Why is the evaporation of water a physical change and not a chemical change?

The evaporation of water is a physical change because it isn't undergoing any chemical reactions. In other words nothing is being combined to form a new substance or substances.

When water turn into gas, it is still water molecules that are gaseous. If you boil a pot of water, the steam that is released is still water. It doesn't change to something else. A chemical change is a change where something completely changed. While a physical property is like a change of state (solid, liquid or gas)