Distinguishing Between Solutions and Colloids

• Coarse mixtures are quite easy to identify as their non-uniform appearance can be observed with the naked eye.

• Solutions and colloids can be harder to differentiate, as they both appear homogenous with the naked eye, and we don’t always have a microscope on hand to help us.

• One way to distinguish between solutions and colloids is to observe how light passes through them.

• Solutions are transparent, which means they are clear, because light passes easily through them.

• Note, transparent is different to colourless – solutions can be colourless, like methylated spirits, or they can be coloured, like petrol; in both cases, light passes through.

Solutions are transparent (clear).

(Image: LHcheM, Wikimedia Commons)

• Colloids, on the other hand, are opaque, which means they are cloudy, because light does not pass easily through them.

• For example, gels, ink and stained glass all appear homogeneous, but have an opaque appearance.

Colloids are opaque (cloudy).

(Images: saponifier, Pixabay; Lolame, Pixabay; Nomoreforme, Wikimedia Commons)

• Another way to distinguish between solutions and colloids is to test whether different phases can be separated.

• One way to test this is to pass the mixture through a fine filter. If nothing gets trapped by the filter, then the mixture is a solution; if some parts get trapped by the filter, then the mixture is a colloid.

• Another way to test this is to spin the mixture very fast in a machine called a centrifuge. If the mixture is a solution, it will look the same after spinning; if the mixture is a colloid, the different phases will separate into layers.

Colloids, such as honey and blood, contain different phases, which can be physically separated.

(Images: Luc Viatour, Wikimedia Commons; Wheeler Cowperthwaite, Wikimedia Commons)