question_answer

(i) Why does the temperature of a substance remain constant during its melting or boiling? (ii) A glass tumbler containing hot water is kept in the freezer compartment of a refrigerator (temperature < 0°C). If you could measure the temperature of the content of the tumbler, which of the following graphs would correctly represent  the  change  in  its temperature as a function of time.  

Answer:

                                                                  During melting, the heat equal to latent heat of fusion and during boiling, heat equal to latent heat of vaporisation is used to overcome the inter particle force of attraction. Therefore, temperature remains constant.                  Fig. (a) represents the change of temperature with time correctly. The temperature of water first decreases up to zero degree celcius then remains constant for sometime (till the ice is formed), then again starts decreasing.