Colloids

Where colloids sit. Mixtures fall along a spectrum by particle size: true solutions have the smallest particles, suspensions the largest, and colloids sit in between. This single fact explains almost every property a colloid shows.

where: $d_{\text{particle}}$ is the diameter of the dispersed-phase particles in the mixture; below $1\ \text{nm}$ the mixture behaves as a true solution, and above $100\ \text{nm}$ it behaves as a suspension.

Two components, always. Every colloid is built from a dispersed phase (the substance present in smaller amount, broken into tiny particles — the "solute-like" role) and a dispersing medium (the substance present in larger amount, surrounding those particles — the "solvent-like" role). Both can independently be solid, liquid, or gas (with one exception: gas-in-gas mixtures always mix as true solutions, never colloids, because gas molecules cannot form separate colloidal particles in another gas).

Four defining behaviours. A colloid does not settle on standing, however long you wait, because the particles are light enough for random molecular collisions (Brownian motion) to keep knocking them back into suspension against gravity. It cannot be separated by ordinary filter paper, since its particles slip through the paper's pores — only a suspension's larger particles get trapped that way. It scatters a beam of light passing through it, called the Tyndall effect, because the particle size is comparable to the wavelength of visible light. And despite all this internal structure, it still looks like a single uniform substance to the unaided eye, which is exactly why colloids are so easy to misclassify as solutions.

Colloidal behaviour is a direct consequence of particle size sitting near the wavelength of visible light, which runs roughly from $400\ \text{nm}$ to $700\ \text{nm}$. A particle much smaller than that wavelength — like a dissolved sugar molecule at well under $1\ \text{nm}$ — cannot deflect a passing light wave in any noticeable way, so the light beam travels straight through and stays invisible from the side; this is why a true solution never shows the Tyndall effect. A colloidal particle, at $1$ to $100\ \text{nm}$, is large enough to interact with the light wave and scatter it sideways in every direction, so an observer looking from the side of the beam sees a glowing, visible path — exactly the torch-through-milk or headlight-through-fog scenario. The same size logic explains settling and filtration. Suspension particles (above $100\ \text{nm}$) are heavy enough that gravity wins over the random jostling of surrounding molecules, so they sink over time and get trapped by ordinary filter paper. Colloidal particles are light enough that Brownian motion — the continuous, random battering by fast-moving molecules of the dispersing medium — keeps them permanently in motion and evenly spread, so they neither settle out nor get caught by filter paper pores. This is also why a colloid is correctly labelled heterogeneous rather than homogeneous: two distinguishable phases genuinely exist side by side, it is only the smallness of the particles that hides this from the naked eye.

Question: A torch beam is passed through three unlabelled beakers: (i) a clear salt solution, (ii) a sample of blood plasma, (iii) river water that has been left undisturbed for two hours after being stirred up. For each, predict whether the beam's path is visible, whether the particles will have settled, and classify the mixture.

Step 1 — salt solution. Dissolved salt ions are far smaller than $1\ \text{nm}$, well below the wavelength of visible light, so they cannot scatter the beam. The torch light passes straight through with no visible path, and since the ions never settle either, this is a true solution.

Step 2 — blood plasma. Blood contains protein and cell-fragment particles in the $1$–$100\ \text{nm}$ colloidal range. These are large enough to scatter light sideways, so the beam's path will be clearly visible (Tyndall effect) as it crosses the sample, while the particles remain evenly spread and do not settle. This identifies blood plasma as a colloid.

Step 3 — settled river water. Freshly stirred river water carries suspended silt particles well above $100\ \text{nm}$. Over the two hours of standing, gravity pulls these heavy particles down faster than Brownian motion can keep them suspended, so the silt settles to the bottom, leaving clearer water above. This confirms river water (before settling) was a suspension, not a colloid.

Step 4 — final classification. Only blood plasma, among the three, is a colloid — confirmed by the visible Tyndall beam combined with the absence of any settling, which together rule out both a true solution and a suspension.

Answer: salt solution = true solution (no scattering, no settling); blood plasma = colloid (Tyndall effect, no settling); river water = suspension (silt settles out).

Competitive stretch

A tricky exam variant swaps in a coloured gel and asks students to name its dispersed phase and dispersing medium. A gel is a liquid dispersed throughout a solid framework, so its dispersed phase is liquid and its dispersing medium is solid — the same size-and-scattering logic applies even though neither component here is the "usual" liquid-in-liquid pairing of milk.

• Calling a colloid "homogeneous" because it looks uniform to the eye. It is correctly classified as heterogeneous — two real phases exist, just at a scale too small to see individually.
• Assuming colloidal particles can be filtered out with ordinary filter paper, like a suspension's particles. Colloidal particles are small enough to pass straight through normal filter paper.
• Mistaking simple surface reflection for the Tyndall effect. The Tyndall effect is specifically the visible path of a beam scattered sideways as it travels through the medium, not light merely bouncing off a surface.
• Assuming any mixture that "does not settle" must be a true solution. A colloid also does not settle, so the settling test alone cannot distinguish a colloid from a solution — the Tyndall (light-scattering) test is needed to tell them apart.

Ask the tutor: Not sure whether a mixture like nail polish, hair gel, or shaving cream is a colloid, a solution, or a suspension? Ask the AI tutor to test you with more real-world examples.