NCERT Solutions for Class 9 Science Chapter 5 Exploring Mixtures and Their Separation (PDF) – 2027

Because the two mixtures hold particles of very different size. Muddy water is a suspension; milk is a colloid.

Evaporation is a surface change that goes on at every temperature; boiling is a bulk change that happens only at one fixed temperature.

Because the air under the tree carries dust, smoke and tiny water droplets, and these particles scatter the sunlight sideways into your eye. This is the Tyndall effect.

Heterogeneous. Oil and water are immiscible — they refuse to mix, and however hard you stir they settle back into two separate layers with a visible boundary between them.

Yes — any mixture in which you can point to more than one part is heterogeneous.

• Solid in liquid: sand in water, chalk powder in water, muddy water, tea leaves in water.
• Liquid in liquid: mustard oil and water, kerosene and water.
• Solid in solid: iron filings and sulfur, sand and common salt, a handful of mixed dal.
• Solid or liquid in gas: smoke (solid particles in air), fog and clouds (water droplets in air), dust in air.

Only in beaker B. The three mixtures are salt in water (A), chalk powder in water (B) and a few drops of milk in water (C).

The path of the beam is invisible in A, brightly visible in B and visible in C.

• Beaker A (salt and water): no change at all. The solution stays clear and uniformly salty from top to bottom.
• Beaker B (chalk and water): the chalk settles as a white layer at the bottom, leaving clearer water above. This settling is called sedimentation.
• Beaker C (milk and water): no change. It stays evenly milky; nothing settles even after hours.

Only beaker B leaves a residue.

They are three different types of mixture. Put the four tests side by side and each beaker gives its own pattern of answers.

There is no single fixed proportion — a solution can be dilute or concentrated. But the proportion actually used can be stated exactly, and that number is called the concentration of the solution.

At Class 9 level it is written as a percentage in one of three ways:

The spray will fail in one of two ways.

• Too little pesticide: the concentration is below what the pest can withstand, so the insects survive and the crop is not protected. Repeated weak doses can even help resistant pests to multiply.
• Too much pesticide: the excess scorches the crop, builds up in the soil, is washed into ponds and rivers, and leaves residues on the grain that reach the people who eat it.

Yes — almost every prepared solution has a proportion that must be respected.

12 g of zinc oxide.

20 % v/v.

Measure 5 mL of glacial acetic acid and add water to it until the total volume of the solution is 100 mL. That gives 5 % v/v vinegar.

Safety first: add the acid slowly to the water (never water to the acid), under an adult’s supervision.

Compound ‘B’ — at every temperature on the graph its curve lies far above that of compound ‘A’.

Less than.

Less than.

The solubility of compound ‘B’ increases more than that of compound ‘A’ with an increase in the temperature.

Part of the dissolved solute will come out of the solution as pure solid crystals. This is the process of crystallization.

The water evaporates from the drop of saturated copper sulfate solution on the glass plate, and small blue crystals of copper sulfate are left behind, spread over the plate.

Yes, small crystals form as the water evaporates — but this is not a good way to experiment. The thin film dries too fast, so the crystals are tiny and badly shaped; all the impurities are left mixed with them; and the result depends on the room temperature and breeze, so it cannot be controlled or repeated. A better method is to filter the hot saturated solution and let it cool slowly and undisturbed (steps 1 – 6).

A controlled experiment in which only the rate of cooling is changed:

1. Prepare a hot saturated copper sulfate solution and filter it while hot.
2. Divide it into two equal parts in two identical beakers.
3. Cool beaker 1 quickly by standing it in ice-cold water; let beaker 2 cool slowly at room temperature. Cover both and do not disturb them.
4. After the same time, filter out the crystals from each beaker, rinse them with a little cold water and dry them.
5. Compare the crystals under a magnifying glass — their size and how well-shaped they are.

