When water vapor is cooled, it re-forms into water droplets. Why does this represent a conservation in mass?
When water vapor is cooled and re-forms into water droplets, it represents conservation of mass because the total mass of the water vapor before cooling is equal to the total mass of the water droplets after condensation.
In other words, the mass of water remains the same throughout the process of evaporation, condensation, and precipitation. Energy is exchanged during these phase changes, but the actual amount of water (mass) does not change. This is due to the principle of the conservation of mass, which states that mass cannot be created or destroyed, only transformed or transferred from one form to another.