Class 6 Science Chapter 9: Methods of Separation in Everyday Life

A Courtyard Full of Grain

Malli and his sister Valli are off on their summer holidays, travelling across India to see relatives. Their first halt is their Nani's house in Haryana, ringed by large fields. In the courtyard, grain is piled high, and the family is busy picking small stones and husk out of it by hand.

What a Mixture Is

That pile is a mixture: two or more substances mixed together. Grain with stones in it, grain with husk in it, flour with bran in it. Every method in this chapter exists because somebody wanted the parts of a mixture apart again. Sometimes both parts are wanted; sometimes only one of them is.

The Simplest Method Has a Name

Picking small stones and husk out of wheat or rice with your fingers is called handpicking. Nani even turns it into a game, challenging the two of them to pick stones out of the grain with their eyes shut.

What It Works On

Handpicking works off differences in the size, the colour and the shape of the pieces. Your eye finds the odd one out, and your fingers do the rest. That is why shutting your eyes makes it so much harder. Take away the looking and you are left guessing by touch alone.

When to Choose It

It is the right choice where only a little has to come out and fingers can get hold of it easily. It is a poor choice for a heap the size of a courtyard, as Valli soon works out.

You Do It Yourself

At lunch, Malli is served steaming vegetable pulao and starts setting the whole black peppers aside on his plate. Valli teases him for it. That is handpicking, and he was doing it without thinking.

Out in the Fields

Mama walks them out to see the harvest: cut wheat stalks tied in bundles on the ground, and a few laid out in the sun to dry. Pick up a stalk and you find grains attached all along it. The grain has to come off before anything else can happen.

How the Grain Comes Off

Over by a heavy wooden log, several farmers are striking bundles of stalks against it. That is what Valli asks about, and Mama explains that the beating is what knocks the grains loose. Freeing grain from stalks in this way goes by the name threshing.

And the Job Is Not Over

The separated grains come away mixed with piles of husk. Valli asks the obvious question. Will the farmers handpick the grain out of all that husk? How long would that possibly take? A courtyard of grain would keep a family busy for days.

Try It With Peanuts

Here is the answer in miniature. Pick up a handful of roasted peanuts and work them between your palms until the skins come away. Now blow gently. The skins go and the peanuts stay where they are.

The Difference That Does the Work

Blown air parts the heavier component from the lighter one. Neither one has been picked out by anybody. The moving air simply carries the light one further than the heavy one.

The Same Thing, Field-Sized

Nana takes them out to watch it done properly. A farmer stands on a raised platform, holding a soop, the bamboo tray, full of threshed wheat. He moves the tray in the direction the wind is blowing, and the two parts land in different places.

The Name for It

Parting the heavy half of a mixture from the light half, using wind or air you blow yourself, goes by the name winnowing. The light husk is carried off to one side, and the heavy grain falls in a heap close by the farmer.

Why the Platform Matters

Standing high up gives the falling mixture a long way to travel in the moving air, and the longer it falls the further apart the two parts land. A tray held at knee height would sort the mixture far less cleanly. Valli, trying to do the same thing in a closed room, gets nowhere. No moving air, no winnowing.

One Machine for Both Jobs

Technology has caught up with the fields. Machines called threshers free the grain from the stalks and part it from the husk as well. They carry out the threshing and the winnowing at the same time, which is two of these three methods in one operation.

Scenario:

A class watches a video of grain being cleaned on a farm. Stones are picked out by hand, the stalks are beaten off, and the husk is blown away in the wind. Nita asks why three different methods are needed for one heap. Their teacher asks what each one separates. Stones differ from grain in size and can be seen, so fingers work. The grain is held on the stalk, so it must be struck loose. Husk differs from grain in weight, so moving air carries it off. The class sees that the method is chosen to match the difference, and that nobody picked three methods at random.

Analysis:

Three methods on one heap because there are three kinds of difference. Each tool answers the property that separates the parts.

Activity:

Mix rice with a few small stones and some husk or paper bits. Separate it using three methods and name the difference each one used.

Title:

Winnow a handful of peanuts

Safety:

Blow gently and across the plate, never down into it, so that nothing flies into your eyes. Ask before taking food from the kitchen.

