid state have sufficient intermolecular attractions to keep them from flying away from each other onto the gas state, but the intermolecular forces are not strong enough to prevent the molecules form sliding around past each other.
When the heat is applied to a liquid, the temperature of the liquid rises until the vapor pressure of the liquid equals the pressure of the surrounding atmosphere. A pure liquid also may change to a gas at temperature below the boiling point through the process of ‘evaporation’. In the gas state there are essentially no intermolecular interactions. Gaseous molecules are ‘free’ from each other.
What is boiling point?