Question

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 .

Answer 1

Well, for example, when you have an ice cube(solid) and you melt it in a cup, the weight of the water after melting(liquid) will be the same as the solid.

Nothing can be created or destroyed without tampering.

The water only changed physically, turning from a liquid into a gas.
The matter did not change at all; it only evaporated and changed its form physically.

Answer 2

Because if you let it melt and there is no evaporation there will be the same amount of water.