Expected result: beaker 1 gives many small, poorly formed crystals; beaker 2 gives fewer, larger, well-shaped crystals. This supports the hypothesis.

The solution of compound ‘B’ will deposit far more solid.

Step 1 — read the solubilities off Fig. 5.6 (in g per 100 g of water):

Step 2 — solid deposited per 100 g of water

Yes. Slow evaporation gives large crystals; fast evaporation gives many small ones.

Salt is obtained from seawater by evaporating the water, which leads to crystallization:

1. Seawater, a dilute solution of salt, is let into wide, shallow pans near the coast.
2. The sun and wind evaporate the water. The salt stays behind, so the solution becomes more and more concentrated until it is a saturated solution.
3. As more water evaporates, the water left can no longer hold all the salt, so the extra salt separates out as salt crystals. These are collected, washed and dried.

Natural crystals grow wherever a mineral-rich solution is left undisturbed for a very long time — in caves, in mines and inside the Earth’s crust.

Sample answer: On a school trip to Sohra (Cherrapunji) in Meghalaya we went into the Mawsmai Cave (Fig. 5.10a). Water dripping through the limestone roof carries dissolved calcium compounds. As each drop hangs at the roof and slowly loses water and carbon dioxide, the solution becomes saturated and a little solid is deposited. Over thousands of years these deposits build up into stalactites hanging down and stalagmites rising from the floor. Quartz (Fig. 5.10b), with its clear six-sided pointed crystals, is another beautiful example found in mines and rock cavities.

Use distillation instead of plain evaporation.

By distillation, provided their boiling points differ by at least about 25 °C.

Heat the mixture in a distillation flask. Acetone, the lower-boiling liquid, boils first at about 56 °C; the thermometer bulb at the mouth of the side arm holds steady near 56 °C while it distils over. Its vapour passes through the water condenser, is cooled and collected in the conical flask as pure acetone. Water, needing 100 °C, stays behind in the distillation flask.

No. Evaporation can give you back only one of the two — and even that one is not obtained pure.

• The liquid that evaporates escapes into the air and is lost; there is no arrangement to catch it. So one component is gone for good.
• Both liquids evaporate at the same time, because evaporation goes on at all temperatures. The liquid left behind therefore still contains some of the other liquid — it is not pure either.

The water creeps steadily upwards through the paper, well above the level in the jar, and when the wet front reaches the ink spot it begins to drag the ink up with it. The single black spot stretches into a streak, and colours start to appear at the edges.

That the “black” ink is not a single substance at all — it is a mixture of several coloured dyes, and each dye travels up the paper at its own speed.

• Because separate coloured spots appear, at least that many different components are present.
• The number of spots tells you how many components the ink contains.
• The spot that has travelled farthest is the component least held by the paper and most attracted to the water.

No. Water works only when the components of the mixture actually dissolve in water. In many cases a different solvent, or a mixture of solvents, must be used.

The mixture, cloudy at first from being poured, soon clears into two sharply separated layers — a yellow layer of mustard oil (5 mL) floating on a colourless layer of water (20 mL), with a flat, clearly visible boundary between them.

Because mustard oil is less dense than water, so an equal volume of it weighs less and it floats.

They will not form one layer above the other. With equal densities neither liquid floats or sinks, so one liquid stays as drops or blobs spread anywhere inside the other (the drops may join into larger blobs). There is no flat boundary between them, so a separating funnel cannot separate them.

Yes. Gases mix freely with one another, so a mixture of gases is almost always homogeneous — but if the other component is a solid or a liquid spread through the gas, the mixture is heterogeneous.

Yes — a layer of white solid camphor collects on the cool inner wall of the inverted funnel, while the sand stays behind in the china dish.

Because they do not dissolve in each other, and they have different densities.

Yes, they are different. Sublimation is solid → vapour directly; evaporation is liquid → vapour from the surface. The key difference is the state you start from and whether a liquid stage appears at all.

Yes — any mixture in which exactly one component sublimes on gentle heating.

