Exploring Mixtures and their Separation: Class 9 Science · Chapter 5

Look at three ordinary things from today. The salt on your plate began as sea water. The petrol in a scooter began as thick, black crude oil. A blood report from a health camp began with a few drops of blood spun in a small machine. Each time, somebody had to pull a mixture apart, and each time they chose a method that suited that mixture.

In Class 6 you sorted, sieved, filtered and evaporated. This chapter goes further. You will measure how strong a solution is, read curves that tell you how much solid can dissolve, and meet the methods that laboratories, factories and hospitals use: crystallization, distillation, chromatography, sublimation, centrifugation and coagulation. You will also learn why a torch beam shows up in fog but not in clean water.

5.1 How Can We Classify Mixtures?

Dissolve a few blue crystals of copper sulfate in a beaker of water and stir. The liquid becomes the same pale blue everywhere: at the top, at the bottom, near the glass and in the middle. Take a drop from anywhere and it holds the same share of copper sulfate. A mixture like this, with the same composition throughout, is a homogeneous mixture (समांगी मिश्रण).

Every solution is homogeneous, and it stays that way: a sealed bottle of soda water or a drip bottle of saline never separates into layers, however long it stands. Air is a homogeneous mixture of gases. So is a stainless-steel spoon, which is iron with chromium, nickel and a little carbon spread evenly through it.

Now stir a spoon of soil into water. You can see the grains swirl, and within minutes they begin to sink. A drop from the bottom of the glass holds far more soil than a drop from the top. Such a mixture, whose composition changes from place to place, is a heterogeneous mixture (विषमांगी मिश्रण). Oil floating on water, a bowl of poha with peanuts and smoke in the air are also heterogeneous.

The two words are only a first sort. Some mixtures look uniform to the eye and still behave differently from a true solution. The next activity compares three such mixtures side by side.

5.2 Solutions

A glass of nimbu pani is a solution with several parts. The sugar, salt and lemon juice are the solutes: the substances that dissolve. Water is the solvent: the substance they dissolve in. A solvent is usually the part present in the larger amount.

Solutions are not always liquids in water. Soda water has a gas (carbon dioxide) dissolved in water. Air is a gas-in-gas solution. Brass is a solid solution of zinc in copper. But knowing what is dissolved is only half the story. We also need to say how much.

5.2.1 Concentration of a solution

Many health workers teach a home recipe for oral rehydration solution (ORS): six level teaspoons of sugar and half a level teaspoon of salt in one litre of clean water. Double the salt and the drink can harm a child who is already weak. Halve the sugar and the body absorbs the water less well. Change the proportion and you have just a sweet, salty drink, not a medicine.

The proportion matters far beyond ORS. Water-supply staff add just enough chlorine to kill germs: too little leaves the water unsafe, and too much makes it smell and irritate. A dyer in Jaipur weighs the dye for every vat, because a weaker bath gives a paler cloth. A nurse checks the strength printed on an injection before using it. In each case we are talking about the concentration (सांद्रता) of a solution.

5.2.2 How do we express concentration?

Read the labels at home. A packet of namkeen lists its salt and fat "per 100 g". A hand sanitiser may say "ethyl alcohol 70 % v/v". A bottle of mouthwash or eye drops gives its medicine as a percentage too. Each label answers one question: how much solute is in every 100 units of the mixture? There are other ways to state concentration, which you will meet in higher classes. Here we use three kinds of percentage, shown in Fig. 5.2.

A. Mass by mass percentage (% m/m or % w/w)

When both solute and solution are easy to weigh, we compare masses. Mass by mass percentage is the number of grams of solute in each 100 g of solution. Remember that the mass of the solution is the mass of the solute plus the mass of the solvent.

Percent by mass is not only for solutions. A bag of NPK fertiliser, a tin of tooth powder and the "per 100 g" nutrition panel on a biscuit packet all use it, even though none of them is a solution.

