Boiling water is a physical change because it involves a change in the state of water from liquid to gas (water vapor) without any change in the chemical composition of the substance. The molecules of water remain H2O in both the liquid and gaseous states, meaning no new substances are formed during boiling. This change is reversible, as the water vapor can condense back into liquid water. The boiling process changes only the physical state, not the molecular structure or chemical nature of water.