Suspensions

A suspension forms when a solid is mixed into a liquid (occasionally a gas) but does not dissolve — the particles stay as distinct solid lumps, just distributed unevenly through the liquid by stirring or shaking. Four features, all consequences of one fact — the particles are large — define it completely.

Particle size sets everything else. A suspended particle is larger than 100 nanometres, easily large enough to be seen with the naked eye as a distinct speck, unlike the invisibly small particles of a true solution or the intermediate particles of a colloid.

where: $d_{\text{particle}}$ is the diameter of a dispersed particle, measured in nanometres ($1\ \text{nm} = 10^{-9}\ \text{m}$). Anything above this rough threshold behaves as a suspension; below it, particles are small enough to belong to a colloid (roughly 1 nm to 100 nm) or a true solution (below 1 nm).

It scatters light. Large particles physically intercept and scatter a light beam, so a suspension shows the Tyndall effect — a visible beam path when torchlight passes through it. This is a feature it shares with colloids, and it is a common source of confusion (see Common traps below).

It settles on standing. Gravity pulls large, dense particles downward faster than random molecular collisions (Brownian motion) can keep them suspended. Left undisturbed, a suspension separates into a solid layer (sediment) and a clearer liquid above. This settling is the single most reliable test that distinguishes a suspension from a colloid, whose particles are too small and light for gravity to win out.

It is separable by ordinary filtration. Because suspended particles are far larger than the pores of ordinary filter paper, filtration cleanly traps the solid while the liquid passes through as filtrate. A colloid's particles are small enough to slip through the same paper. Everyday examples: chalk powder or fine sand shaken in water, muddy river water, paint before stirring, and flour stirred into water.

The three-way comparison worth memorising for an exam:

The whole behaviour of a suspension traces back to one cause: its particles are simply too big for the ordinary jostling of water molecules to keep them dispersed. In a true solution, individual ions or molecules are so small and light that random thermal collisions from surrounding solvent molecules constantly buffet them in every direction, and this ceaseless motion is more than enough to keep them permanently and uniformly mixed — they never settle. In a colloid, the particles are bigger clusters of molecules, but still small and light enough that the same molecular bombardment (Brownian motion) keeps them wandering randomly instead of sinking, so a colloid also never settles under ordinary gravity, even over hours. A suspended particle is large enough — thousands of molecules bound together, or a genuine solid fragment — that gravity's steady downward pull on its comparatively large mass overwhelms the random nudges from surrounding water molecules. There is no delicate balance here: the particle is simply too heavy for Brownian motion to fight gravity, so it sinks, slowly but surely, until it reaches the bottom of the container. Stirring only interrupts this sinking temporarily by giving the particles a burst of mechanical energy to keep them airborne in the liquid; the moment the stirring stops, gravity resumes its unopposed pull and the particles begin settling again. This is also why a suspension can be separated by simple filtration: the particles are large enough, both to settle and to be too big to squeeze through the microscopic pores of filter paper, while water molecules pass through freely. Light scattering (the Tyndall effect) follows the same logic — a particle only needs to be comparable in size to, or larger than, the wavelength of visible light to intercept and redirect it, and both colloidal and suspended particles clear that bar, which is why both show a visible torch-beam. What only the suspension adds on top of that shared light-scattering behaviour is size large enough to also settle and filter out.

Question: A student mixes 5 g of fine sand with 200 mL of water (density 1 g/mL) in a beaker, stirs vigorously, and observes the mixture immediately and again after standing for 30 minutes. She then filters the mixture through ordinary filter paper and dries the residue. Describe what she observes at each stage, calculate the mass fraction of sand in the original mixture, and state the mass of dry sand she should recover.

Step 1 — immediately after stirring. Vigorous stirring temporarily distributes the sand particles through the water, so the mixture looks uniformly cloudy. Because sand particles are far larger than 100 nm, they never actually dissolve — they are only mechanically kept airborne by the stirring.

Step 2 — after 30 minutes undisturbed. With stirring stopped, gravity pulls the large, dense sand particles down faster than Brownian motion can keep them dispersed. The student should see a layer of settled sand at the bottom of the beaker with visibly clearer water above it — the classic settling test for a suspension.

Step 3 — mass fraction of the original mixture. The total mass of the mixture is the mass of sand plus the mass of water: $200\ \text{mL}$ of water at $1\ \text{g/mL}$ gives $200\ \text{g}$ of water, so the total mass is $5\ \text{g} + 200\ \text{g} = 205\ \text{g}$. The mass fraction of sand is $\dfrac{5}{205} \times 100 \approx 2.44\%$.

Step 4 — filtration and recovery. Since sand does not dissolve, its particles are unchanged in mass and simply too large to pass through the filter paper's pores. Filtration traps all the sand as residue while water passes through as clear filtrate. Because no sand is lost to dissolution, the dried residue should weigh essentially 5 g — recovering the full original mass of sand, confirming the mixture was a suspension rather than a solution.

• Assuming "cloudy" always means "suspension." A freshly stirred suspension and a colloid can both look cloudy — the decisive tests are settling on standing and separation by ordinary filtration, not appearance alone.
• Forgetting that a suspension also shows the Tyndall effect. Both colloids and suspensions scatter light; the real distinguishing feature of a suspension is that its particles eventually settle and can be filtered out.
• Believing stirring "dissolves" the suspended solid. Stirring only mechanically redistributes large particles; it does not shrink them, so they settle again the moment stirring stops.
• Thinking a suspension must look heavily "dirty" or concentrated. Even a dilute sand-in-water mixture with a mass fraction under 3%, as in the worked example, is still a full suspension by definition — the criterion is particle size and settling behaviour, not murkiness.

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