Chemistry: Atoms First 2e · Solutions and Colloids
Colloids
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In 30 seconds
Milk, fog, whipped cream, paint, gelatin dessert, smoke, and even blood plasma share a hidden structure: they are colloids — mixtures in which one substance is finely dispersed throughout another without actually dissolving. Colloidal particles range from about 1 to 1000 nanometers in diameter, far larger than individual molecules or ions yet too small to see or settle out.
Colloids sit between solutions and suspensions. The dispersed particles are big enough to scatter light (the Tyndall effect Scattering of light by colloidal particles, making a beam visible. Full entry →) and to jiggle randomly (Brownian motion Random zigzag movement of colloidal particles from molecular collisions. Full entry →), but small enough that gravity cannot pull them down. Understanding what stabilizes a Colloid Mixture with particles of 1–1000 nm dispersed in another phase. Full entry → — surface charge, adsorbed layers, and emulsifiers — explains why milk stays mixed, why mayonnaise doesn't separate, and why soap cleans grease. This topic covers the types of colloids, their characteristic properties, and how they are made and broken.
Why this matters
- Food science: Mayonnaise, milk, butter, ice cream, and salad dressing are all colloids; their texture and shelf life depend on colloid stability.
- Medicine: Blood is a colloidal dispersion of proteins and cells; dialysis and drug delivery systems rely on colloidal behavior and membrane separations.
- Everyday chemistry: Soap and detergents clean by forming micelles — colloidal aggregates that trap grease — and fog, smoke, and aerosols are colloids we breathe.
- Environment and industry: Water treatment coagulates colloidal clay and bacteria so they settle; paints, inks, and cosmetics are engineered colloids.
- Exam value: The Tyndall effect and colloid classification are frequent short-answer and multiple-choice items.
The college version
Core Concepts
What makes a colloid a colloid
A colloid has two parts: a Dispersed phase The particles distributed through a colloid. Full entry → (the particles, 1–1000 nm) and a Dispersion medium The continuous substance surrounding the dispersed particles. Full entry → (the continuous phase that surrounds them). The key contrast:
- Solution: particles are individual molecules or ions (< 1 nm); homogeneous at the molecular level; never separate; no Tyndall effect.
- Colloid: particles are clusters of many molecules (1–1000 nm); appear homogeneous but are actually two phases; do not settle; show the Tyndall effect.
- Suspension: particles are large (> 1000 nm); cloudy; settle out on standing; can be separated by filtration.
Colloidal particles are large enough to scatter visible light but small enough that Brownian motion (random collisions with solvent molecules) keeps them suspended against gravity.
Types of colloids
Colloids are named by the state of the dispersed phase and the dispersion medium. A "sol" is a solid dispersed in a liquid (paint, gelatin dessert); a gel is a liquid dispersed in a solid (jelly, silica gel); an Emulsion Colloid of one liquid dispersed in another (milk, mayonnaise). Full entry → is a liquid in a liquid (milk, mayonnaise); a foam is a gas in a liquid (whipped cream) or a gas in a solid (styrofoam); an aerosol is a solid or liquid in a gas (smoke, fog, spray deodorant); a solid sol is a solid in a solid (stained glass, some alloys).
The Tyndall effect
When a beam of light passes through a colloid, the dispersed particles scatter the light, making the beam visible from the side — the Tyndall effect. Solutions scatter almost no light, so the effect is a quick test: shine a flashlight through a glass of salt water (clear beam path) versus a glass of milk diluted with water (visible beam). Headlights cutting through fog and the blue beam through a dusty room are Tyndall scattering in action.
Brownian motion and why colloids don't settle
Colloidal particles are constantly bombarded by solvent molecules from all sides. Because the particle is small, these random hits are never perfectly balanced, so the particle zigzags — Brownian motion. This motion counteracts gravity (particles are too small to sediment) and is part of why colloids are stable for long periods. The scattering of light also gives colloids their cloudy or opalescent look.
Stabilizing colloids: charge and adsorbed layers
Why doesn't milk separate into fat and water? Three stabilizing mechanisms work together:
- Surface charge: Many colloidal particles acquire an electric charge at their surface (e.g., by adsorbing ions). Like charges repel, preventing particles from clumping into large, settling aggregates.
- Adsorbed layers: A thin film of solvent or other molecules around each particle acts as a cushion that keeps particles apart.
- Emulsifiers: In emulsions, an Emulsifying agent Substance that stabilizes an emulsion by coating droplets. Full entry → (like the casein in milk or lecithin in mayonnaise) coats droplets of one liquid so they cannot merge. Soap molecules have a water-loving head and an oil-loving tail — they surround grease droplets to form stable micelles, which is exactly how soap washes grease away.
