Question

When water boils, temperature remains constant. Give reasons.

Answer 1

Evaporation of water takes place over a wide range of temperatures ranging from its melting point to its boiling point. However water boils only at the temperature equal to its boiling point.

Complete answer:
The boiling point of water is ${\text{10}}{{\text{0}}^o}{\text{C}}$ or ${\text{273 K}}$. The boiling point of a liquid is the temperature at which its vapour pressure is equal to the atmospheric pressure. The atmospheric pressure is ${\text{1 atm}}$ or ${\text{760 mm Hg}}$ .
When water at room temperature is heated, the added heat is used to increase the temperature of the water. This increase in temperature continues till the temperature of ${\text{10}}{{\text{0}}^o}{\text{C}}$ is obtained. At this temperature, if more heat is supplied, then this heat is used to change the phase of liquid water to gaseous water. This additional heat supplied at ${\text{10}}{{\text{0}}^o}{\text{C}}$ is the latent heat of vaporization of water. It is used to overcome the intermolecular forces of attraction between water molecules. Due to this, water molecules become free and can escape as vapours. When all of the liquid water is converted into gaseous water at ${\text{10}}{{\text{0}}^o}{\text{C}}$ then additional heat supplied will be used to increase the temperature of gaseous water.
Thus, during the process of boiling, the temperature of water remains constant at ${\text{10}}{{\text{0}}^o}{\text{C}}$. This is because the added heat supplied is not used to increase the temperature of water. Instead, this heat is used to overcome the intermolecular forces of attraction between different water molecules.

Note: For any substance, the temperature remains constant during phase transition. Phase transition can be from solid to liquid, the corresponding temperature being melting point or liquid to gas, the corresponding temperature being boiling point.