What You Notice:

The skins lift off and drift sideways while the peanuts stay where they are, which is winnowing exactly. Holding the plate high gives a much cleaner result than holding it low, and that is the reason the farmer stands on a platform. If you blow too hard, some peanuts go too, so the strength of the air matters as well.

Threshing machines, called threshers, carry out the beating and the blowing at the same time.

Farmers often sing folk songs while they work at threshing, and every region has its own.

A soop is a bamboo tray, and it has been the winnowing tool in Indian fields for a very long time.

The three methods in order — Fingers, then a beating, then the breeze — Handpicking, threshing, winnowing, which is the order a field follows.

What winnowing needs — One light, one heavy, and air on the move — Take away any one of the three and it stops working.

A Request Before the Train

Malli and Valli are about to board a train to Ahmedabad. Before they leave, Valli asks Mami to make meethi puri for the journey and offers to help knead the flour. Mami says there is something to do first.

Something in the Flour

Bran may be present in the flour, and it has to come out before the dough is made. Valli asks how. The answer is a tool that is probably in your own kitchen: a sieve.

What a Sieve Does

Sieving lets the fine flour particles pass through the holes of the sieve. The bigger pieces, the bran and any small stones, are held back and stay on top. Nobody picks anything out; the mesh does the sorting.

The Difference It Uses

Sieving is used where the parts of a solid mixture have different sizes. That is the one property it works on, just as winnowing works only on weight. Colour and shape make no difference to a sieve.

Look at a Sieve Closely

Pick one up and study the mesh. Are all the holes the same size? They are, and they have to be, or the sorting would be uneven. Every hole is the same gate applied to every particle. A mesh with holes of two different sizes would let some coarse pieces through and hold some fine ones back.

The Question Worth Asking

Now ask what happens if the holes are bigger than everything in the mixture. The bran goes through as easily as the flour, and you finish with the mixture you started with. A sieve whose holes are too small is no better: nothing goes through at all.

So the Holes Decide

The holes have to fall between the two sizes, larger than the fine part and smaller than the coarse part. Choose the mesh to suit the mixture. At construction sites you can watch a much coarser sieve sorting pebbles and stones out of sand.

At the Sabarmati Ashram

In Ahmedabad the children visit the Sabarmati Ashram and learn about the Namak Satyagrah, the Dandi March. Malli asks the obvious question that follows: where does namak, common salt, actually come from?

Out of the Sea

From seawater. What the sea holds is water carrying salts, and other substances besides, dissolved in it. The salt is not floating about in lumps; it has dissolved, which is exactly why no sieve and no strainer will bring it back. There is nothing left of a size for a mesh to catch.

How It Is Collected

The seawater is run into shallow pits and left open to sunlight and air. Over a few days the water evaporates completely, leaving a solid mixture behind in the pit. Common salt is then obtained from that mixture by further purification.

Try It on Black Paper

You can watch the same thing happen in minutes. Half fill a bowl with water and stir in two or three teaspoons of salt until it dissolves. Now spread a few drops of the solution on a piece of thick dark paper, and let it dry. You can make a pattern with it if you like.

What the Patches Are

Patches show up on the paper, and you can feel the salt by touching them. The water has gone into the air and the salt could not follow it. If you have ever seen white patches on dark clothes in hot weather, that is the same thing after sweat has dried.

Boiling It Off Instead

There is a faster way, which a teacher may demonstrate. Put some of the salt solution in a china dish, stand it on a wire gauze over a spirit lamp, and let the water boil away. Then allow the dish to cool.

And the Salt Is Back

What is left in the china dish is the salt, and you can feel it with your fingers. Careful with the heating: a china dish over a flame gets very hot. One caution worth remembering is that this brings back the salt but not the water.

Where Else This Is Used

The method turns up in places you would not expect. In Ayurveda the practice is to shade-dry a medicinal plant's seeds, flowers, leaves or roots. The surplus water evaporates away and what matters in the medicine is left behind. Sambhar Lake in Rajasthan is one Indian water body that holds common salt.

