A physical change alters form; a chemical change makes new substances. Telling them apart is a core skill.

• Physical change: no new substance, easily reversed, mass and composition unchanged (melting, dissolving).
• Chemical change: new substances form, usually hard to reverse, energy is absorbed or released.
• Evidence of chemical change: gas produced, precipitate formed, colour change, temperature change.
• Mass is conserved in both — atoms are only rearranged.

• Precipitate — an insoluble solid formed in solution.
• Conservation of mass — total mass before equals total mass after.