Boiling is used as a method of water purification. The formation of bubbles in Heated liquid is a complex physical process which often involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface. Over boiling reduces the oxygen and carbon dioxide. In distillation, boiling is used to separate mixtures. This is possible because the vapor rising from a boiling fluid generally has a ratio of components different from that in the liquid.