Question

1) Why does bubbles appearwhen a liquid is heated??
2) What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point??
3) How is heat is supplied to a liquid used during boiling at a fixed temperature??
4) Name two ways of changing liquid state to the vapour state and distinguish them??

Answer 1

1)Bubbles are vapours of that liquid. Ideally they can form any where inside the liquid where the temperature is equal to the boiling point. However, they are usually formed at the heating surface like base of vessel while boiling water since in other regions their heat would be dissipated to surroundings


2)As a liquid is heated, the average kinetic energy of its particles increases. The rate of evaporation increases as more and more molecules are able to escape the liquid's surface into the vapor phase. Eventually a point is reached when the molecules all throughout the liquid have enough kinetic energy to vaporize.


3)This is because the temperature is consumed as latent heat of vaporization.

This amount of heat is used to break inter - molecular forces of attraction in liquid and convert it into a gaseous state.

Hence the temperature of liquid doesn't rise further when it is heated at its boiling point unless it converts into gas.

4)

The two ways in which the liquid changes into vapour are vapourisation and evaporation.

Let us first begin by vapourisation, Here the entire mass of the liquid, on reaching the boiling point gets converted into vapour state. This is because of the thermal energy gained by the molecules of the liquid. This energy is passed to the rest of the molecules and the whole liquid gets converted into gaseous state.

In case of evaporation, no excess heat energy is supplied. It takes place at room temperature. The surface molecules of the liquid gain energy and get converted into vapour .

Hence we can say that vaporization is a bulk phenomenon and evaporation is a surface phenomenon