Types of Mixtures
When a substance is mixed with a liquid, the result depends on the size of the particles:
| Mix type | Particle size | Settles? | Can see particles? | Example |
|---|---|---|---|---|
| Solution | Very small (individual molecules/ions) | No | No | Salt water |
| Colloid | Small (1–1000 nm) | Very slowly or no | No — but scatters light | Milk, fog, jelly |
| Suspension | Large (>1 µm) | Yes, over time | Often visible or cloudy | Sand in water |
Suspensions
A suspension is a mixture of a liquid and insoluble solid particles large enough to eventually settle by gravity. If left to stand, the solid sinks to form a layer, and the liquid above becomes clearer.
Examples:
- Muddy river water
- Orange juice (without shaking)
- Chalk in water
- Some medicines (e.g. "shake before use")
Suspensions can be separated by filtering — the insoluble solid stays in the filter paper.
Rivers carry suspended sediment. When the current slows (e.g. where a river meets the sea), the heavier particles settle first. This is how deltas and mudflats form.
Colloids
A colloid has particles larger than those in a solution but small enough that they don't settle quickly. The particles are dispersed throughout the medium.
Common types:
| Dispersed phase | Medium | Type | Example |
|---|---|---|---|
| Liquid in liquid | Liquid | Emulsion | Milk, mayonnaise |
| Solid in liquid | Liquid | Sol | Paint, blood |
| Gas in liquid | Liquid | Foam | Whipped cream |
| Liquid in gas | Gas | Aerosol | Fog, spray deodorant |
| Solid in gas | Gas | Aerosol | Smoke |
The Tyndall Effect
Shining a beam of light through a colloid causes the light to scatter — you can see the beam path. This is the Tyndall effect. It does not occur in a true solution (particles too small to scatter light, and light passes through). It is why a lighthouse beam is visible in foggy air.
Emulsifiers
Emulsions like milk and mayonnaise mix oil and water — two liquids that normally don't mix. Emulsifiers stabilise them. An emulsifier molecule has one end attracted to water and one end attracted to oil, keeping the droplets dispersed.
Lecithin (from egg yolk) is a natural emulsifier used in chocolate, mayonnaise, and ice cream. It stops the oil and water components separating.
Real world: Milk is a colloid stabilised by protein
Milk is an oil-in-water emulsion colloid — tiny fat globules (0.1–10 micrometres) are dispersed in water. Normally oil and water don't mix, but the fat globules are coated with casein proteins that act as natural emulsifiers, keeping them suspended. Homogenised milk has been mechanically processed to break fat globules into even smaller, more uniform sizes so they don't separate. If you leave unhomogenised milk to stand, the fat rises to form a cream layer — the suspension becomes visible as the colloid separates.
Real world: Fog and clouds — aerosol colloids and the Tyndall effect
Fog and clouds are aerosol colloids — tiny water droplets (1–100 micrometres) dispersed in air. They are visible because the droplets scatter light — the Tyndall effect. A lighthouse beam is visible at night because fog scatters the light along its path. This same scattering makes the sky appear hazy on humid days. True solutions of water vapour in air are invisible (humid air looks clear), but once water condenses into droplets (a colloid), the Tyndall effect makes it visible as mist, fog, or cloud.
Real world: Medicines labelled 'shake before use' are suspensions
Many medicines — calamine lotion, some antibiotics, antacid suspensions — contain insoluble solid particles in liquid. The active ingredient settles over time (it's a suspension, not a solution). The "shake before use" instruction ensures the solid is redistributed evenly before you measure a dose, so each spoonful contains the correct amount. If you don't shake it, early doses are dilute and later doses are dangerously concentrated. This is a practical consequence of the difference between a true solution and a suspension.
Where do you see this in real life? Milk as a fat-in-water colloid (cream rises if unhomogenised), fog and clouds scattering light (Tyndall effect), medicines labelled "shake before use" (suspensions settle), paint as a solid-in-liquid colloid, smoke as a solid-in-gas aerosol — most foods and medicines are colloids or suspensions, not true solutions.