Not at room temperature — but yes, if the metals are melted first. The homogeneous solid formed when such a molten mixture solidifies is called an alloy.

Start with the simplest method and add another only if the water is still not clear.

1. Sedimentation and decantation. Let the muddy water stand. The heavier mud particles settle at the bottom; pour off the clearer water carefully without disturbing the sediment.
2. Filtration. Pass it through filter paper or a clean cotton cloth. Particles larger than the pores are held back as residue.
3. Coagulation, then filtration. Add powdered alum (fitkari). It makes the fine particles clump into larger flocs that settle by gravity and can then be removed by decantation or filtration.
4. Centrifugation. Spin a sample at high speed; the centrifugal force drives even very fine particles to the bottom of the tube.

By centrifugation and/or coagulation — the two techniques the chapter introduces for exactly this situation.

Coagulation. Add a pinch of powdered alum, a white crystalline substance used in water purification. Alum makes the fine suspended particles stick together in clumps. These larger, heavier clumps settle rapidly under gravity — the process called sedimentation — and the clear water above can then be decanted or filtered off.

Centrifugation. Take the cloudy water in a tube and spin it at high speed. The tubes swing out to the horizontal, and the centrifugal force acting outwards on each particle is many times greater than its weight, so even the finest particles are driven to the bottom of the tube while clear liquid remains on top.

The same two-person spinning game is played all over India and has a different name in each language.

Sample answer: “In my language we call it ______. We hold each other’s crossed hands and spin, and the faster we spin the harder we feel ourselves being pulled outwards.” Ask your grandparents — they will almost certainly know a local name for it.

The paperfuge is one of a whole family of frugal medical devices: they use everyday materials and no electricity, cost very little, and can be carried to a village that has neither a laboratory nor a reliable power supply.

Mixture chosen: muddy water.

Pass a thick thread through two holes near the centre of a stiff cardboard disc and tie the ends into a loop. Tape two small, tightly capped tubes of muddy water on opposite sides of the disc, near its rim. Spin the disc by pulling the strings apart and letting them come together, again and again, for 2 – 3 minutes.

Result: the mud collects at the outer end of each tube, with clearer water above it — the heavier particles have moved outwards.

Yes — coagulation is at work in the kitchen, at the water works, and even in your own body.

Neither — blood is a colloid. It is not a solution, because its particles (blood cells and large protein particles) are far bigger than dissolved particles: they scatter light and can be separated by centrifugation. It is not like an ordinary suspension such as muddy water either: blood looks uniform, its cells cannot be seen with the naked eye, and its components do not settle out quickly as mud does.

Your blood group is decided by which marker proteins sit on the surface of your red blood cells. In the common ABO and Rh system there are eight groups: A+, A−, B+, B−, AB+, AB−, O+ and O−.

How to find out: a blood group test needs only a drop or two of blood. It is done free at almost every blood donation camp, and at any hospital or pathology laboratory. Some school health check-ups also record it. Write it down and keep it in your school diary — in an emergency it saves valuable minutes.

A colloid is a mixture in which particles of one substance, sized between 1 nm and 1000 nm, stay evenly dispersed through another substance without dissolving and without settling.

Colloids are everywhere in daily life — in the kitchen, in the bathroom shelf, in the sky and in your own body.

Yes — wherever a beam of light passes through air or a liquid that carries suspended particles, its path becomes visible. That is the Tyndall effect.

• A shaft of sunlight entering a dark room through a small hole or a gap in the curtain, picked out by the dust in the air.
• Sunlight streaming through the gaps between the leaves of a dense tree, or through mist in an early-morning forest.
• The beams of floodlights in a sports stadium (Fig. 5.24), and the cone of light from a cinema projector.
• Car headlights on a foggy night; the beam is visible far ahead because water droplets scatter the light.
• A laser or a torch shone through milk diluted with water, or through a beaker of dilute soap solution.

