Chemistry 2e · Solutions and Colloids
Colloids
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In 30 seconds
A colloid A mixture with dispersed particles of ~1–1000 nm that do not settle Full entry → is a mixture in which particles of one substance (roughly 1–1000 nm in diameter) are dispersed throughout another substance without settling out. Colloids sit between solutions (particles below about 1 nm, fully mixed at the molecular level) and suspensions (particles above about 1000 nm, which settle on standing). The dispersed particles are large enough to scatter light and to be blocked by semipermeable membranes, yet small enough that gravity does not pull them out of the mixture.
Every colloid has two parts: the dispersed phase The particles distributed through the mixture Full entry → (the particles) and the dispersion medium The substance the particles are spread through Full entry → (the substance they are spread through). Because either can be a solid, liquid, or gas, colloids come in many forms — fog is a liquid dispersed in a gas, milk is a liquid dispersed in a liquid, and paint is a solid dispersed in a liquid. What unifies them is particle size and the resulting behavior: the Tyndall effect Scattering of a light beam by colloidal particles Full entry →, Brownian motion Random, jerky movement of particles caused by molecular collisions Full entry →, and long-term stability.
Why this matters
Colloids are everywhere in daily life, biology, and industry:
- Food: Milk, mayonnaise, butter, whipped cream, gelatin desserts, and salad dressing are all colloids whose texture depends on stable dispersion.
- Medicine: Blood plasma carries proteins and lipids as colloidal particles; intravenous fat emulsions and some drug suspensions are engineered colloids. dialysis Separation using a membrane that passes small molecules but blocks colloids Full entry → uses semipermeable membranes to remove small waste molecules from blood while keeping colloidal proteins in.
- Environment: Smoke, fog, and dust are atmospheric colloids (aerosols) that affect air quality and visibility.
- Industry: Paints, inks, cosmetics, and adhesives are colloids formulated to stay dispersed for years. Water treatment plants add coagulants that cause colloidal impurities to clump and settle.
- Household science: Soap and detergent work by forming micelles — colloidal clusters that carry grease away in water.
The college version
Core Concepts
Where colloids fit: solutions vs. colloids vs. suspensions
Particle size determines the category and the observable behavior:
| Mixture | Particle size | Settles? | Passes filter paper? | Passes semipermeable membrane? | Light behavior |
|---|---|---|---|---|---|
| Solution | < ~1 nm | No | Yes | Yes | Clear (no scattering) |
| Colloid | ~1–1000 nm | No | Yes | No | Scatters light (Tyndall effect) |
| Suspension | > ~1000 nm | Yes | No | No | Blocks/scatters light |
The boundaries are not razor-sharp — a "colloid" spans about three orders of magnitude in size — but the behavioral tests (settling, membrane passage, light scattering) are what matter in practice.
The Tyndall effect
When a beam of light passes through a colloid, the dispersed particles scatter the light, making the beam visible — this is the Tyndall effect. A true solution does not scatter visible light because its particles are far smaller than the wavelength of light. You see this every day: car headlights cutting through fog, a flashlight beam in dusty air, or a laser pointer shining through milk diluted in water. The Tyndall effect is the classic test that distinguishes a colloid from a solution.
Brownian motion keeps colloids suspended
Colloidal particles are constantly bombarded by solvent molecules, which are far smaller and moving rapidly. The uneven impacts push the particle in random, jerky directions — Brownian motion. This random jostling counteracts the slow downward pull of gravity, which is one reason colloids do not settle the way suspensions do. Brownian motion also demonstrates that molecules are real, moving objects: it was the key evidence for the existence of atoms and molecules in the early 1900s.
Types of colloids
Classify a colloid by identifying the dispersed phase and the dispersion medium:
| Dispersed phase | Dispersion medium | Name | Example |
|---|---|---|---|
| Gas | Liquid | Foam | Whipped cream, shaving foam |
| Gas | Solid | Solid foam | Marshmallow, sponge |
| Liquid | Gas | Liquid aerosol | Fog, mist, hair spray |
| Liquid | Liquid | Emulsion | Milk, mayonnaise |
| Liquid | Solid | Gel | Gelatin dessert, jelly |
| Solid | Gas | Solid aerosol | Smoke, airborne dust |
| Solid | Liquid | Sol | Paint, ink, muddy water |
| Solid | Solid | Solid sol | Some alloys, ruby glass |
Two names deserve special attention: an emulsion Colloid of one liquid dispersed in another liquid Full entry → is a colloid of one liquid dispersed in another liquid, and an aerosol Colloid of liquid or solid particles in a gas Full entry → is a colloid of liquid or solid particles dispersed in a gas.
