Chemistry - States of Matter

Matter is made up of tiny particles (atoms, molecules, or ions) that are in constant motion, a principle known as the Kinetic Molecular Theory.

Solids have a fixed shape and volume because particles are closely packed in a regular lattice and only vibrate about fixed positions.

Liquids have a fixed volume but take the shape of their container because particles are close together but can move past each other.

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

Changes of state occur when energy is added or removed: Melting ($Solid \rightarrow Liquid$), Boiling ($Liquid \rightarrow Gas$), Freezing ($Liquid \rightarrow Solid$), and Condensation ($Gas \rightarrow Liquid$).

Sublimation is the direct change from a solid to a gas without becoming a liquid, such as with Dry Ice ($CO_2$) or Iodine ($I_2$).

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of particles.

The rate of diffusion is affected by temperature and molecular mass; lighter particles, like $NH_3$, diffuse faster than heavier particles, like $HCl$.