A cloud is a colloid — an aerosol, with tiny liquid water droplets or ice crystals as the dispersed phase and air as the dispersion medium.

Because the smoke and dust particles suspended in the city air scatter sunlight in all directions — the Tyndall effect on the scale of a whole city.

Table 5.1: Properties of different types of mixtures

Examples: solution — salt or sugar in water, vinegar, brass; suspension — chalk powder in water, muddy water, sand in water; colloid — milk, blood, smoke, ice cream, cloud.

Not completely — and certainly not by any one method. Lemonade (shikanji) is water with dissolved sugar, salt, lemon juice with its acids and pigments, and often bits of pulp and ice. You can get some components back, but not every one of them in pure form.

The correct option is (iv) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm.

The correct option is (iii) a and c.

Why (a) and (c) show it: both contain particles large enough to turn part of a light beam sideways, so the path of the beam becomes visible from the side.

Why (b) and (d) do not: they are true solutions. The dissolved particles are smaller than 1 nm, far too small to scatter light, so the beam passes through unseen. (The blue colour of copper sulfate solution does not matter — colour is not scattering.)

Table 5.2 completed

(i) All three ingredients are solids, so the appropriate method is the mass by mass percentage (% m/m).

(ii) “70 % copper by mass” means 70 g of copper in every 100 g of brass.

Yes, it forms a separate layer, and the oil floats on top. The two are separated with a separating funnel.

Density of the oil:

Compare: 0.91 g/cm³ < 1.00 g/cm³, and oil does not mix with water, so the oil forms the upper layer.

To separate them:

1. Pour the mixture into a separating funnel fixed on a stand and let it stand until two clear layers form.
2. Open the stopcock slowly and run the lower layer (water) into a flask. Close it when the boundary reaches the tap, and discard the small mixed portion.
3. Collect the upper layer (oil) in a separate, clean container.

The correct option is (iii) A is true, but R is false.

Assertion — true. A true solution does not show the Tyndall effect. Shine a laser through a salt or copper sulfate solution and the path of the beam cannot be seen from the side.

Reason — false, on two counts.

Table 5.3 completed

Separate them by simple distillation.

Procedure

1. Put the mixture in a distillation flask fitted with a thermometer and joined to a water condenser; place a conical flask as the receiver.
2. Heat gently. The thermometer steadies at about 60 °C while liquid A boils; its vapour is cooled in the condenser and collects as pure A in the conical flask.
3. When the temperature starts rising above 60 °C, change the receiver. Liquid B (boiling point 90 °C) remains in the distillation flask.

When to prefer each

• Evaporation — when the solute is a non-volatile solid, you do not need the solvent back, and purity does not matter much. Obtaining common salt from seawater in salt pans is the standard example; the sun does the work free of charge.
• Crystallization — when you need the solid pure and its solubility changes sharply with temperature. Purifying copper sulfate, or separating a compound from the impurities formed along with it, is done this way, and it gives well-shaped crystals as a bonus.
• Distillation — when the liquid is what you want, or when you must recover both components. Separating acetone from water, obtaining drinking water from salty water, and extracting the fragrance of flowers in the Deg-Bhapka method of Kannauj all need distillation.

(i) If blood behaved like a true suspension, its cells would be large enough for gravity to pull them down — and that would be fatal.

• The cells would settle. Wherever the blood slowed down, red cells would sink to the lower side of the vessel, leaving almost cell-free plasma above. The blood would no longer be uniform.
• Oxygen supply would fail. Red blood cells carry oxygen to every tissue. If they settled out, the blood reaching the brain and the muscles would carry very little oxygen.
• Capillaries would block. Settled and clumped cells would jam the finest capillaries, cutting off the supply of nutrients and the removal of wastes from those tissues.
• The heart would be strained as it tried to push a mixture that keeps separating out, and clots could form.