Scenario:

A class sieves a mixture of flour and small pebbles and separates it cleanly. Then Imran tries the same sieve on a mixture of salt and sugar and nothing separates at all. He decides the sieve is faulty. Their teacher asks him to compare the grain sizes. Salt and sugar grains are close enough in size that both pass the same holes. The class works out what sieving actually needs: a difference in particle size big enough for one part to pass and the other to stay. Imran separates the salt and sugar another way instead, by dissolving, and sees that the method has to match the difference available.

Analysis:

The sieve was never faulty. It separates by size, and salt and sugar do not differ enough in size for it to grip.

Activity:

Sieve two mixtures, one with very different grain sizes and one with similar. Record both results and explain the difference.

Title:

Two mixtures, two different answers

Safety:

Use warm water from a tap, never boiling water, and ask before taking anything from the kitchen. Wash the sieve afterwards.

What You Notice:

The rava stays on top while the flour drops through, and the two are apart in seconds. The sieve catches nothing at all from the salt water, which is the whole point of the comparison. The dry patch feels gritty under a finger, so the salt was in the glass the entire time even though you could not see it.

Sambhar Lake in Rajasthan is one of the water bodies in India that holds common salt.

In Ayurveda, roots, leaves, flowers and seeds are dried in the shade so the excess water goes and the medicine stays.

At construction sites, sieves are used to sort pebbles and stones out of sand.

When a sieve will work — Holes between the two sizes — Bigger than the fine part, smaller than the coarse part.

Choosing between sieving and evaporation — Still solid? Sieve it. Dissolved? Boil it off — Once something has dissolved, a mesh has nothing to catch.

Tea Time in Puducherry

The children are now at their Dada and Dadi's house in Puducherry, catching up with their old neighbour Balan. Talking about old times, nobody notices the afternoon go, and it is suddenly Dadi's tea time. Dada offers to make it.

A Question While the Tea Brews

Balan asks how the tea leaves come out at the end. Obviously with a strainer, says Dada, but then he offers something more interesting. Even with no strainer in the house, most of the leaves can still be removed.

Leave It Alone First

The trick is to do nothing for a while. Leave the saucepan of tea undisturbed, and the leaves work their way down to the bottom on their own. Valli sees where this is going at once: the leaves will settle.

The Name for the Settling

That process has a name. A heavier insoluble part working its way down through a still liquid and gathering along its floor is called sedimentation. Insoluble is the key word, since anything dissolved would stay put and never sink at all. Sugar stirred into the same cup would go nowhere.

Then Pour It Off

With the leaves at the bottom, gently pour the tea into a cup. The liquid comes away and the settled leaves stay where they are. Removing the liquid by tilting the vessel in this way is called decantation.

But It Is Not Perfect

Dada admits the limit himself. He still finds a few tea leaves in his mouth, since this way of doing it never takes out every last one. Balan concludes that it cannot be called a proper separation, and Dada agrees with him.

Where Else You Have Seen It

You do this at home without naming it. Decantation is what happens every time the water is poured off rice or pulses after washing them. Oil and water, which form separate layers when left undisturbed, come apart the same way.

Now Use the Strainer

Malli fetches the tea strainer from the shelf, and Dada pours the tea through it. Every leaf is caught in the mesh. Taking the leaves out of the tea this way goes by the name filtration.

A Harder Test

Balan asks whether a tea strainer would filter muddy water. Try it and you find it will not. The mud particles are far smaller than the gaps in the strainer, and they go straight through with the water. The strainer was built for leaves, not for mud.

Try a Piece of Cloth

Dada suggests filtering the muddy water through cloth instead, and the difference is plain. Cloth is woven, and between its threads sit holes far too small to notice, which are called pores. Those pores act as a filter. People in ancient times did exactly this.

Finer Still

If the water is still muddy after that, the answer is a filter with even smaller pores. Filter paper is one such filter, with very fine pores in it, and it catches what cloth lets through. Each step down in pore size catches a smaller particle than the last one did.

Setting It Up

Fold a circle of filter paper in half, then in half again, and open it out into a cone. Sit the cone in a funnel, stand the funnel over a conical flask, and pour the muddy water in. A tripod stand holds the whole arrangement steady while the water works its way down.

