The vapor pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere . As a result, liquids with high vapor pressures have lower boiling points.
Beyond the critical point, a compound's liquid and vapor phases merge into one phase, which may be called a superheated gas. At any given temperature, if a compound's normal boiling point is lower, then that compound will generally exist as a gas at atmospheric external pressure.
For a substance to become vapor it needs to overcome the pressure of the gas above it. If we increase pressure above the solution excess heat has to be supplied to overcome the pressure above it. The boiling point of liquid increases with increase in pressure and decreases with a decrease in pressure.
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