(ii) In a blood sample:

The correct sequence is 1 → 3 → 2, that is sublimation → filtration → evaporation (crystallization).

Because water and acetone are miscible — so no separating funnel can work — but their boiling points are far apart, which is exactly what distillation needs.

On heating, acetone reaches its boiling point long before the water does. The thermometer holds steady near 56 °C while acetone vapour passes into the condenser, is cooled by the circulating water and collects in the receiver as pure acetone. The water, which needs 100 °C, stays behind in the distillation flask. Both liquids are recovered.

(i) 31 g of potassium nitrate.

(ii) She would see crystals of potassium chloride separating out as the solution cools.

(iii) For all four salts the solubility increases as the temperature increases — but by very different amounts (for sodium chloride, only very slightly).

All values are in grams per 100 g of water. The order of increase is potassium nitrate > ammonium chloride > potassium chloride > sodium chloride.

(i) Remember that the denominator is the mass of the solution, that is sugar + water — not the mass of water alone.

(ii) Student C’s solution is the most concentrated, at about 27.3 % m/m.

(i) The technique marked S is distillation (simple distillation).

(ii) Labels

(iii) Distillation will separate (a) water — acetone and (b) water — salt.

What a good demonstration must contain: at least one true solution as a control, several colloids, and the beam observed from the side at right angles each time — so that the difference is a comparison and not just a single observation.

Sample plan

1. Take five identical glass tumblers. Fill them with: (1) plain water, (2) a clear sugar or salt solution, (3) water with two drops of milk, (4) water with a few drops of soap solution shaken with a drop of oil, (5) very dilute starch paste in water.
2. Darken the room. Shine a laser pointer or a narrow torch beam through each tumbler in turn, and look at the beam from the side, at right angles to it.
3. Record for each: is the path of the beam visible? Is it bright or faint?

Method. For each compound, dissolve it in warm water until no more will dissolve — a saturated solution. Filter it while warm, pour it into a clean, undisturbed container, cover it, and let it cool slowly. Leave it for a day or two, then lift out the crystals, rinse them with a little cold water and dry them on a watch glass. Examine each under a magnifying glass and sketch what you see.

Expected answer: yes, most red leaves also contain chlorophyll — the red pigment simply masks it. Paper chromatography can reveal the green.

What you should see: the single spot separates into several bands — usually a red or purple band, a green band of chlorophyll and often a yellow-orange band of carotenoid. The green band proves that the red leaf does contain chlorophyll.

Method. Dissolve a little of the food colour, or soak the coloured coating off a few sugar-coated fennel seeds, in a few drops of water. Spot the coloured liquid on the pencil line of a chromatography strip, dry it, and stand the strip in water (or in 2 % m/v salt solution) with the solvent level below the spot. Let the solvent rise, then dry the strip.

Sample answer — “Separation Snap” (a card game):

• Mixture cards: sand + salt, oil + water, camphor + sand, muddy water, blood, black ink, seawater, acetone + water, brass.
• Technique cards: filtration, sublimation, separating funnel, centrifugation, coagulation, chromatography, distillation, evaporation, crystallization, “cannot be separated by physical methods”.
• Rules: a player wins a mixture card by laying the correct technique card and naming the property used (particle size, solubility, density, boiling point, sublimation). Some mixtures need techniques in the right order; brass is a trap card. Every fifth turn, the group actually performs a separation at the bench.

Sample answer — a pot-and-lid still: Half fill a large pot with the dirty water and stand a small empty cup inside it on a stone, above the water level. Cover the pot with its lid turned upside down, so that the lowest point of the lid is right above the cup, and keep the lid cool with a wet cloth or cold stones. Heat the pot on the camp fire. Steam rises, condenses on the cool lid, runs to its lowest point and drips into the cup as clean water, while the mud and salt stay in the pot.

Not yet — but scientists are working on it, and the difficulty is exactly the one this chapter is about: blood is a colloid with several different jobs, not a single substance you can bottle.