There are many instances when we notice a substance being separated from a mixture of materials. Let us learn different methods for the separation of substances. Tea leaves are separated from the liquid with a strainer while preparing tea. Grain is separated from stalks, while harvesting or perhaps separating impurities from wheat.

 

Seems easy, but what if the materials we want to separate are much smaller. Imagine you are given a glass of sand with salt mixed in it. Even to think of separating salt from this mixture by picking out grains of sand by hand seems impossible!

 

As we know, before we use a substance, we need to separate harmful or non-useful substances that may be mixed with it. Sometimes, we separate even useful components if we need to use them separately. The substances to be separated may be particles of different sizes or materials. These may be solids, liquids, or even gases.

 

We use different methods of separation for separating substances mixed together if they have so many different properties.

 

1. Sieving

 

Sometimes, it happens that we plan to prepare a dish using flour as ingredients. To do so we need to remove impurities and bran that may be present in it. So what do we do? We use a sieve and pour the flour into it.

 

Sieving allows the fine flour particles to pass through the holes of the sieve while the bigger impurities remain on the sieve. In a flour mill, impurities like husk and stones are removed from wheat before grinding it. Usually, a bagful of wheat is poured on a slanting sieve.

 

The sieving removes pieces of stones, stalk, and husk that may still remain with wheat after threshing and winnowing. You may have also noticed similar sieves being used at construction sites to separate pebbles and stones from sand.

 

2. Threshing

After harvesting wheat and paddy crop�s stalks are left in the field in the form of bundles so as to remove the moisture in them. This can be done with a technique called �threshing�. Threshing is a technique in which the stalks are beaten to free grains. Threshing is an important process in harvesting the field. Threshing can be achieved by various methods.

 

The simplest method is to beat the stalks on the floor and collect the grain on a plastic or cotton sheet. Sometimes threshing is done with the help of bullocks. Stalks are trampled under the feet of bullocks. And this is how the seeds of the grain are separated from the stalks. Nowadays threshing machines are also employed to thresh large quantities of grain.

 

3. Handpicking

 

You might have observed that in your daily life you come across some situations where you have to separate the two items. Like mangoes mixed with guavas in a basket, or separating stones from rice, etc. So today we will talk about the simplest technique for the separation of substances called �hand picking�.

 

�Hand-picking� as you can guess from its name, is a technique of separating substances by picking them from hands.

 

4. Filtration

 

Filtration is a process, which helps to remove the insoluble particles of impurities from a liquid
A very common example of filtration that you might have seen in your house is the passing of tea through the strainer before it is served so as to remove the tea leaves.

 

In villages, pond water is filtered through layers of cloth. Cloth has fine pores which do not allow the mud and small impurities which may be present in tap water to pass through it. Therefore water can be filtered to some extent.

 

5. Evaporation

The process of conversion of water into its vapor is called Evaporation. Evaporation occurs only on the surface of the water. Evaporation is a continuous process, as it occurs at all temperatures.

 

Evaporation also finds its application in the purification of water and other liquid. On boiling, water evaporates which helps to remove the impurities present in water. Therefore the method of evaporation is used for various industrial as well as domestic purposes.

 

6. Sedimentation and Decantation

 

Sometimes you find impurities like dust or small size soil particles in pulses or rice, which are difficult to be removed from the processes like winnowing and handpicking. You might have seen that pulses are washed 3-4 times in water before they are used for cooking.

 

In this case, the lighter particles like husk and dust present in it get separated and are washed away, while the heavier components are settled down. When the heavier component in a mixture settles down after water is added to it, the process is called Sedimentation.

 

When the water along with the dust particles is removed then the process is called decantation. This separation process can be applied to the mixture of oil and water. Water and oil, being immiscible in nature form two separate layers and hence can be easily separated by decantation.

 

7. Winnowing

 

You would have observed that while threshing the stalks, we separate the grains from the stalk. But one can still find the wheat husk mixed with the grains. So to further separate them, a method called winnowing is used. Winnowing is a traditional and efficient method used by farmers to remove the husk from grains.

 

Winnowing is a method in which a mixture of wheat and husk is raised to a platform. It is then shaken gently which in turn causes the husk to be blown away and hence separate out from the grain.

 

Use of more than one method of separation of substances

 

We have studied some methods for the separation of substances from their mixtures. Often, one method is not sufficient to separate the different substances present in a mixture. In such a situation, we need to use more than one of these methods.

 

Take a mixture of sand and salt.

 

How will we separate these? We already saw that handpicking would not be a practical method for separating these. Keep this mixture in a beaker and add some water to this. Leave the beaker aside for some time. Do you see the sand settling down at the bottom? The sand can be separated by decantation or filtration.

 

What does the decanted liquid contain?

 

Do you think this water contains the salt which was there in the mixture at the beginning? Now, we need to separate salt and water from the decanted liquid. Transfer this liquid to a kettle and close its lid. Heat the kettle for some time.

 

Do you notice steam coming out from the spout of the kettle? Take a metal plate with some ice on it. Hold the plate just above the spout of the kettle as shown. What do you observe? Let all the water in the kettle boil off.

 

Condensation

 

When the steam comes in contact with the metal plate cooled with ice, it condenses and forms liquid water. The water drops that you observed falling from the plate, were due to the condensation of steam. The process of conversion of water vapor into its liquid form is called condensation.

 

Did you ever see water drops condensed under a plate that has been used to cover a vessel containing milk that has just been boiled? After all the water has evaporated, what is left behind in the kettle? We have thus, separated salt, sand, and water using processes of decantation, filtration, evaporation, and condensation.

 

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