The law of conservation of matter applies to physical changes as well as to chemical changes. if this is true, then explain why the law of conservation of matter holds true if the amount of water in a glass that was left out overnight decreases .

The law states that matter is conserved. It is not randomly llost or gained, but transformed.

This holds true because when a glass of water is left out over night, the water evaporates. The water doesn't magically disappear. It is just transformed into water vapor.