Separation of Substances

 

There are many instances when we notice a substance being separated from a mixture of materials. Tea leaves are separated from the liquid with a strainer, while preparing tea (Fig.1).

Fig.1.

 

Grain is separated from stalks, while harvesting. Milk or curd is churned to separate the butter . we gin cotton to separate its  seeds from the fibre.Perhaps you might have eaten salted daliya or poha. If you found that it had chillies in it, you may have carefully taken them out before eating.Suppose you are given a basket containing mangoes and guavas and asked to separate them. What would you do? Pick out one kind and place them in a separate container, right? Seems easy, but what if the materials we want to separate are much smaller than mango or guava? Imagine you are given a glass of sand with salt mixed in it. Impossible, even to think of separating salt from this mixture by picking out grains of sand by hand!

 

Activity 1:

  Why do we separate substances?

Table .1 :

Separation process	Purpose for which we do the separation	What do we do with the separated components?
1) Separate stones from rice	a) To separate two different, but useful components.	i) We throw away the soild component.
2) Churning milk to obtain butter	b) To remove non-useful components.	ii) We throw away the impurities.
3) Separate tea leaves	c) To remove impurities or harmful components.	iii) We use both the components.

In Column 1 of Table.1, are given a few processes of separation. The purpose of separation and the way separated components are used is mentioned in Column 2 and 3 respectively. However, the information given in Columns 2 and 3 is jumbled up. Can you match each process with its purpose and the way separated components are used?  

 

The substances to be separated may be particles of different sizes or materials. These may be solids, liquids or even gases.

So, how do we separate substances mixed together if they have so many different properties?

 

Methods of Separation

We will discuss some simple methods of separating substances that are mixed together. You may come across some of these methods being used in day to day activities.

 

1. Hand Picking

 Activity 2 :

Bring a packet of grain purchased from a shop to the classroom. Now, spread the grain on a sheet of paper. Do you find only one kind of grain on the sheet of paper? Are there pieces of stone, husks, broken grain and particles of any other grain in it? Now, remove with your hand the pieces of stone, husks and other grains from it. 

This method of handpicking can be used for separating slightly larger sized impurities like the pieces of dirt, stone, and husk from wheat, rice or pulses (Fig. 2).

Handpicking is a convenient method of separating substances, when the quantity of  impurities is usually not very large.

 

 

 

 

                                                                                        

 

                                                                                     Fig.2.

 

2. Threshing

You must have seen bundles of wheat or paddy stalks lying in fields after harvesting the crop. Stalks are dried in the sun before the grain is separated from them. Each stalk has many grain seeds attached to it. Imagine the number of grain seeds in hundreds of bundles of stalk lying in the field! How does the farmer separate grain seeds from those bundles of stalks? 

One may pluck mangoes or guavas from the trees. But, grain seeds are much smaller than mangoes or guavas. So, plucking them from their stalks would be impossible. How does one separate grain seeds from their stalks? 

The process that is used to separate grain from stalks is threshing.

In this process, the stalks are beaten to free the grain seeds (Fig. 3). Sometimes, threshing is done with the help of bullocks. Machines are also used to thresh large quantities of grain.

Fig.3.

 

 3. Winnowing

 

Activity 3 :

Make a mixture of dry sand with sawdust or powdered dry leaves. Keep this mixture on a plate or a newspaper. Look at this mixture carefully. Can the two different components be made out easily? Are the sizes of particles of the two components similar? Would it be possible to separate the components by handpicking? 

Now, take your mixture to an open ground and stand on a raised platform. Put the mixture in a plate or sheet of paper. Hold the plate or the sheet of paper containing the mixture, at your shoulder height. Tilt it slightly, so that the mixture slides out slowly. 

What happens? Do both the components — sand and sawdust (or powdered leaves) fall at the same place? Is there a component that blows away? Did the wind manage to separate the two components? 

This method of separating components of a mixture is called winnowing. Winnowing is used to separate heavier and lighter components of a mixture by wind or by blowing air. This method is commonly used by farmers to separate lighter husk particles from heavier seeds of grain (Fig.4)

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Fig.4.

 

The husk particles are carried away by the wind. The seeds of grain get separated and form a heap near the platform for winnowing. The separated husk is used for many purposes such as fodder for cattles.

 

4. Sieving

Sometimes, we may wish to prepare a dish with flour. We need to remove impurities and bran that may be present in it. What do we do? We use a sieve and pour the flour into it (Fig. 5).

 

Sieving allows the fine flour particles to pass through the holes of the sieve while the bigger impurities remain on the sieve. Sieving is used when components of a mixture have different sizes.
 

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.                                                                                              

                                                                                                                                                                                             Fig.5

 You may have also noticed similar sieves being used at construction sites to separate pebbles and stones from sand (Fig. 6).

 

 

 

 

 

 

 

 

                                                                                                                                                          Fig.6.

 

SUMMARY

• Handpicking, winnowing, sieving, sedimentation, decantation and filtration are some of the methods of separating substances from their mixtures.

• Husk and stones could be separated from grains by handpicking.

• Husk is separated from heavier seeds of grain by winnowing.

• Difference in the size of particles in a mixture is utilised to separate them by the process of sieving and filtration.