You can tell because there are 2 moles of H2 on the reactants side and 1 mole of O2, and on the products side we see the same amount, meaning that no moles of H2 or O2 were added or destroyed, meaning that it follows the law of conservation of matter.
Evaporation occurs when water molecules at the surface of a pool have enough kinetic energy that the attractive forces holding them to the liquid can be overcome