Two Names for Two Halves

Watch what happens. The mud particles do not pass through, and you are left with mud on the paper and clear water below. The solid held back is called the residue. The liquid collected in the flask is called the filtrate.

Filters Everywhere

On a boat ride with Dada and his friend Otukkam, a fisherman, Valli watches the net come up. Water drains out through the mesh while the fish stay in, and she realises it is filtration again. What comes up with the fish is less welcome: plastic bags, broken bottles, food wrappers, and a large fish with a straw caught in its mouth.

Choosing the Right One

Many materials besides filter paper will serve: cotton, charcoal and sand among them. Which one you reach for turns on how big the particles are that have to come out. Tea bags make the point neatly. They began as silk, moved to gauze or muslin, and are mostly filter paper today.

Scenario:

A class is given muddy water and asked to clear it. One group pours it straight through filter paper and the paper clogs almost at once. A second group lets the jar stand for twenty minutes first, pours off the clear upper liquid, and then filters only that. The second group finishes far sooner with a cleaner result. Their teacher asks the class to explain the difference in time. Most of the mud had already settled to the bottom in the second jar, so the paper had far less to hold back. Sana writes the order down: let it settle, pour it off, then filter what is left.

Analysis:

Both groups used filtering and only one used it well. Settling first removes the bulk, so the paper is asked to do the small part of the job.

Activity:

Make muddy water and clear it twice, once by filtering straight away and once after letting it settle. Time both and compare.

Title:

Clear a glass of muddy water three ways

Safety:

Use garden soil, not anything from a drain, and do not drink any of the water at any stage. Wash your hands afterwards.

What You Notice:

Ten minutes of standing still does a surprising amount, and the layer at the bottom is clearly heavier material. Pouring carefully gets most of the water away, but some cloudiness always crosses over with it. The strainer changes almost nothing, which is the point: its gaps are far wider than the particles. The cloth leaves a brown mark on itself, and that mark is the residue.

Tea bags were first made of silk, then of gauze or muslin, and today mostly of filter paper.

A fishing net drains the water and keeps the fish, which is filtration on a river.

Cotton, charcoal and sand all serve as filters, and the choice depends on what has to be removed.

The two that go together — It settles, then you pour it off — Sedimentation first, decantation second.

Naming the two halves of a filtration — Residue stays, filtrate runs — One is held on the paper, one drips into the flask.

A Painting in a Dhaba

The train brings Malli and Valli to Bhopal, and the sun is climbing. On the way to their Maasi's house they stop at a dhaba for chhach, buttermilk. Malli notices a large painting on the wall and asks about it. It shows something being made rather than a place or a person.

What the Painting Shows

The shopkeeper explains. The picture shows a woman working curd round and round with a big mathni, a churner, to get the butter out of it. That process is called churning.

Where the Butter Goes

Here is what makes it work. Butter is the lighter part of the two, so as it gathers together it floats up to the top of the vessel. The buttermilk is left behind below it, which is exactly what the children are drinking. So the glass in Malli's hand is one half of a separation, and the butter on the table is the other.

One Mixture, Two Useful Parts

Notice that nothing here is being thrown away. The butter is wanted and the buttermilk is wanted too. Some separations remove a nuisance; this one divides a mixture into two things a kitchen will use.

It Has a Modern Form Too

You may have a churner at home that plugs into the wall. An electric mixer set to the right speed does the same job to the same curd, working it round until the butter gathers. The idea has not changed; only the arm turning it has.

Onward to Shillong

After a happy stay in Madhya Pradesh the two of them travel on to Shillong, the capital of Meghalaya, and their Bua's house. A carpenter is at work there, making a wooden door, and the floor around him is deep in sawdust.

A Small Accident

While working he knocks over a few iron nails, and they disappear into the sawdust. He kneels down and starts picking them out by hand, one at a time, which is slow and not very successful. Sawdust and nails look much the same colour once they are mixed, and the nails keep sinking out of sight.

The Children Have a Better Idea

They tell him to hold on, fetch a magnet from their Bua, and ask him to move it through the sawdust. Every nail jumps to it. The sawdust is left exactly where it was.

