The boiling point is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure of the liquid and the liquid is converted to vapour.
Thus, while the boiling point of water is commonly taken as 100°C, this is only true at a pressure of one standard atmosphere (1.013 bar) and, by varying the pressure, the boiling point can be changed Which is show on the below Table . 

Pressure (bar)
Boiling point (°C)

This pressure–temperature property can be shown graphically

The boiling point changes with altitude and that’s why when we go to mountain areas i.e. at higher altitudes cooking food takes more time because the pressure decreases and therefore because of this it takes more time in cooking food at hilly areas.

​Boiling point is also known as saturation temperature.In 1982 IUPAC, defined standard boiling point which is the temperature at which liquid boils under the pressure of 1 bar. Boiling differs from evaporation. Evaporation is a surface phenomenon that occurs at any temperature in which molecules at the liquid edge escape as vapor because there is not enough liquid pressure on all sides to hold them.The boiling point of a substance is limited by the critical temperature at the upper end, beyond which it cannot exist as a liquid, and by the triple point at the lower end, which is at the freezing temperature. Between these two limits, if the liquid is at a pressure higher than its boiling pressure, it will remain a liquid and will be subcooled below the saturation condition, while if the temperature is higher than saturation, it will be a gas and superheated. If both liquid and vapour are at rest in the same enclosure, and no other volatile substance is present, the condition must lie on the saturation line.

At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas. The liquid zone to the left of the boiling point line is subcooled liquid. In refrigeration the term saturation is used to describe the liquid/vapour boundary, saturated vapour being represented by a condition on the line and superheated vapour below the line.

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