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Colloids are like solutions that never quite made it to the finish line. They are similar to solutions in that their particles are evenly distributed within each other, but their difference resides in the size of their particles. Colloidal particles are fairly large, which makes it hard for them to be dissolved in each other the way solvents and solutes are. The size of these particles also means that, unlike in solutions, light is visible when shone through a colloid (see Tyndall effect).

Some things you should know about colloids:

Dispersion

The particles in a colloid are said to be dispersed in each other. Colloidal particles are not dissolved in each other, however, so they are at best mixtures. They are evenly distributed and appear homogeneous, but are in fact neither homogeneous nor heterogeneous. Instead, colloids are by definition in between homogeneity and heterogeneity.

Colloids have a continuous phase and a dispersed phase, sort of like how solutions have solvents and solutes, respectively. The dispersed phase particles are smaller than the continuous phase particles, which allows the former to be suspended in the latter.

Classification

Colloids are sometimes classified by their appearance, which depend on whether the continuous and dispersed phases are vapor, liquid, or solid. Every combination has a different appearance and thus a different classification. The only exemption is a gaseous dispersed phase combined with a gaseous continuous phase, which does not exist, as all gases are mutually miscible.

Tyndall Effect

The “Tin Doll” effect, sadly, has nothing to do with tin dolls but everything to do with colloids. The Tyndall effect describes the suspension of light particles by colloidal particles. Because of the colloidal particles’ large size, light particles scatter and are suspended in the mixture, which we see as a light path shining through the colloid.

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