Why That Worked

It works because of a property the children already knew from another chapter. Anything a magnet draws towards itself is called a magnetic substance, and iron is the common example of one. Wood is not magnetic at all, so the sawdust ignores the magnet completely.

The Name for the Method

Using a magnet to part the magnetic things in a mixture from the non-magnetic ones goes by the name magnetic separation. The nails are magnetic and the sawdust is not, so one comes away and the other does not.

What It Does Not Do

This method is powerful but very narrow. It acts on one property alone. Try a magnet on a mixture of sand and sugar, or on tea leaves in tea, and it will come out of the mixture carrying nothing whatever. Not one of the nine other methods in this chapter is fussier about what it will accept.

So It Is a Test as Well

That narrowness is useful in itself. Run a magnet through an unknown heap and whatever follows it is magnetic. A carpenter who wants his nails back learns something about the nails at the same time.

The Same Idea, Industrial Scale

In the recycling yards, magnets are what pull iron articles out of a waste heap. In many industries the waste material contains scrap iron, and that scrap comes out of the heap using magnets fitted to a crane. A crane can sweep a whole yard in the time a person would take to sort one barrow-load.

And Then It Is Used Again

The scrap iron collected this way can be recycled and reused, which is the whole point of pulling it out. A separation method here is not tidying up; it is recovering something valuable from a heap nobody else wanted.

Nine Methods, One Journey

That completes the tour. Malli and Valli have watched handpicking, threshing, winnowing, sieving, evaporation, sedimentation, decantation, filtration, churning and magnetic separation, in kitchens, fields, workshops and on a river. The next topic asks how you choose between them.

Scenario:

A class visits a dairy and watches curd being churned in a mathni. Butter gathers at the top and is lifted off. Arjun asks why the butter rises instead of sinking. Their teacher asks what else the class knows that floats. Oil on water, and wood. All of them are lighter than what surrounds them. The class then asks why churning is needed at all, since the butter is already in the curd. The answer is that it is spread through it in tiny amounts, and churning brings those together until they are large enough to gather. Nita sees that the method does not create butter, it collects it.

Analysis:

The butter was in the curd all along. Churning does not make it, it gathers scattered fat until it is big enough to float clear.

Activity:

Ask at home how butter or ghee is made. Write the steps in order and mark the one that separates rather than changes anything.

Title:

Fish the iron out of a mixture

Safety:

Pins are sharp, so count them in and count them out, and keep the magnet away from phones, cards and anything with a screen.

What You Notice:

The pins come away almost at once and hardly any sawdust travels with them, so the method is both fast and clean. Most people underestimate how quick it is. The second test is the interesting one: the magnet comes out of the sand and sugar with absolutely nothing on it, which tells you the method needs a magnetic part to work on.

Recyclers use magnets to pull iron articles out of heaps of waste, and some of those magnets are fitted to cranes.

Scrap iron pulled out of waste in this way can be recycled and used again.

A big wooden churner used for curd is called a mathni.

What churning gives you — Butter up, buttermilk down — The lighter part rises and the rest stays below.

When a magnet will help — No iron, no use — The method works only if one part is attracted to a magnet.

Nine Methods, One Question

Malli and Valli have watched nine methods on their journey through India. The next question is the useful one. Handed an unfamiliar mixture, how do you decide which of the nine to reach for? Nothing on the packet tells you, and there is no list to look it up in.

Ask What Differs

Every method acts on one property. So look at the mixture and ask which property tells its parts apart. Size points to a sieve, weight to winnowing, attraction to a magnet, and being dissolved or not points to evaporation.

Then Ask How Much

The property is only half of it. Quantity decides too. A few stones in a bowl of rice can be picked out by hand. The same stones spread through a courtyard heap need something that works on everything at once.

Why Are You Separating at All?

There is a second question worth asking, and it splits every job in this chapter into two kinds. Sometimes you want two different but useful parts. Sometimes you only want to remove one part that is no use to you.

Two Useful Parts

Churning is the clearest case of the first kind. Butter and buttermilk are both wanted, and one mixture has given a kitchen two things. Salt from seawater is another, and so is grain and husk, since husk is used as fodder.