Making and breaking colloids
Colloids are made by dispersion (grinding or shaking large particles down to colloidal size, as in a colloid mill for paint) or by condensation (growing small particles, e.g., precipitation reactions). They are broken by Coagulation Clumping of colloidal particles into larger, settling aggregates. Full entry → — neutralizing the surface charge so particles clump and settle. Heating, adding electrolytes (like the salt that separates curds from whey in cheese-making), or adding an oppositely charged colloid (as in water treatment with alum) all cause coagulation.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Colloid | Solution | Solution particles are < 1 nm molecules/ions (no Tyndall effect, never separate); colloid particles are 1–1000 nm clusters (Tyndall effect, Brownian motion). |
| Colloid | Suspension | Suspension particles settle on standing and can be filtered; colloids stay dispersed and pass through ordinary filters. |
| Emulsion | Solution | An emulsion is two immiscible liquids held apart by an emulsifier; a solution is one homogeneous phase. |
| Brownian motion | Sedimentation | Brownian motion keeps particles suspended; sedimentation pulls them down. Stability = motion beats gravity. |
| Tyndall effect | Fluorescence | Tyndall scattering redirects the same wavelength of light; fluorescence re-emits light at a longer wavelength. |
| Emulsifier | Solute | An emulsifier coats and separates droplets at interfaces; a solute dissolves molecularly into one phase. |
| Coagulation | Precipitation of a solute | Coagulation clumps pre-existing colloidal particles; precipitation forms a new solid from dissolved ions. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a stadium full of tiny balloons, each one floating on its own because they all push each other away. They're too big to be invisible like air molecules, but too small to fall to the floor. Shine a flashlight and you see the beam because the balloons scatter the light. That's a colloid — tiny particles that stay floating because they're constantly being bumped around and push each other apart, like the fat droplets in milk.
Worked example
Worked example 1 — classify by the evidence. You have three unlabeled samples: A (clear, yellow liquid), B (cloudy white liquid that separates after an hour), and C (milky liquid that stays mixed for weeks and shows a light beam when a flashlight is shined through). Classify each.
- Sample A is a solution: clear, no beam, particles < 1 nm (molecules/ions).
- Sample B is a suspension: it settles, meaning particles > 1000 nm.
- Sample C is a colloid: it doesn't settle (Brownian motion) and shows the Tyndall effect — particles in the 1–1000 nm range.
Worked example 2 — why does salad dressing separate but mayonnaise doesn't? A simple oil-and-vinegar dressing separates on standing because the oil droplets are large enough to coalesce and float; there is no emulsifier to coat them. Mayonnaise contains lecithin (from egg yolk), an emulsifying agent whose molecules have a water-loving head and an oil-loving tail. The tails embed in oil droplets while the heads face the water, coating each droplet with a charged, repelling shell. The droplets can no longer merge, so the emulsion stays stable for weeks. This is the same chemistry as soap: one molecule, two personalities — one end loves water, the other loves oil.
Worked example 3 — coagulation in water treatment. A river is muddy because colloidal clay particles carry negative surface charges and repel each other, so they never settle. A treatment plant adds alum (aluminum sulfate), which supplies Al³⁺ ions. The positive ions neutralize the negative surface charges; with the repulsion gone, particles clump into larger aggregates that settle out, and the water clears. Salt does the same job on curds in cheese-making — ions neutralize the charge on casein micelles, and the curd precipitates.
Key takeaways
- Colloid particle size: 1–1000 nm — between solution molecules (< 1 nm) and suspension particles (> 1000 nm).
- Colloids do not settle and cannot be separated by ordinary filtration; they show the Tyndall effect; solutions show neither.
- Brownian motion (random bombardment by solvent) keeps colloidal particles suspended.
- Stability comes from surface charge (repulsion), adsorbed layers, and emulsifiers — not from dissolving.
- An emulsion is a liquid-in-liquid colloid (milk, mayonnaise); emulsifying agents stabilize it (casein, lecithin).
- Soap works by forming micelles that trap grease droplets.
- Coagulation (adding electrolyte, heating, or oppositely charged particles) destroys colloids; water treatment and cheese-making both use it.
- Distinguish by particle size: solution < colloid < suspension; the Tyndall effect is the quick test.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
A student shines a laser through salt water (no beam) and through diluted milk (visible beam). What do the results tell you?
Show answer
Salt water is a true solution (particles < 1 nm scatter no visible light); diluted milk is a colloid whose 1–1000 nm particles scatter light — the Tyndall effect.
Why don't colloidal particles settle out over time, even though they are denser than the medium?
Show answer
Brownian motion: random collisions with solvent molecules constantly jostle the particles, counteracting gravity so they never settle.
List the three main mechanisms that keep colloids stable.
Show answer
Surface charge (like charges repel), adsorbed solvent/layer cushions, and emulsifying agents that coat droplets.
Milk is an emulsion of fat droplets in water. Why doesn't it separate into layers in the refrigerator?
Show answer
Casein and other milk proteins act as emulsifiers, coating fat droplets so they cannot coalesce into a floating layer.
How does adding salt to a colloid cause it to coagulate?
Show answer
The ions neutralize the surface charges on the particles; with repulsion gone, particles collide, clump, and settle (coagulation).
A muddy river is a suspension or a colloid? What evidence would settle the question?
Show answer
If it settles on standing, it's a suspension; if it stays cloudy and shows a Tyndall beam, it's a colloid. (Rivers are typically suspensions of larger sediment plus colloidal clay.)
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Colloid
- Mixture with particles of 1–1000 nm dispersed in another phase.
- Dispersed phase
- The particles distributed through a colloid.
- Dispersion medium
- The continuous substance surrounding the dispersed particles.
- Tyndall effect
- Scattering of light by colloidal particles, making a beam visible.
- Brownian motion
- Random zigzag movement of colloidal particles from molecular collisions.
- Emulsion
- Colloid of one liquid dispersed in another (milk, mayonnaise).
- Emulsifying agent
- Substance that stabilizes an emulsion by coating droplets.
- Micelle
- Spherical cluster of soap/surfactant molecules trapping grease.
- Coagulation
- Clumping of colloidal particles into larger, settling aggregates.
Sources & references
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