Describe what happens to both the potential energy and the average kinetic energy of the molecules in the H2O sample during interval AB

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What happens in the interval AB is simple heating of water.

From kinetic theory of gases, we know that kinetic energy of gas particles is directly proportional to its temperature. As we can see that temperature of the gas is increasing we conclude that kinetic energy also increases.

There are no changes in potential energy.

at an sample interval AB, there will be no change in the potential energy and there will be increase in average kinetic energy of the molecules in H2O.