B. Mass by volume percentage (% m/v or % w/v)

Pharmacists and laboratory staff rarely weigh a liquid: pouring it into a measuring flask is much faster. So for a solid dissolved in a liquid they often use mass by volume percentage, the grams of solute in each 100 mL of solution. Drips, injections and many syrups carry this label.

C. Volume by volume percentage (% v/v)

If the solute is itself a liquid, it is simplest to measure both parts in millilitres. Volume by volume percentage gives the millilitres of solute in each 100 mL of solution. Look for it on sanitisers, mouthwashes and the alcohol content of some tonics.

In shops and factories you will often see % w/w (weight by weight) instead of % m/m. In daily use, weight and mass are treated as the same thing here, so a label of 5 % w/w means exactly the same as 5 % m/m.

5.2.3 Solubility of substances

Keep adding sugar to a glass of water, stirring each time. At first every spoonful disappears. Sooner or later the crystals stay at the bottom however hard you stir. The water now holds as much sugar as it can at that temperature. It has become a saturated solution (संतृप्त विलयन). Before that point, while it could still take more, it was unsaturated.

The mass of solute that just saturates 100 g of a solvent is the solubility (विलेयता) of that solute. A value of solubility always needs a temperature beside it. Warm the glass of sugar water and the crystals at the bottom dissolve, because the solubility of most solids rises as the temperature rises. That is why sugar vanishes quickly in hot chai but lingers in iced tea.

Gases behave the other way round. A warm bottle of soda fizzes over when opened, because warm water holds less carbon dioxide. In summer, ponds hold less dissolved oxygen, and fish can be seen gulping at the surface. So: solids usually dissolve more in warmer water, gases dissolve less. Differences in solubility are one of the main tools for separating mixtures, as you will see next.

5.3 Methods of Separation of Homogeneous Mixtures

In a homogeneous mixture the parts are mixed right down to single particles. No sieve, filter paper or magnet can pick them out. To separate them we must use a property in which the parts differ: how solubility changes with temperature, how easily each part boils, or how strongly each part clings to paper.

5.3.1 Crystallization

Go back to compound P in Fig. 5.3. At 60 °C, 100 g of water can hold 130 g of P. Suppose we make exactly that saturated solution and let it cool to 40 °C. Now the water can hold only 80 g. The extra 50 g has nowhere to go, so it comes out of the solution as a solid. If the cooling is slow, that solid appears as neat, glittering crystals.

In a crystal, the particles sit in a neat arrangement that repeats again and again, like bricks in a well-built wall. That inner order shows on the outside as flat faces and straight edges. You have seen many: lumps of mishri hung on a thread, chunks of alum (fitkari) used after a shave, the six-armed shapes of snowflakes, and the grains that sparkle in a granite kitchen slab.

Growing crystals from a saturated solution in a laboratory is called crystallization (क्रिस्टलीकरण). It is the standard way to purify a solid. When a new compound is made, it usually comes with small amounts of unwanted substances. Dissolve the impure solid in a hot solvent and let it cool: the main substance is present in large amount, so it saturates and crystallizes. The impurities are present in tiny amounts, so they never reach saturation and stay dissolved. The method works because solubility depends on temperature.

Why not simply boil the solution dry? There are two reasons. First, some solids change or break down when heated strongly: sugar syrup boiled to dryness turns brown and then burns. Second, when every drop of solvent is driven off, the soluble impurities dry onto the solid as well. In crystallization the impurities stay behind in the leftover liquid, so the crystals are much purer.

5.3.2 Distillation

When salt water dries in a saucer, we get the salt but lose the water into the air. Sometimes the liquid is the part we want. A ship far out at sea needs drinking water from sea water. A laboratory may need to recover an expensive solvent. And when two liquids mix completely, such as alcohol and water, evaporation alone is useless: both liquids simply escape into the air.

