NCERT Class 9 Science Chapter 5 Activities — Exploring Mixtures and their Separation
NCERT Class 9 Science — Exploring Mixtures and their Separation, Activities 5.1 to 5.9 on pages 73 to 88.
Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.
Nine activities, the most of any Class 9 chapter in scope, and they split cleanly into two halves that match the chapter’s title.
Activities 5.1 to 5.4 are about what a mixture is — telling solutions, suspensions and colloids apart, and understanding solubility well enough to grow a crystal from a solution. Activities 5.5 to 5.9 are about taking mixtures apart, one technique per activity: chromatography, a separating funnel, sublimation, centrifugation, and then a summary table pulling it all together.
Key insight. Every separation technique in the second half works by finding one property the components do not share — solubility in the moving solvent for chromatography, density and immiscibility for the separating funnel, the ability to sublime for camphor and sand, density again for the centrifuge. Asking “what is different about these two substances?” before choosing a method is the skill this chapter is teaching, and it is what examination questions actually test.
Activity 5.1
Divide the class into three groups. Group A adds one spatula of common salt to 50 mL of water in a beaker and stirs; Group B does the same with chalk powder; Group C with a few drops of milk. Label them A, B and C. Compare their appearance, shine a torch through each, leave them to stand, and finally filter each one.
What you should observe. Three mixtures that look superficially similar behave differently on every test.
| A — salt | B — chalk | C — milk | |
|---|---|---|---|
| Appearance | transparent | opaque | translucent |
| Light beam through it | not visible | visible | visible |
| On standing | stays mixed | settles | stays mixed |
| On filtering | passes through | residue on the paper | passes through |
| Type | solution | suspension | colloid |
Why the filtration step is the one that separates B from C. Milk and chalk-water both scatter light and both look cloudy, so the first two tests cannot tell them apart. Only filtering does: chalk particles are large enough to be caught by the paper, milk’s are not. That is why the chapter puts filtration last and then says “these are indeed different types of mixtures”.
Why it is set as a group activity. The three have to be compared side by side. Done one after another by the same group, the differences in transparency are much harder to judge — a beaker of milk-water looks cloudy in isolation but obviously different next to salt solution.
What goes wrong. Too much milk. A few drops in 50 mL gives a translucent colloid; a splash gives something as opaque as the chalk, and the middle column of the table collapses.
Activity 5.2
Consider water as the solvent and compounds A and B as solutes. A graph of solubility against temperature is a solubility curve. Study Fig. 5.6 and answer: which compound is more soluble at 20 °C? Which deposits more solid when a saturated solution is cooled from 80 °C to 60 °C?
What you should observe. The two questions have different answers, and that is the point.
The second question is decided by the steepness of the curve, not its height. Between 80 °C and 60 °C, compound B’s solubility falls by about 70 g per 100 g of water while A’s falls by only about 10 g. So B deposits far more solid on cooling — even where A is the more soluble one at the starting temperature.
Why the slope and not the height. The mass that comes out of solution is the difference in solubility between the two temperatures. A compound can be highly soluble and still deposit almost nothing if its curve is nearly flat, and a less soluble one with a steep curve will drop far more. Reading the graph for a difference rather than a value is the skill here.
What it sets up. This is the reasoning behind Activity 5.3. Crystallisation works precisely because solubility changes with temperature — and it works best on substances with steep curves.
What goes wrong. Reading off one temperature and comparing heights. That answers the first question and not the second, and the two get conflated constantly.
Activity 5.3
Collect copper sulfate (blue vitriol) — common salt will do if it is unavailable. Prepare a hot saturated solution, filter it, and leave the filtrate to cool slowly and undisturbed. The book flags that copper sulfate is toxic and the experiment needs adult supervision.
What you should observe. Well-formed blue crystals grow as the solution cools.
Crystals of copper sulfate form as the solution cools, because a saturated solution at a high temperature holds more solute than it can hold when cool — the excess comes out as crystals. This is crystallisation.
