Suspensions and Colloids

Not all mixtures are solutions. If the particles of one substance are too big to dissolve completely in another, you get either a suspension (particles big enough to eventually settle out) or a colloid (particles small enough to stay suspended but big enough to scatter light). Sand in water is a suspension; milk is a colloid. Both are surprisingly common in everyday life and are sometimes confused with true solutions. Understanding the difference matters for chemistry, cooking, biology and even how the sky gets its blue colour.

  • Particle size: solution<1 nmIndividual molecules or ions
  • Particle size: colloid1 to 1,000 nmBigger than molecules
  • Particle size: suspension>1,000 nmSettle out over time
  • Solution exampleSalt waterLooks clear, never settles
  • Colloid exampleMilk, fog, mayonnaiseCloudy, never settles
  • Suspension exampleSand in waterSettles to the bottom

Suspensions

A suspension is a mixture where particles of one substance are scattered through another but are big enough to settle out over time. Stir sand into a glass of water and you get a suspension. Wait a few minutes and the sand sinks to the bottom.

Common suspensions include:

  • Sand in water.
  • Flour in water (until it settles or is cooked).
  • Some medicines that say "shake well before use".
  • Muddy river water.
  • Italian salad dressing (oil and vinegar separate when left to stand).

Colloids

A colloid is a mixture with particles that are smaller than a suspension but much bigger than individual molecules. The particles are too small to settle out (they stay floating forever) but big enough to scatter light, giving colloids their characteristic cloudy or milky appearance.

Common colloids include:

  • Milk: tiny fat droplets in water.
  • Fog and mist: tiny water droplets in air.
  • Smoke: tiny solid particles in air.
  • Mayonnaise: oil droplets in vinegar held together by egg yolk.
  • Jelly: water trapped in a gelatine network.
  • Whipped cream: air bubbles in cream.
  • Paint: solid pigments in a liquid.
  • Blood: cells suspended in plasma (technically a colloid).

How to tell them apart

  • Solutions look clear (even if coloured) and never settle.
  • Colloids look cloudy or milky and never settle.
  • Suspensions look murky and gradually settle.

There is a classic chemistry test called the Tyndall effect: shine a torch through the mixture. If the beam shines through invisibly, it is a solution. If the beam lights up the mixture in a visible glowing path, it is a colloid (or a suspension). The visible beam is light being scattered by the larger particles.

Fact The blue colour of the sky is caused by the same effect that makes colloids look cloudy. The atmosphere is mostly a clear mixture of gases, but tiny dust particles and clusters of air molecules scatter sunlight (especially blue light). When you look up, the scattered blue light reaches your eyes from every direction across the sky, making it appear blue. The same effect makes sunsets red: as the Sun sets, its light passes through more atmosphere, and all the blue gets scattered away, leaving only the red light to reach you.

Why colloids do not settle

You might expect colloid particles to eventually sink (like sand in water), but they do not. Several things keep them suspended:

  • They are very small: small enough that random collisions with surrounding molecules (called Brownian motion) are strong enough to keep bouncing them around.
  • They often have an electric charge: many colloid particles have the same charge as their neighbours, so they repel each other and stay spread out.
  • The molecules of the surrounding solvent stick to the colloid particles and help support them.

Emulsions: special liquid colloids

An emulsion is a colloid where one liquid is suspended in another. Oil and water do not naturally mix, but with the help of an emulsifier (usually a molecule that can stick to both oil and water), the oil can be broken into tiny droplets that stay suspended.

  • Mayonnaise: oil suspended in vinegar, held together by egg yolk (a natural emulsifier).
  • Milk: tiny fat droplets in water, stabilised by milk proteins.
  • Butter: water droplets in fat, the opposite kind of emulsion.
  • Hand cream and moisturisers: emulsions of water and oils held by chemical emulsifiers.
Did you know? Mayonnaise is one of the trickiest things to make from scratch because you need to break the oil into thousands of tiny droplets and keep them suspended in vinegar. If you whisk too slowly, the emulsion "breaks" and the oil separates back out. Adding lecithin (from egg yolks) as an emulsifier makes the droplets stable. This is why most mayonnaise recipes call for egg yolks: not for flavour, but as a natural emulsifying agent.
Deeper dive: aerosols, gels and other less obvious colloids

There is a whole zoo of different kinds of colloid, depending on what is suspended in what.

  • Sol: solid in liquid. Examples: paint, ink, blood, mud.
  • Emulsion: liquid in liquid. Examples: milk, mayonnaise, hand cream.
  • Foam: gas in liquid. Examples: whipped cream, beer head, bubble bath.
  • Solid foam: gas in solid. Examples: bread, polystyrene, marshmallows.
  • Aerosol: liquid or solid in gas. Examples: fog, mist, smoke, hairspray.
  • Gel: liquid trapped in a solid network. Examples: jelly, hair gel, jam.
  • Solid sol: solid in solid. Examples: ruby gemstones (chromium in corundum), coloured glass.

Many everyday products are mixtures of multiple colloid types. Ice cream, for example, is a complex mixture of a solid sol (sugar crystals in fat), an emulsion (fat droplets in water), and a foam (air bubbles in the mix). Different ice creams have different ratios, which gives them their characteristic textures.

The study of colloids and surfaces is called colloid chemistry and is hugely important in modern industry. From the formulation of paints and inks to the production of cosmetics, medicines, food and electronics, almost every modern product involves colloid chemistry. Designing a new mascara, for example, requires balancing colloid particles, binders, oils and pigments so the product spreads evenly, sticks to lashes, dries quickly and stays put all day.

For more, see solutions and separating mixtures.