What are solutions, colloids, and suspensions?

2 Answers
Dec 23, 2016

These are three types of mixtures that are distinguished by the sie of the particles being mixed, and other properties (discussed below).


A solution is a mixture of two or more pure substances that is homogeneous. This means it is perfectly uniform everywhere, because the mixing occurs at a molecular level. The solution will be transparent, and will not separate over time. You cannot separate the components by filtering.

In a suspension, larger pieces are matter are combined (most agree on #10^(-5) m# or greater as being the particle size in a suspension. As a result, the mixture will be cloudy or even opaque, and generally separates upon standing. Filtering will generally result in separation is the materials.

A colloid is sort of "in-between" these two. Particle size is smaller (but not on the molecular level). A true colloid should not separate if left standing, Light passing through a colloid will tend to be scattered, with some light passing through, giving a translucent appearance.

Dec 23, 2016

Different numbers of phases and stabilities....


Solutions comprise a solute that is solvated by a solvent. Solvation means that the ions in a solution are surrounded by or complexed by molecules of the solvent (its a slightly more complex process for things such as polymer solutions, so I won't go into that here). The fact that the solute ions are completely surrounded by and/or complexed with solvent molecules mean that solutions are single phase and homogenous.

In a suspension, however, one phase is simply dispersed into small particles that float around in the solvent. There is no surrounding of, or complexing of, any species at the molecular or ionic level. Suspensions are therefore two phase (either solid/liquid, solid/gas, gas/liquid or liquid/imiscible-liquid) and are heterogeneous.

Many suspensions will separate on standing (solid particles fall to the bottom of the container, or gas bubbles coalesce and rise to the top, or two immiscble liquids separate into two layers). Colloids are a specific class of suspensions where the particle size is between 1 and 1000 nanometres (1 x #10^-9# to 1x #10^-6# m). They tend to be much more stable than regular dispersions and do not readily separate on standing. A classic example would be milk.