What is Boiling Point?

"It is the temperature at which the vapour pressure of the liquid equals the atmospheric pressure".

In simple terms in a solid state the molecules are tightly packed and in the liquid phase this arrangement breaks down and in a gas the molecules are freely moving about with the least attractions.

In other words melting a solid or vapourising a liquid is simply pulling the molecules apart against their inter-molecular forces of attractions. The greater the attractions, more energy is needed and hence higher will be the boiling point. Hydrogen bonding between molecules(inter mlecular) is one such attractive force.

• Water has many unique properties because of inter molecular hydrogen bonding.
• Water is probably the only compound, which exists as a liquid in a wide range of temperatures from 0 to 100^oC.
• H₂O and H₂S are comparable / are similar molecules in many respects with one major difference that is, there is no significant hydrogen bonding possibilities in H₂S due to the low electronegativity of sulphur.
• Conversion of liquid phase to gaseous phase is simply pulling the molecules apart against inter molecular forces of attraction, the greater the attraction the more energy that is needed.
• The boiling point of water is 100^oC while that of H₂S is somewhere near –60^oC, a vast difference. In water because of the hydrogen bonding attraction between molecules greater energy is needed to separate them from against their inter molecular attraction, therefore higher boiling point.
• Water is possibly the only liquid whose volume increases when it freezes (lower density), making ice float on water.
• In some countries in winter when lakes and rivers freeze aquatic life is still possible below a thick layer of ice on the top.
• Absence of hydrogen bonding in trimethyl amine explains its low boiling point( 277 K) in comparison with dimethyl amine (288 K) in spite of a larhe molecular mass of the former