Is the amount of water on earth always changing or is it a constant amount?

The amount of water on the Earth is constant, or nearly so. Actually, the amount is increasing ever so slightly due to volcanic eruptions expelling water vapor into the atmosphere, but, for all practical purposes, the amount of water (as a gas, liquid and as snow and ice) can be considered to be constant.

Explanation:

particle   p is an electron and particle q is a neutron

the dissolution of polymer can be thermodynamically described in 2 steps.