In which of the following cases the heat energy is given out?
(i) When water boils
(ii) When ice melts
(iii) When water freezes to ice
(iv) When water vapour condenses to water
(A) In (i), (ii) & (iii)
(B) In (iii) only
(C) In (iv) only
(D) In both (iii) & (iv).
Ans is D but how?
Ice is the solid form, water is the liquid form and vapour is the gaseous form.
Solid to liquid - This process is called melting.
On increasing temperature, the particles of solid gain kinetic energy and move vigorously Melting occurs when the particles gain enough energy to overcome the forces of attraction holding them together at fixed positions.At this point, solid changes into liquid.
It means for the conversion of ice to water heat energy is required.
Liquid to gas- This process is called boiling.
On increasing temperature, the particles of liquid gain kinetic energy and move more vigorously and move farther from each other with more speed. Boiling occurs when the particles gain enough energy to overcome the forces of attraction holding them together in liquid state and the particles break free from one another and escape from the surface of the liquid.At this point, liquid changes into gas.
It means conversion of water to water vapours heat energy is equired.
Gas to liquid: This is called condensation.
On decreasing temperature, the particles of gas lose kinetic energy and move less vigorously and come closer to each other than before.The forces of attraction become strong than before and the particles are held a little close.At this point, the gas changes into liquid. It means in this no heat energy is required but heat energy is given out. Conversion of water vapour to water heat energy is given out.
Liquid to solid - This is called freezing.
On decreasing temperature, the particles of liquid lose kinetic energy and move very slowly and come closer to each other than before.The forces of attraction become strong than before and the particles are held tightly.At this point, the liquid changes into solid. It maens conversion of water to ice heat energy is given out.
Thus the correct option is (D).