Physical and chemical changes
A physical change alters the form of a substance but not what it is. Melting, boiling, dissolving, cutting and bending are physical changes. Ice, liquid water and steam are all H2O.
A chemical change produces one or more new substances with different properties. Burning, rusting and cooking an egg are chemical changes.
Signs that a chemical change may have happened:
- a gas forms (bubbles) without boiling
- a solid forms when two solutions are mixed
- the color changes unexpectedly
- heat or light is released or absorbed
These are clues, not proof. The real test is whether a new substance formed.
Worked example: boiling water and burning wood
Boiling turns liquid water into water vapor. The molecules are still H2O, so this is a physical change.
Burning wood combines it with oxygen to form carbon dioxide, water vapor and ash. These are new substances, so this is a chemical change.
Common mistake
Treating dissolving as a chemical change. When sugar dissolves in water the sugar molecules spread out but stay sugar. Evaporate the water and the sugar is still there.