The answer is to boil the liquid off and then catch it again. Distillation (आसवन) works in three stages: boil, cool, collect. Heat the mixture, and whichever liquid has the lower boiling point escapes first as vapour. Lead that vapour through a condenser, a glass tube inside a jacket of cold running water. Chilled, the vapour becomes liquid again and drips out as the distillate. Anything that never boiled, such as dissolved salt or a liquid with a higher boiling point, never leaves the heated flask.

Look at the thermometer in Fig. 5.8. Its bulb sits at the side arm, so it measures the temperature of the vapour leaving the flask. While methanol is distilling, the reading stays near 65 °C. When the methanol is used up, the reading climbs towards 100 °C, which tells us to stop or to change the collecting flask. Notice too that cooling water enters the condenser at its lower end and leaves at the upper end, so the jacket stays completely full of cold water.

Simple distillation separates two liquids well only if their boiling points differ by about 25 °C or more, like methanol (65 °C) and water (100 °C). If the gap is smaller, both liquids boil off together in large amounts, and the distillate is still a mixture. It also works for a liquid with a solid dissolved in it: distil sea water and pure water drips out while the salt stays in the flask.

5.3.3 Paper Chromatography

Have you ever spilled water on homework written in a green sketch pen? The ink runs across the wet page, and the edge of the smear may look yellow or blue instead of green. The "green" was really two dyes. The water dragged one of them farther than the other, and the mixture began to come apart on the paper. That accident is the idea behind paper chromatography.

This method of separating the dissolved parts of a mixture is paper chromatography (क्रोमैटोग्राफी). It depends on two pulls acting on each substance: the moving solvent carries it along, and the paper holds it back. Because each substance responds differently, the parts end up at different heights. It needs only a tiny sample, so it is ideal for colours: checking which dyes are in a food colour, separating the green and yellow pigments of a leaf, or comparing ink from a forged note with the ink in a suspect's pen.

Water is not always the right solvent. A permanent marker's ink does not dissolve in water at all, so its dot would stay on the line. Such inks and leaf pigments need spirit (alcohol) or acetone, sometimes blended together.

5.4 How Can We Separate the Components of Heterogeneous Mixtures?

In a heterogeneous mixture the parts stay as separate bits, drops or layers, even when they are too small to see. That makes separation easier in principle: we only need a property in which the bits differ, such as density, size, magnetism or how easily they turn into vapour.

5.4.1 Separation of two immiscible liquids

Pour a spoon of cooking oil into a glass of water and stir hard. For a moment the oil breaks into droplets. Within a minute the droplets rise and join into a layer on top. Liquids that do not mix like this are immiscible (अमिश्रणीय). Kerosene and water, and petrol and water, are other pairs. Their mixtures are heterogeneous: you can see where one liquid ends and the other begins.

The layers form in order of density. Most oils are less dense than water (about 0.9 g for every millilitre, against 1.0 g for water), so oil floats on top. To separate the layers we use a separating funnel: a pear-shaped glass vessel with a stopper at the top and a tap, called a stopcock, at the bottom. We let the lower, denser liquid run out through the tap and keep the upper one behind.

What about mixtures that contain a gas? The smell of leaking cooking gas reaches every corner of a kitchen within minutes, because gas particles race about and spread evenly among other gases. So air and other gas-only mixtures are homogeneous. Add solid specks or liquid drops to a gas, though, and the result is heterogeneous: the black exhaust of an old diesel truck, the spray from a perfume atomiser or the dust storms that sweep across Rajasthan in May.

Solids mixed with solids, such as rice with tiny stones or iron filings with sand, usually keep their separate grains. To pull them apart, look for something one solid does and the other does not: iron is pulled by a magnet, husk blows away in the wind, small grains fall through a sieve. Some solids have one more trick: they turn straight into vapour.

5.4.2 Sublimation

Mothballs (naphthalene balls) in a cupboard get smaller month by month, yet they never leave a puddle. Camphor tablets on a puja plate also slowly disappear. Such solids skip the liquid stage altogether and go straight from solid to gas: this is sublimation (ऊर्ध्वपातन). Cool that gas on a cold surface and it can skip the liquid stage on the way back too, forming solid directly; this is deposition.