Why cool slowly. The chapter sets this as a hypothesis to test: cool a hot saturated solution rapidly in ice-water and you get small, poorly formed crystals; cool it slowly at room temperature and you get large, well-formed ones. Slow cooling gives the ions time to find their places in the growing lattice.
Why it purifies as well as separates. Impurities are usually present in much smaller amounts and stay in the solution, which remains unsaturated in them. The crystals that grow are therefore purer than the solution they came from — which the chapter gives as the reason newly prepared compounds are crystallised.
What goes wrong. Disturbing the beaker. Every knock starts new crystals growing, so you end up with a powder of tiny ones rather than a few large ones. Put it somewhere it will not be moved, and do not stir. The book’s alternative — evaporating a couple of millilitres on a glass plate — is quicker but gives worse crystals, and it asks you to say why.
Activity 5.4
Look at Fig. 5.9, which shows how salt crystals are obtained from seawater, and describe the process in your own words.
Seawater is let into shallow pans; the Sun’s heat evaporates the water, leaving a saturated solution; as evaporation continues the salt can no longer stay dissolved and crystallises out, and the crystals are raked up and dried.
Why evaporation here and cooling in Activity 5.3. Two different routes to the same end. Cooling reduces how much solute the solvent can hold; evaporation reduces how much solvent there is. Either way the solution passes its saturation point and the excess comes out as solid. Sea salt uses evaporation because sunshine is free and cooling seawater is not practical.
What goes wrong. Describing it as “boiling the water off”. Nothing is boiled — it is sunlight and wind over a large shallow area, which is why salt pans are laid out the way they are.
Activity 5.5
Take a 3 cm wide strip of chromatographic or filter paper and draw a pencil line 2 cm from the bottom. Put a small drop of black ink on the line and let it dry. Hang the strip so the bottom dips into water in a beaker, with the ink spot above the water level. Leave it and watch the water rise.
What you should observe. The single black spot separates into several coloured bands up the paper.
Black ink separates into several coloured components, each carried a different distance up the paper. This is chromatography, and it works because the dyes have different solubilities in the moving solvent and stick to the paper to different extents.
Why the line is drawn in pencil. Pencil is graphite — it does not dissolve and will not run. A pen line would separate along with the sample and ruin the result. This is the detail most often skipped, and it is in the instructions for a reason.
Why the spot must sit above the water. If the ink is below the surface it simply dissolves into the beaker, and nothing travels up the paper at all. The water has to climb to the spot and then carry it.
What goes wrong. A spot that is too large, which gives broad overlapping bands instead of distinct ones. Use a fine point, and build it up by letting each application dry before adding the next.
Activity 5.6
Pour a mixture of 5 mL of mustard oil and 20 mL of water into a 50 mL separating funnel. Let it stand undisturbed. Then open the stopcock and run off the lower layer.
What you should observe. Two clean layers, with the oil on top.
Two separate layers — mustard oil above, water below — because the two are immiscible and the oil is less dense. Opening the stopcock runs the water off first; close it the moment the boundary reaches the tap, and the oil is left behind.
Why this method and not filtration. Both components are liquids and both pass straight through filter paper. What differs is density and the fact that they refuse to mix, and the separating funnel is built to exploit exactly that.
What goes wrong. Closing the tap too late, so the first of the oil runs into the water. Watch the boundary rather than the flow, and slow the tap right down as it approaches. Shaking the funnel first is also a mistake here — it makes an emulsion that takes far longer to settle.
Activity 5.7
Put a spatula full of crushed camphor mixed with sand into a clean dry china dish on a wire gauze on a tripod. Plug the nozzle of a clean dry glass funnel with cotton, invert it over the dish, and heat gently for a few minutes. Then look at the inner wall of the funnel.
What you should observe. White solid appears on the inside of the funnel, well away from the heat.
White solid camphor deposits on the inner wall of the funnel, while the sand remains in the china dish. Camphor changes directly from solid to vapour without melting — sublimation — and then deposits as a solid again on the cool glass.
Why the cotton plug. It stops the vapour escaping through the nozzle. Without it much of the camphor is lost to the room and the deposit is thin — and the smell fills the laboratory.
