A colloid is a heterogeneous system in which one substance (the dispersed phase) is spread as fine particles through a continuous medium (the dispersion medium). Particle size is the key. In a true solution particles are smaller than $1\ \text{nm}$, the system is one phase, transparent, passes through filter paper and parchment, and does not scatter light. In a colloid particle size is $1$–$1000\ \text{nm}$, the system is two-phase, passes through ordinary filter paper but not parchment, and scatters light. In a suspension particles are larger than $1000\ \text{nm}$, are visible, settle on standing and are retained by filter paper.
Classification. By the affinity of the dispersed phase for the medium, sols are lyophilic (solvent-loving, e.g. starch, gum, gelatin in water — reversible and very stable) or lyophobic (solvent-hating, e.g. sols of metals and metal sulphides — irreversible and needing a stabiliser). By particle type they are multimolecular (aggregates of many small atoms/molecules, e.g. gold sol, sulphur sol), macromolecular (single large molecules, e.g. proteins, starch, cellulose, plastics) or associated colloids / micelles (substances behaving as normal electrolytes at low concentration but forming aggregates above the critical micelle concentration, e.g. soaps and detergents). By physical state of the two phases we get sols, gels, aerosols, foams and emulsions; e.g. fog is a liquid-in-gas aerosol, milk is a liquid-in-liquid emulsion, and cheese is a liquid-in-solid gel.
Preparation. Lyophilic sols form simply by mixing the substance with the medium. Lyophobic sols are made by condensation methods (building particles up to colloidal size — oxidation, reduction, hydrolysis or double decomposition, e.g. $\text{FeCl}_3$ + hot water gives a $\text{Fe(OH)}_3$ sol) or by dispersion methods (breaking large particles down, e.g. Bredig's arc for metal sols and peptisation, the conversion of a fresh precipitate into a sol by adding a small amount of a suitable electrolyte). Purification removes excess electrolyte by dialysis (diffusion of small ions through a parchment membrane), electrodialysis (dialysis speeded by an applied potential) or ultrafiltration.
Properties. The Tyndall effect is the scattering of a beam of light by colloidal particles, making the path of the beam visible; it distinguishes a colloid from a true solution. Brownian motion is the continuous zig-zag movement of colloidal particles caused by unequal bombardment by molecules of the medium; it keeps particles suspended. Electrophoresis is the movement of charged colloidal particles toward an oppositely charged electrode under an applied field, proving that colloidal particles carry charge. The charge arises from preferential adsorption of ions and gives stability through mutual repulsion; the layer of adsorbed ions plus the oppositely charged layer is the electrical double layer (zeta potential).
Coagulation (or flocculation) is the aggregation and settling of colloidal particles, brought about by adding an electrolyte, by mutual mixing of oppositely charged sols, by heating or by prolonged dialysis. The Hardy-Schulze rule states that the coagulating power of an ion increases sharply with its charge: the effective ion is the one carrying the charge opposite to that of the sol. For a negative sol (e.g. $\text{As}_2\text{S}_3$) the order is $\text{Al}^{3+}>\text{Ba}^{2+}>\text{Na}^+$; for a positive sol (e.g. $\text{Fe(OH)}_3$) it is $[\text{Fe(CN)}_6]^{4-}>\text{PO}_4^{3-}>\text{SO}_4^{2-}>\text{Cl}^-$.
Emulsions are colloidal dispersions of one liquid in another immiscible liquid. The two types are oil-in-water (o/w, e.g. milk, vanishing cream) and water-in-oil (w/o, e.g. butter, cold cream). They are stabilised by emulsifiers (soaps, detergents, proteins) and can be broken by heating, centrifuging or adding an electrolyte (demulsification). Applications of colloids include water purification by alum (coagulating the negatively charged clay sol), the Cottrell electrostatic precipitator for removing carbon/dust from smoke, formation of deltas where river (colloidal clay) meets sea (electrolyte), cleansing action of soaps (micelle formation), and many foods, medicines and rubber/tanning processes.