Matter and Chemical Change - States of Matter

All matter is composed of tiny particles ($atoms$ or $molecules$) that are in constant motion, according to the Kinetic Molecular Theory.

Solids have a fixed shape and volume because particles are closely packed in a regular lattice and only vibrate in fixed positions due to strong intermolecular forces.

Liquids have a fixed volume but take the shape of their container; particles are close together but can move past one another, allowing them to flow.

Gases have no fixed shape or volume and are highly compressible because the particles are far apart and move rapidly in random directions.

Phase changes occur when thermal energy is added or removed: Melting ($Solid \rightarrow Liquid$), Freezing ($Liquid \rightarrow Solid$), Evaporation/Boiling ($Liquid \rightarrow Gas$), Condensation ($Gas \rightarrow Liquid$), Sublimation ($Solid \rightarrow Gas$), and Deposition ($Gas \rightarrow Solid$).

During a phase change, the temperature remains constant even though heat is being added or removed, as the energy is used to break or form intermolecular bonds rather than increasing kinetic energy.

Density is a characteristic property of matter that describes the mass per unit volume of a substance: $\rho = \frac{m}{V}$.