Cooks seldom deal with pure chemical compounds or even single phases. Foods are mixtures of different molecules, different phases, and even different kinds of mixtures! Here are brief definitions of mixtures that are important in the kitchen.
A solution is a material in which individual ions or molecules are dispersed in a liquid. Salt brines and sugar syrups are simple culinary examples.
A suspension is a material in which a substance is dispersed in a liquid in clusters or particles consisting of many molecules. Nonfat milk is a suspension of milk-protein particles in water. Suspensions are usually cloudy because the clusters are large enough to deflect light rays (individual dissolved molecules are too small to do so, so solutions are clear).
An emulsion is a special kind of suspension, one in which the dispersed substance is a liquid that can’t mix evenly with the containing liquid. Cream is an emulsion of butterfat in water, and an oil-and-vinegar dressing is usually an emulsion of vinegar in oil.
A gel is a dispersion of water in a solid: the molecules of the solid form a sponge-like network, and the pockets of water are trapped in the network. Examples are savory or sweet jellies made with gelatin, and fruit jellies made with pectin.
A foam is a dispersion of gas bubbles in a liquid or solid. Soufflés, bread, and the head on a glass of beer are all foams.