When a substance changes from one state to another, we use a special name for the change. A solid turning into a liquid is melting. A liquid turning into a solid is freezing. A liquid turning into a gas is evaporating, or boiling when it happens throughout the liquid at its boiling point. A gas turning into a liquid is condensing.
Some solids change straight into a gas without becoming a liquid first. This is called sublimation. Naphthalene, the solid in mothballs, does this.
These changes of state are physical changes. No new substance is made, so water is still H2O whether it is ice, liquid or steam. Physical changes are reversible, and the mass stays the same. This is different from a chemical reaction, where bonds break and new substances form, such as when magnesium burns in air.
