Colloids and Suspensions

12 min read Updated Mar 26, 2026

Not every mixture is a true solution. Some mixtures have particles too large to dissolve at the molecular level but too small to settle out immediately. These intermediate mixtures - colloids and suspensions - have unique properties that show up on the MCAT.

The Three Types of Mixtures

Property	True Solution	Colloid	Suspension
Particle size	< 1 nm	1 - 1,000 nm	> 1,000 nm
Settles on standing?	No	No (or very slowly)	Yes
Filterable?	No (passes through filter)	No (passes through filter)	Yes (caught by filter)
Tyndall effect?	No	Yes	Yes (but settles)
Appears	Transparent	Translucent/opaque	Opaque, cloudy
Example	Saltwater, sugar water	Milk, fog, blood	Muddy water, sand in water

Three beakers comparing a true solution (transparent, particles less than 1 nm), a colloid (translucent, particles 1-1000 nm), and a suspension (opaque and cloudy, particles greater than 1000 nm). The solution is clear, the colloid scatters light, and the suspension has visible settling particles. — Comparison of a true solution, colloid, and suspension. Particle size determines transparency, settling behavior, and whether light is scattered (Tyndall effect). *Credit: OpenStax Chemistry 2e, CC BY 4.0*

The Tyndall Effect

The Tyndall effect is the scattering of light by colloidal particles. Shine a flashlight through a true solution and the beam is invisible (particles too small to scatter light). Shine it through a colloid and the beam becomes visible as a glowing path through the mixture.

This is why:

Car headlights are visible in fog (fog is a colloid of water droplets in air)
A flashlight beam is visible in a dusty room (dust is a colloid of solid particles in air)
Saltwater appears clear (true solution, no scattering)
Milk appears white (colloid, scatters all wavelengths)

Photograph demonstrating the Tyndall effect: a beam of light is visible as it passes through a colloidal mixture because the particles scatter the light, while the beam is invisible in a true solution because the particles are too small to scatter light. — The Tyndall effect: light beams become visible when passing through a colloid because colloidal particles (1-1000 nm) are large enough to scatter light. True solutions do not show this effect. *Credit: OpenStax Chemistry 2e, CC BY 4.0*

Brownian Motion

Brownian motion is the random, zigzag movement of colloidal particles suspended in a fluid. It is caused by constant bombardment from surrounding solvent molecules. The particles are small enough to be jostled by individual molecular collisions but large enough to be observed under a microscope.

Brownian motion:

Is evidence for the kinetic molecular theory (molecules are in constant random motion)
Keeps colloid particles suspended (prevents settling)
Increases with temperature (faster-moving solvent molecules hit harder)

Types of Colloids

Colloids are classified by the phases of the dispersed substance and the medium:

Type	Dispersed Phase	Medium	Example
Sol	Solid	Liquid	Paint, blood
Gel	Liquid	Solid	Gelatin, jelly
Emulsion	Liquid	Liquid	Milk, mayonnaise
Foam	Gas	Liquid	Whipped cream, shaving cream
Aerosol (liquid)	Liquid	Gas	Fog, hairspray
Aerosol (solid)	Solid	Gas	Smoke, dust
Solid foam	Gas	Solid	Styrofoam, pumice
Solid sol	Solid	Solid	Colored glass, certain alloys

Emulsifying Agents

Oil and water form a suspension (they separate on standing). But add an emulsifier - a molecule with both hydrophilic and hydrophobic parts - and the oil is dispersed into tiny droplets that stay suspended as a colloid (an emulsion).

Examples of emulsifiers:

Soap and detergent - clean grease by emulsifying it in water
Bile salts - emulsify dietary fats in the small intestine so lipase can digest them
Lecithin (in egg yolk) - why mayonnaise stays mixed (oil emulsified in vinegar)

The emulsifier coats the oil droplets, with its polar head facing water and nonpolar tail embedded in the oil. This prevents the droplets from merging back together.

Coagulation and Flocculation

Colloidal particles often carry surface charges that create electrostatic repulsion, keeping them dispersed. Adding electrolytes (like salts) neutralizes these charges and causes the particles to clump together and settle out. This process is called coagulation or flocculation.

This is why:

Adding alum to murky water causes impurities to clump and settle (water treatment)
River water clears when it meets salty ocean water (salt neutralizes colloidal clay charges)

A student shines a laser pointer through three beakers: one with saltwater, one with milk, and one with muddy water. In which beaker(s) will the beam be visible?

Click to reveal answer

Milk and muddy water. Saltwater is a true solution (particles < 1 nm, no Tyndall effect - beam invisible). Milk is a colloid (fat droplets 100-500 nm, Tyndall effect - beam clearly visible). Muddy water is a suspension (particles > 1000 nm, scatters light but also blocks it). The Tyndall effect is the defining test for colloids: visible beam = colloid.

Bile salts are amphiphilic molecules that emulsify dietary fats. What type of colloid is formed, and why is this important for digestion?

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An emulsion (liquid-in-liquid colloid). Bile salts coat fat droplets with their nonpolar tails facing the fat and polar heads facing the aqueous intestinal fluid. This prevents the fat droplets from coalescing and increases the surface area available for pancreatic lipase to digest the fat. Without emulsification, large fat globules have too little surface area for efficient enzymatic digestion.