States of Matter & Phase Changes
Solids hold shape and volume, liquids hold volume but flow, gases fill their container; phase changes — melting, freezing, vaporization, condensation, sublimation, deposition — are physical changes driven by adding or removing energy.
Sublimation skips the liquid phase entirely — dry ice going straight to gas is the standard example, and deposition is the reverse (frost). Signs of a chemical rather than physical change include gas production, color change, and new precipitate.