Solutions, suspensions and colloids
Stir a spoon of sugar into water and it vanishes, leaving a clear liquid. Stir the same spoon of chalk powder into water and it stays cloudy, with white grit settling at the bottom by evening. Stir milk into water and it stays hazy forever, yet no grains are ever visible. Same action — stirring a solid or a liquid into water — three completely different outcomes. The difference is not what the eye first suggests; it is the size of the particles doing the mixing. This article sorts every such water mixture into one of three families — true solution, suspension, and colloid — and gives you the practical, lab-bench tests (clarity, settling, filterability) that ICSE Class 8 expects you to use to tell them apart, without needing to measure a single particle.
Mixtures of a substance in water fall into solutions, suspensions, or colloids depending on how large the mixed-in particles are, and that size alone decides whether the mixture looks clear or cloudy and whether it settles on standing.
The core idea
When a solid or liquid is mixed into water, the particles it breaks into can be roughly sorted into three size bands. Nothing here is measured in numbers at this level — instead, particle size is inferred indirectly from three everyday observations: how clear the mixture looks, whether it settles when left alone, and whether ordinary filter paper can catch the particles.
- True solution — the solute particles are so small that no instrument used in a school lab (not even a hand lens) can see them. They spread evenly and permanently through the water. The mixture is completely clear/transparent and never settles, no matter how long it stands. Example: salt or sugar dissolved in water.
- Suspension — the dispersed particles are large enough to see with the naked eye. They do not truly dissolve; gravity eventually pulls them down, so the mixture separates into a settled layer and clearer liquid above. The mixture usually looks cloudy or muddy while stirred. Example: chalk powder () or sand stirred into water.
- Colloid — the particles are in between: too small to see individually, yet large enough to scatter light and keep the mixture permanently hazy or milky rather than glass-clear. They never settle on standing. Example: milk, or starch mixed with water.
The three families sit on one continuous ladder of particle size, with solutions at the small end and suspensions at the large end:
where: each term is a relative size ranking, not a measured number — ICSE Class 8 does not require numeric particle-size ranges, only the ordering and the observable consequences of that ordering (clarity, settling, filterability).
Why it works
There is no formula behind this classification — it is a reasoning chain from particle size to three observable behaviours, and ICSE Class 8 expects you to apply that chain rather than recall a number.
Size decides visibility. True-solution particles (single dissolved ions or molecules) are far too small for any school-lab instrument to resolve, so the liquid looks perfectly clear. Suspension particles are clumps of many molecules, large enough for the unaided eye to see as specks. Colloid particles sit in between: too small to see individually, but large enough that they scatter light passing through the liquid, which is why colloids look milky or hazy rather than transparent, even though no single particle is visible.
Size decides settling. Gravity pulls every particle down, but for very small particles the constant jostling of surrounding water molecules is strong enough to keep them permanently suspended — this is true for both solution and colloid particles. Suspension particles are heavy and large enough that gravity wins, so they sink out over time, leaving clearer liquid above a settled layer.
Size decides filterability. Ordinary filter paper has pores far bigger than solution or colloid particles, so both pass straight through and cannot be separated this way. Suspension particles are large enough to be physically trapped by the same paper, so filtration does separate a suspension into solid residue and clear filtrate. Put together, the same underlying property — particle size — independently explains why a solution is clear, why a colloid is hazy but permanent, and why only a suspension settles and filters out. None of the three tests (clarity, settling, filtration) needs a microscope or a measured particle size; they are the practical stand-ins for a size difference too small to see directly.
Worked example
Question: A student prepares three beakers — (A) sugar stirred into water, (B) fine chalk powder stirred into water, (C) milk mixed with water — and leaves them undisturbed overnight. Classify each mixture and justify the classification using the observations.
Step 1. Beaker A, immediately after stirring, is completely clear and no particles are visible even against a bright light. Overnight, nothing changes — no layer forms at the bottom. Clear plus no settling means the particles are too small to see and too small for gravity to pull down: this is a true solution.
Step 2. Beaker B, immediately after stirring, looks cloudy with visible white specks swirling in it. Left overnight, a distinct white layer of chalk collects at the bottom, and the liquid above turns noticeably clearer than it was right after stirring. Visible particles plus settling means the particles are large: this is a suspension.
Step 3. Beaker C, immediately after stirring, looks uniformly milky, but no individual particle can be picked out by eye. Left overnight, it is still uniformly milky throughout — no layer has formed anywhere in the beaker. Hazy but not settling means the particles are intermediate in size: this is a colloid.
Step 4. Collecting the three results: Beaker A is a solution (clear, does not settle), Beaker B is a suspension (visible, settles, would also be caught by filter paper), and Beaker C is a colloid (hazy, does not settle, would pass through filter paper like a solution). The classification of A, B, C is solution, suspension, colloid, decided entirely by clarity and settling behaviour, not by any measured particle size.
Competitive stretch
If Beaker B and Beaker C were both freshly stirred and looked equally cloudy at that instant, settling alone — not initial appearance — is the reliable test: only the suspension (Beaker B) separates into a settled layer and a clearer liquid above after standing; the colloid (Beaker C) stays uniformly hazy no matter how long it is left, which is exactly the test used in Worked Example 2 of the source lesson to tell chalk-in-water apart from milk.
Common traps
- Assuming any cloudy liquid is automatically a suspension — colloids can look just as hazy immediately after mixing; the real test is whether it settles on standing, not how it looks the moment you stir it.
- Forgetting that a coloured liquid can still be a true solution — copper sulphate solution is blue but completely see-through, so colour is not the same property as clarity/transparency.
- Trying to judge particle size by eye for solutions versus colloids — both are invisible to the naked eye at this level; the working tests are settling behaviour and translucency, not direct particle-size measurement.
- Confusing "does not settle" with "cannot be filtered by ordinary filter paper" — both properties happen to be true together for solutions and colloids, but they are two separate tests; a suspension is the only one of the three that both settles and gets caught by filter paper.
Ask the tutor: Want to practise sorting a few more everyday mixtures — starch paste, muddy river water, fog, or paint — into solutions, suspensions, and colloids using just these three tests?
If I cannot see the particles in either a solution or a colloid, how am I supposed to tell them apart just by looking
You are right that direct viewing will not work for either one — that is exactly why the settling test and translucency are used instead. A true solution stays perfectly clear, like plain water with nothing added, even though something is dissolved in it. A colloid, by contrast, stays permanently hazy or milky because its slightly larger particles scatter light passing through, even though you still cannot pick out any single particle. So look at clarity first: perfectly clear points to a solution, persistently hazy points to a colloid, and in both cases nothing ever settles out.
Why does a suspension eventually settle but a colloid never does, if neither one dissolves
It comes down to how heavy and large the dispersed particles are relative to the constant bombardment from water molecules around them. Suspension particles, like chalk or sand grains, are large and heavy enough that gravity steadily pulls them down faster than the water's jostling can keep them up, so they sink into a visible layer over time. Colloid particles are smaller and lighter, and the continuous random collisions with water molecules are enough to keep them permanently spread through the liquid, so they never accumulate at the bottom no matter how long you wait.
Is muddy river water a suspension or a colloid, since it looks brownish and cloudy but takes a while to clear up
Muddy river water is a suspension. The tell-tale sign is exactly the "takes a while to clear up" part — if you let a jar of muddy water stand undisturbed, the sand and silt particles gradually sink and settle at the bottom, leaving progressively clearer water above, which is the defining suspension behaviour. A true colloid, like milk, would stay just as hazy after standing as it did the moment you mixed it, with no settled layer ever forming, however long you wait.