Colloids and Suspensions: Mixtures That Don't Quite Dissolve
Milk looks like a single uniform liquid, but it isn't a true solution the way salt water is. It's actually millions of microscopic fat droplets suspended in water, small enough that they never settle out under normal conditions, but large enough that they aren't truly dissolved at the molecular level either. This in-between category is called a colloid, and understanding where it sits between a true solution and a simple suspension explains everything from why fog scatters headlight beams to why salad dressing needs to be shaken before use.
The Three Categories of Mixtures, by Particle Size
Mixtures can be classified along a spectrum based entirely on the size of the particles distributed within them:
- True solutions: particles are individual ions or molecules, roughly less than 1 nanometer in size. They never settle, can't be filtered out, and don't scatter light. Salt water and sugar water are classic examples.
- Colloids: particles range from about 1 to 1,000 nanometers, small enough to stay permanently suspended (they don't settle under gravity) but large enough to scatter light and be distinguished from the surrounding medium under the right conditions. Milk, fog, and gelatin are all colloids.
- Suspensions: particles are larger than about 1,000 nanometers, visible to the naked eye or under a simple microscope, and will eventually settle out under gravity unless continuously stirred or shaken. Muddy water and a mixture of sand in water are suspensions.
The Tyndall Effect: How to Tell a Colloid From a True Solution
One of the simplest ways to distinguish a colloid from a true solution is the Tyndall effect: shining a beam of light through the mixture and observing whether the light's path becomes visible. In a true solution, particles are far too small to scatter light noticeably, so a light beam passes through invisibly. In a colloid, particles are large enough to scatter light in all directions, making the beam's path visible as it travels through the mixture, exactly like how a car's headlights become visible beams when driving through fog, but not when driving through clear air.
This is a practical, visual test: a laser pointer aimed through a glass of salt water produces no visible beam within the liquid, while the same laser aimed through a glass of milk (diluted with water) produces a clearly visible glowing path.
Types of Colloids
Colloids are further categorized by the physical state of their two components: the dispersed phase (the substance distributed throughout) and the dispersion medium (the substance it's distributed in).
| Dispersed Phase | Dispersion Medium | Type | Example |
|---|---|---|---|
| Liquid | Gas | Aerosol | Fog, mist |
| Solid | Gas | Aerosol | Smoke |
| Gas | Liquid | Foam | Whipped cream, shaving foam |
| Liquid | Liquid | Emulsion | Milk, mayonnaise |
| Solid | Liquid | Sol | Paint, blood |
| Liquid | Solid | Gel | Gelatin, jelly |
| Solid | Solid | Solid sol | Colored glass, some alloys |
Why Colloids Don't Separate on Their Own
Colloidal particles are small enough that random molecular collisions with the surrounding medium (a phenomenon called Brownian motion) keep them in constant, chaotic motion, counteracting the pull of gravity that would otherwise cause them to settle. Many colloids are also stabilized by electrical charge: dispersed particles often carry the same electrical charge as one another, causing them to repel each other and resist clumping together into larger particles that would eventually settle out.
This is why an emulsion like oil-and-vinegar salad dressing separates relatively quickly (the droplets are on the larger end of the colloidal range and lack a stabilizing charge), while a well-formulated emulsion like mayonnaise, which includes egg yolk as an emulsifier to stabilize the tiny oil droplets within it, can remain stable and uniform for a very long time.
Everyday Examples of Colloids
- Milk: an emulsion of fat droplets dispersed in water, stabilized by proteins.
- Fog and clouds: an aerosol of tiny water droplets dispersed in air.
- Whipped cream and shaving foam: a foam of gas bubbles dispersed in a liquid.
- Blood: a sol in which blood cells and proteins are dispersed in plasma (the liquid component).
- Paint: a sol of pigment particles dispersed in a liquid binder.
- Gelatin desserts: a gel where a liquid is dispersed throughout a solid, semi-rigid network.
FAQ
No, ordinary filter paper has pores far too large to catch colloidal particles, which pass right through along with the dispersion medium, the same way they'd pass through in a true solution. Separating colloidal particles typically requires more specialized techniques, such as centrifugation at very high speeds or a specially designed semipermeable membrane.
Smoke is generally classified as a colloid (specifically an aerosol), since its solid particles are small enough to remain suspended in air for extended periods and to scatter light visibly, rather than settling out quickly the way a coarser suspension would.
Oil and vinegar (which is mostly water) are both polar-incompatible in a specific sense relevant here: oil droplets in the dressing are on the larger end of colloidal size and, without an emulsifier to stabilize them, gradually merge back together (a process called coalescence) and separate under gravity. Adding an emulsifier, like mustard or egg, provides molecules that surround the oil droplets and prevent them from recombining, keeping the emulsion stable much longer.
Both are colloids, but they differ in which phase forms the continuous structure. In a sol, solid particles are dispersed throughout a liquid that remains free-flowing. In a gel, the dispersed and continuous phases essentially trade roles in terms of rigidity: a liquid is trapped within a solid, interconnected network, giving the overall mixture a semi-solid, often jelly-like consistency.
Not exactly a single fixed size, but they fall within a defined range, roughly 1 to 1,000 nanometers, which is what distinguishes them from true solutions (smaller) and suspensions (larger). Within that colloidal range, particle size can still vary, and it affects properties like how strongly the mixture scatters light.
Conclusion
Colloids occupy the middle ground between a true solution, where particles are far too small to ever settle or scatter light, and a coarse suspension, where particles are large enough to see and settle under gravity. Milk, fog, whipped cream, and paint all owe their distinctive, uniform-looking appearance to particles sitting in that narrow, in-between size range, small enough to stay suspended indefinitely through Brownian motion and electrical repulsion, but large enough to visibly scatter light via the Tyndall effect. Once you know what to look for, colloids turn out to be one of the most common mixture types in daily life, hiding in plain sight in nearly every kitchen and bathroom cabinet.
Here are some useful references if you want to go deeper:
- Khan Academy – Solutions, Suspensions, and Colloids — free lessons distinguishing the three mixture categories.
- Chemguide – Colloids — a detailed explanation of colloid types and stabilization.
- LibreTexts Chemistry – Colloids — an open textbook resource covering the Tyndall effect and colloid classification.
