Boiling point ( Effect of temperature and pressure on Boiling point)

Boiling point

 The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.

 The standard boiling point is now (as of 1982) defined by IUPAC as the temperature at which boiling occurs under a pressure of 1 bar.

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    A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmospheric pressure.

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  A liquid in a high pressure environment has a higher boiling point than when the liquid is at atmospheric pressure.

  Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules located near the vapor/liquid surface escape into the vapor phase. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Relation between the normal boiling point and the vapor pressure of liquids:

The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.

The liquids with the highest vapor pressures have the lowest normal boiling points.

For example, at any given temperature, propane has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point(-42.1 °C), which is where the vapor pressure curve of propane (the purple line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapor pressure.
In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough thermal energy to overcome the various intermolecular attractions binding the molecules as liquid (e.g. dipole-dipole attraction, instantaneous-dipole induced-dipole attractions, and hydrogen bonds) and therefore incur a phase change into the next phase (gas). Therefore the boiling point of a liquid is also an indicator of the strength of weak attractive forces between the liquid's molecules.