Solid carbon dioxide, called dry ice, also sublimes. It is about −78 °C cold and leaves no wet mess, so it keeps frozen foods and some vaccines cold during transport, and makes stage "fog". Whenever one solid in a mixture sublimes and the other does not, sublimation can separate them.

Mixing metals: alloys

Metal workers have long used a trick. Two metals that will never mix as solid lumps blend completely once both are molten, much as sugar disappears into water. Pour the blend into a mould and it hardens into one uniform metal. Such a uniform blend of metals, sometimes with a non-metal such as carbon added, is called an alloy.

Pure copper is soft and pure iron rusts; blending fixes such weaknesses. Brass, used for taps, keys and musical instruments, is mostly copper with some zinc. Bronze, used for bells and the famous Chola Nataraja statues, is copper with some tin; the kansa (bell metal) thali is a tin-rich bronze. Kitchen stainless steel is iron with about 18 % chromium, about 8 % nickel and a trace of carbon. Physical methods such as filtration, distillation or sublimation cannot separate the metals in an alloy.

5.4.3 Suspensions

Read the label on a bottle of antacid or calamine lotion: "Shake well before use." The medicine is a powder that does not dissolve. Shaking spreads it through the liquid, but it slowly sinks again. A heterogeneous mixture like this, in which undissolved solid particles are spread through a liquid for a while, is a suspension (निलंबन).

Compared with dissolved particles, these grains are giants: you can see them with your eyes or a hand lens. Their size explains two more clues. They sink when the mixture stands, and filter paper catches them. Soil in water, chalk powder in water and the flour in Activity 5.1 are suspensions.

A bucket of pond water after a storm shows the problem. Overnight, sand and grit sink to the bottom (sedimentation), and the water above can be poured off gently (decantation). A muslin cloth catches a little more dirt. Even so, a faint brown haze can hang in the water for days: the finest clay particles slip through the gaps in the cloth and are too light to sink quickly. Two methods deal with such stubborn particles: spinning them out, or clumping them together.

A. Centrifugation

Sit on a fast merry-go-round and you feel pushed outwards. A washing machine's spin cycle uses the same effect: the clothes are pressed against the drum while the water is flung out through its holes. In a laboratory centrifuge, tubes of the mixture hang from a rotor that whirls round thousands of times a minute. As it speeds up, each tube swings outwards until it lies almost flat. Everything in the tube is now flung away from the centre, and the denser particles win: they pack into the end farthest from the centre, which is the bottom of the tube. Lighter liquid is left nearer the open end. Separating a mixture this way is centrifugation.

Dairies use centrifuges called cream separators: the lighter cream collects near the centre of the spinning bowl and is drawn off. Pathology laboratories spin blood samples to get the pale yellow plasma away from the cells, and factories use giant centrifuges to pull solids out of liquids.

B. Coagulation

In many Indian homes, a lump of alum (fitkari) is swirled in a bucket of cloudy water and the bucket is left to stand. A few hours later the dirt lies at the bottom and the water above is clear. Alum works by making the fine particles stick to one another. Hundreds of them join into fluffy clumps called flocs, heavy enough to sink. Once they have settled, the clear water is poured off or filtered. Any process that gathers tiny suspended particles into clumps is coagulation (स्कंदन); a substance added to do it, such as alum, is a coagulant.

Water-treatment plants use the same trick on a huge scale before they filter and disinfect the water. Coagulation happens in sweet shops too. To make chhena for rasgullas, the halwai adds a pinch of citric acid (nimbu sat), or some sour whey from the last batch, to boiling milk. The milk proteins clump into soft white curds, which are strained through muslin. And when you cut your finger, the blood near the cut coagulates, or clots, to seal it.