Hydrophilic vs. hydrophobic colloids and micelles
Colloidal particles interact with water in two different ways. Hydrophilic colloids (starch, gelatin, proteins) have polar or charged groups on their surfaces, so water molecules cluster around each particle and keep it dispersed — these are stable in water almost on their own. Hydrophobic colloids (oil droplets, metal particles) have no such affinity for water; they stay dispersed only because their surfaces carry an electric charge. Like charges repel, so the particles cannot clump.
Soap molecules exploit both behaviors. A soap molecule has a long nonpolar hydrocarbon "tail" (hydrophobic) and an ionic "head" (hydrophilic). In water, many soap molecules cluster into a micelle Cluster of soap/detergent molecules with nonpolar tails inward and ionic heads outward Full entry → — tails pointing inward to trap grease, heads pointing outward toward the water. The micelle is a colloid-sized particle that carries the grease into the rinse water.
Emulsions and emulsifying agents
Oil and water normally separate because their molecules do not mix. An emulsifying agent is a substance that coats the droplets of one liquid so they cannot merge — for example, egg yolk (containing lecithin) stabilizes mayonnaise, and casein protein stabilizes milk fat droplets. Without the emulsifier, the droplets would collide, coalesce, and separate into two layers.
Coagulation and dialysis
Colloidal stability can be destroyed. Adding an electrolyte such as salt introduces many ions that neutralize the surface charges on hydrophobic particles; the particles then clump into larger aggregates (coagulation) that settle out. This principle is used in water treatment, where coagulants cause suspended clay and microbes to settle, and it explains why adding salt or acid can curdle milk.
Dialysis is the separation of small dissolved molecules from colloidal particles using a semipermeable membrane: small solutes and ions diffuse through, colloidal particles do not. This is the physical principle behind kidney dialysis machines, which clear metabolic waste from the blood while retaining blood proteins.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Colloid | Solution | Colloid particles (1–1000 nm) scatter light (Tyndall effect); solution particles (< ~1 nm) do not. Both are clear enough to pass filter paper |
| Colloid | Suspension | Suspension particles (> ~1000 nm) settle on standing and are trapped by filter paper; colloid particles stay suspended and pass filter paper |
| Tyndall effect | Fluorescence | Tyndall effect is scattering of light by particles; fluorescence is emission of light after absorption. A glowing beam ≠ glowing colloid itself |
| "Colloidal particles are molecules" | Particle clusters | A colloid particle contains many molecules or atoms clustered together; it is not a single molecule |
| "Milk is a solution" | Emulsion | Milk is an emulsion — liquid fat droplets dispersed in water, not dissolved molecules |
| "Gel is a liquid" | Gel structure | A gel is liquid dispersed in a solid network (e.g., jelly) — it holds its shape |
| "Soap dissolves grease" | Micelle formation | Soap does not dissolve grease; micelles surround and carry away nonpolar droplets in water |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A colloid is like a glass of chocolate milk: the tiny cocoa bits are too small to sink to the bottom and too big to disappear completely. If you shine a flashlight through it, the beam glows because the bits bounce the light around. Fog, milk, whipped cream, and smoke are all cousins — different ingredients, but all made of tiny bits floating inside something else without settling.
Worked example
Example 1: Particle size and the Tyndall effect
A colloid contains particles with an average diameter of 200 nm. Convert this to meters and compare it with the wavelength of visible light (about 400–700 nm) to explain why the colloid scatters light.
Formula first (unit conversion):
200 nm × 1 m109 nm = 2 × 10-7 m
(Unit check: nm × m/nm = m ✓)
Visible light has wavelengths of 400–700 nm, or 4 × 10-7 to 7 × 10-7 m. The 200 nm colloid particle is comparable in size to these wavelengths, so light is deflected when it hits a particle — the beam becomes visible (Tyndall effect). A solute molecule of, say, 0.5 nm is hundreds of times smaller than the wavelength, so it deflects essentially nothing and the solution looks clear.
