True Solutions, Colloids and Suspensions

Not every mixture of two substances is a true solution. Depending chiefly on the size of the particles of one substance dispersed through the other, a two-component mixture falls into one of three broad categories: a true solution, a colloidal solution (or colloid), or a suspension, summarized in Table 1.

A true solution is a homogeneous mixture in which the solute is dispersed as individual ions or molecules, each smaller than about $1\ \text{nm}$ in diameter — small enough that the particles cannot be seen by any optical means, do not scatter light noticeably, never settle out under gravity, and pass freely through both ordinary filter paper and even the finest ultrafilters. Common salt dissolved in water is a true solution.

A colloidal solution, or colloid, occupies the intermediate size range, roughly $1\ \text{nm}$ to $1000\ \text{nm}$ (that is, up to about a micrometre) in diameter. This size is small enough that colloidal particles, like those in a true solution, do not settle out under ordinary gravity and pass through ordinary filter paper, yet large enough that they scatter a visible beam of light passing through them (the Tyndall effect, covered later in this chapter) and are retained by an ultrafilter (a very fine membrane, such as parchment or an animal bladder, with pores smaller than colloidal particles but larger than solvent molecules). Milk, a starch solution, and a gold sol are all colloids; the term was coined by the British chemist Thomas Graham, who first systematically distinguished colloids ("glue-like" substances that diffused only very slowly) from ordinary crystalline solutes.

A suspension contains still larger particles, generally above $1000\ \text{nm}$, coarse enough to be visible to the naked eye or under an ordinary microscope, and heavy enough that they settle out of the mixture on standing under gravity alone; suspensions are consequently opaque or cloudy in appearance and are readily retained by ordinary filter paper. Chalk powder shaken up in water, or muddy river water, are common suspensions. …