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

Why do we separate things?

Before rajma goes into the cooker, someone at home often spreads the dry beans on a steel thali. A finger runs through them, and out come a tiny grey stone, a bit of twig and a shrivelled bean. Why take the trouble? Because one small stone can crack a tooth!

That bag of beans was a mixture. A mixture is made of two or more different things mixed together. In a mixture, each part keeps its own nature: the stones stay hard and grey, and the beans stay smooth and red. Because the parts stay different, we can pull them apart again.

We separate mixtures for a few reasons. Sometimes we remove a part that is useless or harmful, like stones in beans or mud in water. Sometimes both parts are useful, and we want each one on its own, like butter and buttermilk from curd. And sometimes we simply sort things into groups, such as big and small mangoes for the market.

Over thousands of years, people have found clever ways to separate things. Each way uses one difference between the parts. It may be size, colour, shape or heaviness. It may be whether a part dissolves in water, or whether a magnet pulls it. In this chapter you will meet these methods one by one. For each one, ask yourself: which difference does it use?

Handpicking

The simplest way to separate things is to use your eyes and fingers: look, spot, pick. This is handpicking. When you sort rajma, three clues help you spot the odd ones. A stone has a different colour, grey instead of red. A twig has a different shape, long and thin instead of kidney-shaped. A bad bean has a different size: it is small and shrivelled.

Handpicking works well when two things are true: the unwanted part is big enough to hold, and there is only a little of it. Picking five stones out of a kilogram of rice takes a few minutes. Picking a kilogram of rice out of a heap of stones would take all day! And you could never handpick sugar out of flour. The bits are far too tiny.

You can see handpicking all around you. A vegetable seller lifts the squashed tomatoes out of a crate. A gardener pulls the weeds that grow between the seedlings. A tailor sorts a jar of loose buttons by colour. Each time, a person uses their eyes and fingers to take one part away from the rest.

Threshing

Where does the rice in your kitchen come from? When paddy (the rice plant) is ripe, farmers cut it and dry the stalks in the sun. At this stage every grain is still fixed to the plant. Each stalk can carry a hundred grains or more. Pulling them off one at a time would take for ever.

So farmers knock them all off at once. In many villages they grab a bundle of dry stalks and beat it hard against a wooden board or a flat stone. Every whack shakes the grains loose, and they shower down. In some places bullocks walk round and round over stalks spread on the ground, and their hooves do the same job. This is threshing.

Look closely at what lies under the board. It is not clean grain yet. It is a brand-new mixture of grains, loose husk and bits of broken straw.11Each rice grain also keeps a tight husk of its own. A rice mill strips it off later, before the rice reaches your kitchen. The next method takes care of the loose bits.

Winnowing

Now we have a heap of grain mixed with husk. The husk bits are roughly the size of the grains, so a sieve would not help much. And there are far too many of them to pick by hand. But there is one big difference between them: grain is heavy and husk is light. What could push light things away more easily than heavy ones?

Farmers use exactly this idea. On a breezy day, a farmer fills a wide, shallow basket called a soop (सूप) with the threshed mixture. She lifts it up high and tips it slowly, so that the mixture falls out in a thin stream. The breeze carries the light husk to one side. The heavy grain drops in a clean heap close to her feet. This is winnowing.

Winnowing needs moving air. On a still day, the husk simply falls down with the grain and nothing is separated. Many farmers wait for the afternoon breeze, or winnow in front of a fan. The winnowing basket goes by many names across India: muram in Tamil, kulo in Bengali and chhaj in Punjabi.

Sieving

Before sowing seeds in a tray, a gardener wants soil that is soft and fine, with no pebbles, twigs or lumps. She tips the soil into a sieve and shakes it over a tub. Fine soil pours through like rain. Pebbles, roots and lumps are left behind on top.

A sieve is a frame with a mesh of wires or threads stretched across it. The mesh has many holes, all about the same size. When you shake a dry mixture on it, every bit smaller than the holes slips through. Every bit larger than the holes stays on top.

Sieving works only when the parts have different sizes, and when the holes are in between those sizes. If the holes are bigger than every bit, everything falls through. If they are smaller than every bit, nothing gets through at all. That is why many kitchens keep more than one sieve: a very fine one for flour and a coarser one for rava.

Sieves come in every size. A potter sieves dry clay powder so that no grit spoils the pot. At the beach, a child shakes sand through a toy sieve to find tiny shells. In a flour mill, huge shaking sieves separate fine flour from bran, the coarse outer layer of the grain.

Evaporation

If you have ever swallowed a mouthful of seawater while swimming, you know it is terribly salty. Several salts are hidden in it, mostly common salt. Yet you cannot see a single grain. The salt has dissolved: it has spread through the water as bits far too tiny to see. So no sieve, and not even a filter, can catch it. How do we get it out?

Travel along the coast of Gujarat or Tamil Nadu in the dry season. You may see workers raking gleaming white heaps out of flat fields. Weeks earlier, those fields were wide, shallow pans full of seawater. Day after day, sun and dry wind turned the water into vapour. The salt could not turn into vapour, so it was left behind as a crust of crystals.

You met evaporation in the chapter on the states of water. Water evaporates faster when it is warm, when the air is dry and windy, and when it is spread out thin. Salt makers use all three tricks: sunny weather, open windy land and very shallow pans. Before it reaches your kitchen, the raw salt is washed and cleaned in a factory.

