In chemistry, the terms “mixture” and “solution” actually refer to two quite different types of samples. According to Charles Ophardt, a mixture is a combination of substances (at least two, but often more) which have not reacted chemically and may or may not possess a uniform physical composition and visual appearance, whereas a solution is a particular type of mixture in which several substances have combined to form what appears to be, both visually and in terms of its uniform makeup, a single substance. More generally, a solution is actually a type of mixture; however, while a solution is a type of mixture, a mixture may or may not be a solution.
A mixture involves the combination, but not chemical reaction or dissolution, of multiple substances. Unlike solutions, other mixtures can theoretically be separated back into their constituents through manual physical separation. (Solutions may still be separable, for instance through boiling off the solvent). In theory, a mixture does not have to be in a liquid, although we generally associate it with such; one could, for example, have a mixture of blue-and red-coloured balls within a container.
There are essentially two types of mixtures: homogeneous and heterogeneous. Homogeneous mixtures look entirely the same in terms of their appearance and makeup, and are generally referred to by their own classification, solutions. Heterogeneous mixtures, by contrast, are ones in which the different constituents have not mixed thoroughly, and one has not dissolved into the other. Such mixtures can usually be identified because they visibly contain more than one substance.
Sometimes, mixtures may seem relatively homogeneous, but are not: in such cases, or “suspensions,” larger particles may hang briefly for a time within a liquid, but will eventually settle to the surface or bottom and become differentiated again. Pouring a small amount of fine beach sand into a pail of water can create this type of mixture; certain types of juice power or juice crystals also form suspensions.
Solutions contain, at minimum, two components: a substance (generally the larger quantity) called the solvent, and a second (usually lesser), called the solute, which dissolves into the first. In everyday experience, most solutions consist of substances dissolved in water. Salt water, for example, is a solution: sodium chloride (table salt) has dissolved into water to form this solution. Other examples of solutions include vinegar, carbonated water, and air pollution, or smog.
Because of the degree of homogeneity required in a solution, generally speaking the solvent in the solution is either a liquid (like water) or perhaps a gas; the solute, by contrast, may be a solid, provided that it is capable of dissolving. Acids and bases are examples of solutions in which particular substances – respectively, those with give off hydrogen ions and raise the hydronium (H3O+) content of water, and those which give off hydroxide ions and decrease the hydronium content of water.