Colloids
What is a colloid?
A colloid is any type of cloudy mixture where a substance or substances are dispersed in another. Derived from the Greek term 'to form glue', colloids contain particles too small to be filtered and not dense enough to settle. Particles can be solid, droplets of liquids or bubbles of gas and the dispersion medium (the substance holding the particles) can be solid, liquid or gas. See image 1.
Some examples of colloids include: mist, where water droplets suspend themselves in air; muddy water, which is clay particles in liquid; and smoke, where tiny carbon particles suspend themselves in air.
However, colloids do not just occur between substances in different physical states (although a gas-gas colloid is not possible). Water and oil, both liquid, can form a colloid with the addition of a substance (known as an emulsifier) that stops the water and oil from separating. A product such as mayonnaise contains an emulsifier.
The particles in colloids do not dissolve, which makes them different from solutions. All gases are soluble, which is why it is impossible to have a gas-gas colloid. If you shine light through a colloid, you will be able to see the particles (known as the Tyndall effect) whereas in a solution, the dissolved particles are too small to identify. Other examples of the Tyndall effect include: sunlight coming through a window, illuminating dust; or car headlights beamed through fog. See animation 1.
It was mentioned before that muddy water is a colloid and that a colloid cannot be filtered. Fortunately, it is possible to use chemistry to treat a colloid and separate its substances, making it possible to extract clean water from muddy water. In this case, chemicals (such as potassium aluminium sulphate) are added to the muddy water, which cause the particles to cluster and settle to the bottom.
Gels
'Gel' is a word often used as a substitute for the full term, 'gelatinous colloid' from the Latin 'gelatus' meaning 'frozen, immobile'. Gel is a colloid of a solid and a liquid, though there is some dispute about whether it is a solid dispersed in a liquid or vice versa. The proportions of solid to liquid aspects in gel indicate that it is mostly liquid, yet it is viscous (thick) enough to behave like a solid.
Some examples of gel include: edible jelly (made of a protein called gelatin), where the protein particles hold the liquid together; and paint, where the particles of pigment form a solid when the paint dries. See image 2.
