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The boiling point of a substance is the temperature at which the substance boils, or enters a state of rapid evaporation. For pure water this is 100° Celsius or 212° Fahrenheit. This is measured at one atmosphere, that is, the air pressure at sea level.
The boiling point of a liquid depends on the pressure of the surrounding air. An increase in air pressure increases the boiling point; a decrease decreases the boiling point. In the low pressure environment at the top of Mt Everest for example, water boils at only 69 °C. (156.2 °F). It can also be defined in terms of vapour pressure as the temperature at which vapour pressure of liquid becomes equal to atmospheric pressure