Why heat gently. The chapter specifies heating below camphor’s melting point. Too fierce a flame melts the camphor instead of subliming it, and a liquid mixed into the sand cannot be separated this way at all.
What goes wrong. A damp funnel. Any moisture condenses on the inside and the camphor deposit is mixed with water, which makes it hard to see and impossible to collect cleanly. “Clean and dry” appears twice in the instructions.
Activity 5.8
Make your own centrifuge from a cardboard disc and thick thread, and watch how the heavier particles move outwards.
When the disc is spun fast, the denser particles move outwards and collect at the far edge, while the lighter liquid stays nearer the centre. That is what a centrifuge does — it separates by density, much faster than letting a suspension settle under gravity alone.
Why spinning beats waiting. A suspension will separate on its own eventually; spinning produces a much larger effective force than gravity, so what would take hours takes seconds. Nothing new is happening chemically — the separation is simply hurried along.
Why the chapter puts a paperfuge beside it. The box next to this activity describes a hand-powered device built from a button-and-string toy that spins blood samples fast enough to detect malaria and anaemia without electricity. It is worth showing alongside, because it makes the point that the physics of the toy and the physics of the laboratory instrument are the same.
What goes wrong. Not enough speed, or a disc that wobbles. The string must be wound and pulled smartly, and the load must be balanced — an unbalanced disc wobbles instead of spinning and separates nothing.
Activity 5.9
Complete Table 5.1, reviewing what you have learnt about solutions, suspensions and colloids.
| Property | Solution | Suspension | Colloid |
|---|---|---|---|
| Nature | homogeneous | heterogeneous | heterogeneous |
| Particle size | smallest | largest | intermediate |
| Visible to the eye | no | yes | no |
| Settles on standing | no | yes | no |
| Separated by filtration | no | yes | no |
| Scatters a beam of light | no | yes | yes |
| Example | salt in water | chalk in water | milk |
The two rows that do the work. Filtration separates the suspension from the other two; light scattering separates the solution from the other two. Between them, any mixture can be placed — which is exactly the sequence of tests Activity 5.1 ran at the start of the chapter.
Why the scattering row matters beyond this table. The chapter states it plainly: “scattering of light occurs when it passes through a colloid or a suspension but not when it passes through a transparent solution”. That is the Tyndall effect, and it is why a torch beam is visible in milk-water and invisible in salt solution — and why sunbeams show up through dusty air or in stadium floodlights.
What goes wrong. Writing “yes” for a colloid settling. It does not — the particles are too small to fall out under gravity, which is why milk keeps indefinitely without separating and why a colloid needs a centrifuge rather than patience.
Common mistakes
- Deciding Activity 5.1’s three mixtures on appearance alone. Chalk-water and milk-water both look cloudy. Only filtration tells them apart, which is why the chapter ends the activity with it.
- Answering Activity 5.2 from the height of the curve. What deposits on cooling is the change in solubility between two temperatures — the slope, not the value.
- Stirring or moving the beaker in Activity 5.3. Every disturbance starts new crystals, so you get powder instead of the large well-formed crystals the activity exists to grow.
- Drawing the chromatography line in pen. It runs, and its dyes separate along with the sample. Pencil, every time.
- Putting the ink spot below the water line in Activity 5.5. The ink dissolves into the beaker and nothing climbs the paper.
- Heating too hard in Activity 5.7. Camphor melts instead of subliming, and a liquid in the sand cannot be separated by this method at all.
- Saying a colloid settles on standing. It does not, which is exactly what distinguishes it from a suspension in Table 5.1.
Practise next
- The end-of-chapter questions, Revise, Reflect, Refine, where the separation techniques of Activities 5.5 to 5.8 are applied to unfamiliar mixtures and you have to choose the method yourself.
- The in-text Pause and Ponder questions of this chapter, ten across five boxes, including the solubility-curve reading that Activity 5.2 prepares for.
- Chapter 9, Atomic Foundations of Matter, which takes the pure substances left behind by these separations and asks what they are made of.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.