You can check that dissolved salt stays behind with a simple experiment of your own.

You can spot the same thing in everyday life. After a swim in the sea, let your arms dry in the sun. They soon feel gritty, with a fine white dust of salt. Steel tumblers left to dry in a rack sometimes show little white spots for the same reason. The water went into the air; what was dissolved in it stayed behind.

Evaporation in the sun takes days. Heating speeds it up, and with one clever extra step, we can get back the water as well as the salt.

Sedimentation and decantation

After a heavy monsoon shower, water scooped from a puddle looks like milky tea. Fill a tall glass jar with it, stir it and leave it on a shelf. Come back later, and the jar has changed all by itself.

The heaviest bits sink first: grains of sand reach the bottom within minutes. Finer mud takes hours. By the next morning, most of it lies at the bottom as a soft layer called the sediment. The water above is far clearer. This settling of heavy bits that do not dissolve is called sedimentation.

Now tilt the jar slowly and let the clear water run into another glass. Stop just before the sediment starts to slide. Pouring off a liquid and leaving the settled solid behind is called decantation.

Kitchens use this pair of methods all the time. When potatoes have boiled, the water is poured off and the heavy potatoes stay in the pan. When eggs are boiled, the hot water is tipped out the same way. Quick and easy!

Decantation is fast, but it is rarely perfect. As you pour, a little cloudy water near the sediment sneaks out too, and some clear water has to stay behind. It works best when the solid is heavy and settles firmly. If you need the liquid to be perfectly clear, decanting is only the first step.

Filtration

Sedimentation and decantation made our water clearer, but not perfectly clear. To catch the fine bits too, we need a filter. A filter is a material with holes, or pores, so small that the solid bits cannot pass but the liquid can. Letting a mixture run through a filter is called filtration.

You filter things at home without even thinking about it. A steel strainer catches the lemon seeds as you pour nimbu pani into a glass. A muslin cloth holds back grated coconut while its milk drips through. A sugarcane juice seller pours the juice through a strainer to catch the stringy fibres.

Why does cloth work as a filter? Hold a cotton cloth up to a bright window. Light shines through countless tiny gaps where the threads cross. Water can squeeze through these gaps, or pores, but bits bigger than them are held back. Fold the cloth into several layers and the gaps no longer line up, so even finer bits get caught. Lab filter paper is finer still: its pores are far too small to see.

The solid that a filter holds back is called the residue. The liquid that runs through is the filtrate. Anything dissolved in the liquid goes along with the filtrate.

Filtering works on size, just like sieving. A filter catches a solid only if its pores are smaller than the solid bits, while still letting the liquid through. This is also why dissolved salt slips through every filter: its bits are far smaller than any pore. To get dissolved salt back, you have to evaporate the water.

Water from a pond or a village tank may carry leaves, sand, mud and tiny living things. Big water-treatment plants clean it in stages, and one stage lets it trickle through deep beds of sand and gravel. Your model will use four trusted filter materials. Gravel and sand trap grit and mud. Charcoal can also take away some colour and smell. Cotton wool holds everything in place.

Nets and screens are filters too. Some towns fix metal screens or floating booms across big drains. These catch thermocol plates, sachets and even lost slippers before they reach a river. The traps fill up again and again, which points to the real fix: keeping plastic out of drains in the first place.

Churning

In many homes, butter is still made the old way. Curd goes into a deep pot with a little cold water. A wooden churner called a mathani (मथानी) stands in the pot, with a rope wound round its stick. Pull one end of the rope, then the other, and the churner spins quickly one way, then back the other way.

Curd holds countless drops of fat, far too small to see. The spinning makes these drops bump into one another, and when they bump, they stick. Small drops join into bigger ones, and bigger ones into soft yellow lumps. Butter is lighter than the liquid around it, so the lumps float up to the top. Someone scoops them off, and what remains in the pot is buttermilk, or chhach.

Today many families churn curd with an electric mixer or a hand blender, which spins far faster than any rope. Big dairies separate cream from milk in a similar way. Machines spin the milk very fast. The light cream and the heavier skimmed milk gather in different places and flow out through different pipes.

Magnetic separation

By evening, the floor under a tailor's table is covered with snipped cloth, threads, buttons and dropped pins. Picking out the tiny steel pins by hand is slow, and they prick your fingers. A magnet can do the job in seconds.

Remember from the chapter on magnets that a magnet pulls things made of iron, nickel or cobalt, and most kinds of steel. The pins are made of steel, but cloth, thread and plastic buttons are not attracted. Sweep a magnet over the heap, and only the pins jump up to it. Using a magnet to pull magnetic things out of a mixture is called magnetic separation.

Here is a neat trick. Slip the magnet into a thin plastic bag first. The pins cling to the outside of the bag. Hold the bag over a box and pull the magnet out. The pins lose their grip and drop neatly into the box, and your magnet stays clean.

Choosing and combining methods

Every method in this chapter uses one difference between the parts of a mixture. So before you choose a method, ask: how are the parts different? The table sums it up.

People separate things for two main purposes. Sometimes one part is thrown away. At other times both parts are kept because each is useful. Play this quick game to sort some examples.

Many real mixtures have more than two parts. Then one method is not enough. We use several methods, one after another, and the order matters.