Methods of Separation in Everyday Life

Why This Matters

Look around your kitchen. Things are mixed together all the time.

Rice often has tiny stones hidden in it. Flour has bits of hard skin called bran. Tea has tea leaves floating in it. Sea water has salt mixed in it.

But you do not want to eat a stone with your rice. You do not want tea leaves in your cup. So somebody has to take the unwanted bit out.

That is what this chapter is about. It is the art of separating things that are mixed together.

You already do this without thinking. When you pick the curry leaves out of your dal before eating, you are separating. When your mother pours tea through a small strainer, she is separating. Farmers do it on a huge scale to clean grain.

Once you learn the few simple methods in this chapter, you will understand the why behind all of it. And you will never look at a kitchen sieve the same way again.

The Big Idea

When two or more things are mixed together, we can pull them apart only if they are different in some way. One may be bigger, or heavier, or it may dissolve in water while the other does not. Every method of separation works by using one of these differences. So the trick is always the same: first find what is different between the things, then choose the method that uses that difference. No difference means no way to separate. A difference is your tool.

Let’s Break It Down

Why do we separate things at all?

We separate things for three simple reasons.

One — to throw away the useless or harmful bits. Stones in rice are useless and can crack your tooth. So we remove them. This makes the food fit to eat.

Two — to keep the part we actually want. When we make tea, we want the tasty liquid, not the soggy tea leaves. So we keep the liquid and throw the leaves away.

Three — to separate two useful things. Sometimes both parts are useful. When milk is churned, we get butter and buttermilk. We use both. So here we are not throwing anything away. We are just splitting one mixture into two useful things.

A mixture is just two or more things put together, where each thing keeps its own nature. Rice mixed with stones is a mixture. The rice is still rice and the stone is still a stone. They are only sitting together.

Handpicking — using your hands

The simplest method of all is to just pick the unwanted bits out with your fingers. This is called handpicking.

You handpick when the unwanted bits are few in number and easy to see and grab. Picking small stones, husk, or insects out of a bowl of rice or dal is handpicking. Taking the whole black peppercorns out of your pulao is also handpicking.

It works because the bits you want to remove are different in size, colour, or shape. A grey stone stands out among white rice. A black peppercorn stands out in yellow pulao. You spot the difference, then your fingers do the rest.

But handpicking is slow. Imagine a farmer with a huge pile of grain mixed with husk. He cannot pick out every tiny piece of husk by hand. It would take days. So for big jobs, we need faster, cleverer methods.

Threshing — knocking the grain off the stalks

When a crop like wheat or rice is cut, the grain does not come off on its own. It is still stuck to the dry stalks of the plant. To free it, farmers beat or hit the harvested stalks. This knocks the grain loose from the stalks. Separating the grain from the stalks in this way is called threshing.

Farmers do threshing in a few ways. They may beat the stalks against a hard surface by hand. They may spread the stalks out and let bullocks walk over them. Or they may use a machine that does the beating for them. However it is done, the goal is the same — shake the grain free from the stalks.

But even after threshing, the grain is not clean yet. It is still mixed with light husk and bits of dry stalk. So threshing is followed by winnowing, which blows those light bits away.

Winnowing — and exactly why it works

After the grain is threshed off the plant, it is still mixed with husk. Husk is the light, papery, dry skin that covers the grain. There is a huge amount of it. Handpicking is hopeless here.

So farmers use winnowing. Winnowing means separating heavier and lighter parts of a mixture by letting wind blow through it.

Here is how it is done. The farmer stands up high, holds a tray (called a soop) full of the grain-and-husk mixture, and slowly pours it down while the wind blows. Figure 9.1 below shows what happens.

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Now the important question: why does the wind separate them? This is the part the textbook expects you to just accept. Let us not just accept it. Let us understand it.

The grain is heavy. The husk is light. When the wind pushes on both of them, the same gentle push is not enough to move the heavy grain much, but it easily blows the light husk away. Think of it like this: a strong breeze can blow a dry leaf right across the road, but it cannot move a stone lying next to the leaf. Same wind, different result, because the leaf is light and the stone is heavy.

So the grain barely changes direction and drops straight down. The husk gets carried off to the side. They land in two separate piles. The difference being used here is weight.

Sieving — separating by size

Now think about flour. Flour from the mill often has bigger bits in it — pieces of bran (hard grain skin) and the odd small stone. You want the smooth fine flour, not the rough bits.

Here the two parts are different in size. The flour bits are tiny. The bran and stones are bigger. When the difference is size, we use sieving.

A sieve is a tray with many small holes in its bottom. You put the mixture in and shake it. Figure 9.2 below shows what happens.

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Why does it work? The holes act like tiny doorways. Anything smaller than a hole can pass through. Anything bigger is stopped. So the fine flour rains down into the bowl, and the big bits are trapped on the sieve. The difference being used here is size.

One thing to notice: sieving only works if the two parts are clearly different in size, and the holes are just the right size. If the holes were bigger than the bran, the bran would fall through too, and nothing would be separated.

You have surely seen this beyond the kitchen. At a building site, workers shake sand through a big sieve to keep out pebbles and stones. Same idea, bigger sieve.

Sedimentation and decantation — letting the heavy stuff settle

Now think about muddy water, or tea with tea leaves in it. Here a solid is mixed in a liquid, and the solid does not dissolve. It just floats around or sinks.

