BOILING: is when a liquid rapidly becomes gaseous to the point that bubbles form in the liquid. Molecules enter the gas phase throughout the liquid as its vapour pressure matches the pressure outside the liquid. Consequently, water boils more easily at the top of a mountain (where the atmospheric pressure is lower) than at sea level (where the atmospheric pressure is higher). This video shows simplified molecules rapidly entering the gas phase (i.e. they are jumping out of the liquid because of the heat applied). Water is unusual in that hydrogen bonds link the molecules together and make the boiling point of much higher than it would otherwise be. However, when water boils the molecules get so much kinetic energy (they are moving so fast) that they escape these bonds and become gas.