<P> Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere . There are two main types of boiling; nucleate boiling where small bubbles of vapour form at discrete points, and critical heat flux boiling where the boiling surface is heated above a certain critical temperature and a film of vapor forms on the surface . Transition boiling is an intermediate, unstable form of boiling with elements of both types . The boiling point of water is 100 ° C or 212 ° F but is lower with the decreased atmospheric pressure found at higher altitudes . </P> <P> Boiling water is used as a method of making it potable by killing microbes that may be present . The sensitivity of different micro-organisms to heat varies, but if water is held at 70 ° C (158 ° F) for ten minutes, many organisms are killed, but some are more resistant to heat and require one minute at the boiling point of water . </P>

What kind of change happens to water when it is boiled