Colloids are heterogeneous mixtures in which very fine particles of one substance are dispersed in a continuous medium of another substance. Colloids are classified based on the state of the dispersed phase and the dispersion medium. - Gel: Dispersed phase is solid, dispersion medium is liquid (e.g., butter, cheese, jelly). - Emulsion: Dispersed phase is liquid, dispersion medium is liquid (e.g., milk, mayonnaise). - Sol: Dispersed phase is solid, dispersion medium is liquid (e.g., paint, ink, blood). - Aerosol: Dispersed phase is solid or liquid, dispersion medium is gas (e.g., smoke (solid in gas), fog (liquid in gas)).