Watch what happens if you leave a glass of muddy water still for a while. The mud slowly sinks to the bottom. The clear water stays on top. Figure 9.3 below shows this in two steps.

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The settling of the heavy solid at the bottom is called sedimentation. The “sediment” is just the stuff that settles down.

Why does the mud sink? Because a spoonful of mud is heavier than the same amount of water — that is, mud is heavier for its size. This is exactly why some things sink and others float, which you met in Chapter 6 — Materials Around Us. Given time and stillness, all the mud collects at the bottom and the water above turns clear.

Once it has settled, you gently tilt the glass and pour the clear water off into another glass. This careful pouring of the upper clear liquid is called decantation. The mud stays behind because it has settled and is too heavy to flow out with the water (as long as you pour gently).

This is exactly how your Dada removes tea leaves without a strainer. He lets the saucepan stand, so the leaves sink, then pours the tea out gently. But notice — a few tea leaves always sneak into the cup. So decantation does not separate things fully. It is quick, but not perfect. For a clean result, we need filtration.

Filtration — straining out the solid

Filtration also separates a solid from a liquid, but it does the job properly. It catches even the tiny bits that decantation misses.

The idea is to pass the mixture through a material full of very tiny holes called pores. The holes are big enough to let the liquid through, but too small to let the solid through. So the solid is trapped and the liquid passes. Figure 9.4 below shows filtration with a filter paper.

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Let us learn the two words this method gives us:

The clear liquid that passes through is called the filtrate.

The solid left behind on the filter is called the residue.

Why does it work? Same reason as sieving, but with much, much smaller holes. The pores of a filter paper are so fine that even tiny mud bits cannot squeeze through, while the tiny bits of water slip through easily. The difference being used is again size — but at a very tiny level.

You use filters every day. A tea strainer is a filter. A piece of clean cloth has small pores between its threads, so it works as a filter too. People in villages often pour drinking water through a folded cloth to catch dirt. The finer the holes, the cleaner the result.

Evaporation — getting salt back from salt water

Now the trickiest case. What if the solid has dissolved in the liquid?

When salt is stirred into water, it disappears. You cannot see it. This is called a solution — the salt has dissolved completely and spread evenly through the water. You cannot filter it out, because the salt is now in pieces far too small for any filter paper to catch. You cannot let it settle either, because it is not sitting at the bottom; it is mixed right into the water.

So how do we get the salt back? We use evaporation. We get rid of the water instead, and the salt is left behind. Figure 9.5 below shows how.

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This is how common salt is actually made. Sea water is let into shallow pits near the coast. The hot sun and air slowly dry it up. The water evaporates and flies off into the air. After some days, the water is gone and a layer of salt is left behind in the pit.

Why does only the water leave and not the salt? Because water turns into vapour much more easily than salt does. Heat gives the water enough energy to escape into the air as an invisible gas (water vapour). Salt needs far, far more heat to do the same. So with gentle heating or sunshine, the water leaves and the salt stays put.

Choosing the right method

So we have many methods. How do you pick the right one? The answer is the one big rule from the Big Idea: find the property in which the two things differ, then use the method that works on that property.

• Different in size? Use sieving (or filtration, if a solid is in a liquid).
• Different in weight? Use winnowing (light versus heavy, like husk and grain).
• One dissolves in the liquid? Use evaporation to remove the liquid.
• A solid that does not dissolve sits in a liquid? Use sedimentation and decantation, or filtration for a cleaner result.

Figure 9.6 below puts this whole decision into one picture.

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Let us practise this thinking on a real problem.

Here is a quick table you can use to remind yourself of every method, what it separates, and the difference it uses.

Common Mistakes

Let us clear up the ideas that trip up most students.

Quick Check

Let us see if the key ideas have stuck.

Practice Problems

Easy

Medium

Challenge

Summary

• We separate things to remove useless or harmful bits, to keep the part we want, or to split a mixture into two useful things.
• We can only separate two things if they are different in some way — in size, weight, or whether they dissolve. The difference is the tool.
• Handpicking removes a few large or odd-looking bits by hand, using differences in size, colour or shape.
• Threshing is beating harvested stalks to knock the grain loose from the stalks it is stuck to.
• Winnowing separates a light part from a heavy part using wind — the light husk is blown aside while the heavy grain falls straight down.
• Sieving separates small bits from big bits using a tray with holes — small bits fall through, big bits stay.
• Sedimentation is the settling of a heavy, non-dissolving solid at the bottom of a liquid; decantation is then gently pouring off the clear liquid on top.
• Filtration passes a mixture through tiny pores — the liquid passes through as the filtrate, the solid stays behind as the residue.
• Evaporation removes a liquid by turning it into vapour, leaving a dissolved solid (like salt) behind.

What’s Next

You now know how to take mixtures apart in the kitchen, the field and the lab. Next, in Chapter 10 — Living Creatures: Exploring their Characteristics, we turn from non-living things to living ones. You will learn what makes something alive — how living creatures grow, move, breathe, eat and respond to the world — and how scientists tell living things apart from non-living things. Just as this chapter taught you to spot differences between things in a mixture, the next one teaches you to spot the special features that all living creatures share.