heat capacity of H2O at its boiling point is
1) 3 cal mol -1K-1
​2)5 cal mol-1 K-1
3) 7 cal mol-1 K-1
4) Infinite
ANS : 4) how?

Dear User,
Heat capacity of a system is defined as the quantity of heat required to raise the temperature of the system by one degree. 

Heat capacity = dqdT
Boiling point of water = 100º C
Heat capacity of water will be the amount of heat required to raise the temperature from 100​º C to 101 ​º C. At 100º C the temperature of  the water becomes constant even after the addition of infinite amount of heat. This is because the heat supplied after attaining 100​º C is utilised in changing the phase of water from liquid to vapour . This is called as latent heat. Therefore the amount of heat supplied is constant i.e. q=∞ and hence heat capacity becomes infinite.
So, the answer is (d)


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