Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Boiling Water in a Bowl kept in another Bowl

Objective

To understand heat transfer during boiling process.

Apparatus

a small and a big bowl, a stove

Procedure

Put some water in the smaller bowl and float it in the larger bowl (pot) containing water. Place them on the stove. Soon water in larger pot starts boiling. Would water in the smaller bowl also boils?

Discussion

When water is heated, the heat is utilized in raising its temperature. After reaching a particular temperature, the heat is utilized in changing its state (latent heat to change state from water to steam). Here, the temperature of water stops rising and remains equal to the boiling point of water.

As water in larger bowl heats up, it starts transferring heat to the smaller bowl. Once water in the larger bowl attains boiling point, its temperature stops rising. When water in the smaller bowl also reaches this temperature, transfer of heat stops. So water in the smaller bowl does not get any more heat to sustain boiling process (latent heat). So, water in small bowl does not boil.

Now add a cupful of salt to water in the larger bowl. This raises boiling point of water. So, temperature of water in the larger bowl rises. Now water in the smaller bowl also starts boiling but at a lower rate.