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.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.