Removing the Nuisance

The second kind is more familiar. Stones out of rice, bran out of flour, mud out of water, nails out of sawdust. Here one part is the point and the other is simply in the way. The method may be the same either way, but the reason for reaching for it is not.

Sorting the Nine

The nine fall into groups, which makes them easier to hold in mind. Handpicking, sieving and winnowing separate a solid from a solid, using shape and colour, then size, then weight.

The Liquid Group

Sedimentation, decantation and filtration all take an insoluble solid out of a liquid. They differ in how thoroughly and how quickly. Evaporation stands apart, because it is the only one that reaches a solid which has dissolved.

And the Two Specialists

Churning uses weight, but inside a liquid rather than in moving air. Magnetic separation uses attraction to a magnet, a property nothing else in the chapter touches at all. Between them the nine cover a surprising amount of a kitchen, a field and a workshop.

When One Method Is Not Enough

Now take a harder case. You are handed potatoes, salt and sawdust mixed together. No single method will do, because three components mean at least two separations, and probably three. So the first thing to produce is not an answer but a plan.

Plan the Order First

Pick out the potatoes by hand, since they are large and few. Add water and stir: the salt dissolves, the sawdust does not. Filter, and the sawdust stays on the paper as residue. Evaporate the filtrate and the salt comes back.

Why That Order and Not Another

Take out what is easy first, so that less is left to handle. Leave the dissolved part until last, because evaporation destroys nothing but takes the longest. Boil the water off too early and you are left with salt, sawdust and potatoes in one dry heap.

Filters You Did Not Choose

Some separations are working on you already. The hair inside your nose traps dust from the air you breathe in. The masks everybody wore during the COVID-19 pandemic were fabric filters, holding back droplets and particles.

A Net That Cannot Choose

On the boat ride, Valli saw the limit of a filter. A fishing net keeps whatever is bigger than its mesh. Up came plastic bags, broken bottles, food wrappers and a fish with a straw caught in its mouth, along with the catch. A filter has no opinion about what it stops.

One Last Everyday Example

Sludge cleared from drains is left in heaps beside them for three or four days before it is carted to a garden or field. The water evaporates in that time, so the load grows lighter and cheaper to move, and safer for the people handling it.

Scenario:

Valli comes back from a nature walk with a jar of pond water. It is brown, it has bits of leaf and grit floating in it, and it smells faintly of the pond. Her class decides to build a water filter from low-cost materials and try it. Three suggestions are made. Arundhati says one layer of muslin will be enough, since cloth was good enough for people in ancient times. Ibomcha wants sand and charcoal packed into a cut bottle, with cloth at the bottom. Ritu says whatever they build, the water should be clear at the end, and clear water is safe to drink. Their teacher writes one line on the board: what has your filter actually removed?

Analysis:

Three claims are on the table and only one of them is straightforwardly right. The other two are each wrong in an instructive way, one about what a single layer can catch and one about what clear means.

Activity:

Build the layered filter and pour one jar of pond water through it. Draw the filter in your notebook, label every layer, and write beside each layer what it caught. Then hold the collected water up to the light and write one sentence on what you still cannot tell about it.

Title:

Separate a three-part mixture

Safety:

Use warm water from a tap rather than boiling water, and do not taste anything at any stage. Wash your hands and the cloth afterwards.

What You Notice:

Most people plan the right order on the second attempt rather than the first. The beans come out in seconds. The sand sits on the cloth in a visible layer, which is your residue. The collected water looks like plain water, which is the interesting part, and only the dry saucer proves the salt was in it all along.

The hair inside your nose traps dust from the air you breathe in, which is a filter you were born with.

The masks worn during the COVID-19 pandemic were fabric filters, holding back droplets and particles.

Fishing nets bring up plastic bags, bottles and wrappers along with the fish, because a net cannot tell them apart.

How to pick a method — What differs, and how much of it? — The property chooses the method; the quantity chooses whether hands will do.

Order for a many-part mixture — Easy first, dissolved last — Take out the obvious pieces early and boil off the liquid at the end.