Melting and Boiling Point

What is Boiling Point?

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure of the liquid’s environment. At this temperature, the liquid is converted into a vapour.

The boiling point of the liquid depends upon the pressure of the surrounding. When the liquid is at high pressure, it has a higher boiling point than the boiling point at normal atmospheric pressure. The boiling point of different liquids is different for a given pressure. In 1982, IUPAC defined the standard boiling point of a liquid as the temperature at which the liquid boils under a pressure of 1 bar.

The boiling point of a substance is dependent on the pressure of its surroundings. In mountainous terrains (where the altitude is high), the pressure of the atmosphere is relatively lower than the atmospheric pressure at sea level. This is the reason why food cooks at a slower pace in mountainous areas (the lower atmospheric pressure causes water to boil at temperatures below 100^oC).

What is the Melting Point of a Substance?

The temperature at which solid changes its state to liquid at atmospheric pressure is called the melting point of that liquid. This is the point at which both liquid and solid phase exists at equilibrium. The melting point of the substance also varies with pressure and is specified at standard pressure.

The term ‘freezing point’ is used to denote the temperature at which a liquid is converted into a solid and can, therefore, be viewed as the opposite of the term ‘melting point’. However, substances can be cooled below their freezing points without the formation of a solid. Such liquids are known as supercooled liquids.

Melting and Boiling Points of Some Elements

A tabular column listing the melting and boiling points of some important elements is provided below.

Name of the substance	Boiling point(K)	Melting point(K)
Aluminium	2740	932
Copper	1460	1359
Gold	2933	1336
Hydrogen	20.3	13.8
Mercury	630	234