5.4.4 Colloids

Now look again at the diluted milk from Activity 5.1. It looked uniform, it never settled, and it passed straight through the filter paper, just like a solution. Yet the light beam showed up inside it, just like a suspension. Milk is neither. It belongs to a third group, the colloids (कोलॉइड). Whipped cream, mayonnaise, jelly, shaving foam, fog and, as this book treats it, blood are colloids too.

What really sorts solutions, colloids and suspensions is the size of the particles spread through them (Fig. 5.14). Sizes are measured in nanometres (nm); one nanometre is a millionth of a millimetre. In a solution the dissolved particles are smaller than about 1 nm. In a colloid they are about 1 to 1000 nm across. In a suspension they are larger than about 1000 nm. For comparison, a human hair is roughly 70 000 nm thick.

Leave a colloid for a week and it still will not settle. Its particles are so small that the water molecules constantly bumping into them from every side keep them on the move and spread out, just as in a solution. Only a strong push, such as a centrifuge or a coagulant, makes them come out.11Blood is a special case. Its proteins form a colloid in the plasma, but its red cells are about 7000 nm across, the size of suspension particles. Over many hours they do settle slowly, and doctors measure this settling rate (the ESR) in a common blood test.

5.5 Tyndall Effect

Recall your table from Activity 5.1. The beam vanished inside the sugar water but lit up the flour mixture and, surprisingly, the clear-looking diluted milk as well. What was going on?

We see a beam from the side only if something inside the liquid sends some of its light sideways, into our eyes. The particles of a colloid or a suspension are big enough to do this: light strikes them and spreads out in all directions. This spreading is called scattering. Dissolved particles in a solution are far too small to scatter light noticeably, so the beam passes through unseen. Physicists call this the Tyndall effect (टिंडल प्रभाव), honouring John Tyndall, an Irish scientist who investigated it in the 1860s.

You see the Tyndall effect often once you look for it. Car headlights on a foggy winter night seem to draw solid cones of light. A projector beam in a cinema shows up when there is dust or smoke in the hall. In a forest or an old temple, shafts of sunlight slant down through gaps where the air holds mist, dust or smoke. In each case tiny droplets or particles in the air are scattering light.

Every colloid has two partners. The dispersion medium is the continuous background substance, like the water in milk. The dispersed phase is what is scattered through it in tiny bits, like the fat droplets in milk. They play roles similar to a solvent and a solute, except that the scattered bits are far bigger than dissolved particles. Table 5.3 gives more examples.

Shake oil with water and the droplets soon join up and float. Yet milk and butter, which also contain both fat and water, stay smooth for days. They are emulsions: colloids in which one liquid is spread as tiny droplets through another liquid it would not normally mix with. If oil droplets sit in water, it is an oil-in-water emulsion, like milk. If water droplets sit in fat or oil, it is a water-in-oil emulsion, like butter. What keeps the droplets apart is an emulsifying agent, a substance that coats each droplet. Egg yolk does this in mayonnaise, natural proteins do it in milk, and soap does it in the wash.

Cod-liver oil and some vitamin syrups are sold as emulsions. A spoonful of tiny oil droplets in flavoured water is far easier to take than a spoonful of plain oil.

Separation beyond the laboratory

Every method in this chapter uses a difference in some property: size, density, solubility, boiling point or the ability to sublime. Some mixtures resist all of them. Straining the tea leaves out of masala chai is easy; getting the sugar, milk and water back separately is not.

Your body runs its own separation plant. The kidneys filter about 180 litres of fluid out of the blood each day, take back almost all of it along with the useful substances, and pass out roughly a litre and a half as urine. Cities run larger plants: a sewage-treatment works uses screens, settling tanks, coagulants and filters, and some Indian cities sell the cleaned water to power stations for cooling. The simplest separation of all happens at the kitchen dustbin. Keeping food scraps apart from packaging lets one become compost and the other be recycled. E-waste recyclers go further, recovering copper, gold and lithium from old phones. Every separation done well means less waste sent to landfills.