(a) The molecules begin to move farther apart decreasing their intermolecular forces and they escape the liquid state to enter the gas state.
Explanation:
As we put tea pot full of water on stove the heat transfer starts from surface of tea pot to the closest water molecule near the surface of tea pot, after becoming hot the water rises up and cooler water comes down. When water begins to boil their intermolecular forces begin to decrease and they start to convert in gaseous or vapor state.