Example 2: Classifying everyday colloids
Classify each of the following by identifying the dispersed phase and dispersion medium: (a) fog, (b) milk, (c) smoke, (d) whipped cream, (e) gelatin dessert.
Use the rule: name = dispersed phase + medium, with gas/liquid/solid combinations:
| Item | Dispersed phase | Dispersion medium | Type |
|---|---|---|---|
| (a) Fog | Liquid water droplets | Gas (air) | Liquid aerosol |
| (b) Milk | Liquid fat droplets | Liquid (water) | Emulsion |
| (c) Smoke | Solid particles | Gas (air) | Solid aerosol |
| (d) Whipped cream | Gas (air bubbles) | Liquid (cream) | Foam |
| (e) Gelatin dessert | Liquid (water) | Solid (gelatin network) | Gel |
Notice the same substance can appear in different roles: in whipped cream air is the dispersed phase, while in fog water droplets are the dispersed phase and air is the medium.
Example 3: Why does salt curdle milk?
Milk is an emulsion of fat droplets stabilized by casein proteins, which carry a surface charge that keeps droplets apart. When salt (an electrolyte) is added, the dissolved ions neutralize those surface charges. Droplets can then collide and merge into large clumps — the milk coagulates into curds. The same charge-neutralization principle is used deliberately in water treatment, where coagulants make fine suspended particles clump so they can be filtered or settled out. (General principle; laboratory steps are not part of this study guide.)
Key takeaways
- Colloid particle size: roughly 1–1000 nm — between solution (< ~1 nm) and suspension (> ~1000 nm).
- Colloids do not settle, pass through filter paper, but do not pass through semipermeable membranes.
- Tyndall effect (light scattering) is the classic test: colloid scatters light, true solution does not.
- Brownian motion (random jostling by solvent molecules) is a major reason colloids stay suspended.
- Colloid type is named by dispersed phase + dispersion medium: foam, solid foam, liquid aerosol, emulsion, gel, solid aerosol, sol, solid sol.
- Emulsion = liquid-in-liquid colloid; aerosol = liquid or solid in gas.
- Hydrophilic colloids are stabilized by water attraction; hydrophobic colloids by surface charge.
- Micelles (soap clusters with tails in, heads out) carry grease away in water.
- Adding an electrolyte causes coagulation — particles clump and settle.
- Dialysis uses a semipermeable membrane to separate small molecules from colloidal particles.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What particle-size range defines a colloid, and what two other mixture categories sit on either side?
Show answer
Colloids have particle diameters of roughly 1–1000 nm; solutions are below about 1 nm and suspensions above about 1000 nm.
A clear liquid scatters a flashlight beam; a second clear liquid does not. Which is more likely a colloid, and what is the test called?
Show answer
The liquid that scatters the beam is the colloid; the effect is called the Tyndall effect.
Why don't colloid particles settle out over time?
Show answer
Brownian motion — continuous random bombardment by solvent molecules — jostles the particles and counteracts settling; surface charges also keep particles from clumping.
Name the colloid type for each: milk, fog, smoke, whipped cream, gelatin dessert.
Show answer
Milk = emulsion, fog = liquid aerosol, smoke = solid aerosol, whipped cream = foam, gelatin dessert = gel.
Why does adding an electrolyte cause a hydrophobic colloid to coagulate To clump together and settle out of a colloid Full entry →?
Show answer
The ions neutralize the surface charges that keep particles apart; particles then collide, clump, and settle (coagulation).
In dialysis, what passes through the membrane and what does not?
Show answer
Small dissolved molecules and ions diffuse through the semipermeable membrane; colloidal particles (e.g., blood proteins) are retained.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- colloid
- A mixture with dispersed particles of ~1–1000 nm that do not settle
- dispersed phase
- The particles distributed through the mixture
- dispersion medium
- The substance the particles are spread through
- Tyndall effect
- Scattering of a light beam by colloidal particles
- Brownian motion
- Random, jerky movement of particles caused by molecular collisions
- emulsion
- Colloid of one liquid dispersed in another liquid
- aerosol
- Colloid of liquid or solid particles in a gas
- micelle
- Cluster of soap/detergent molecules with nonpolar tails inward and ionic heads outward
- coagulate
- To clump together and settle out of a colloid
- dialysis
- Separation using a membrane that passes small molecules but blocks colloids
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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