question_answer 33)

                  Refer to the plot of temperature versus time (fig.) showing the changes in the state of ice on heating (not to scale).                 Which of the following is correct? (a) The region AB represents ice and water in thermal equilibrium. (b) At B water starts boiling. (c) At C all the water gas converted into steam. (d) C to D represents water and steam in equilibrium at boiling point.                

Answer:

                  (a, d)                 (a) In region AB, a phase change takes place from solid (ice) to liquid (water) without a change of temperature. The heat supplied is used to increase the energy of the molecules so that they can break loose from their bonds in the solid structure and change into a liquid.                 (d) In the region CD, a phase change takes place a from liquid (water) to a vapour state (steam) without a change in temperature. The heat supplied is used to break loose the molecules of water free molecules in the steam phase.