Class 6 Science Chapter 3 ‘Separation of Substances’ Notes by Sciक्षक

Before starting with Class 6 Science Chapter 3 i.e. ‘Separation of Substances’, I am hoping that you have completed the 2nd chapter. If not, then you can go through its Notes and NCERT Exercise Solutions whose links have been provided below. ⤵️

Table of Content

Introduction

Have you ever seen your parents/caregivers separating tea leaves from the liquid tea with the help of a strainer?

Or, if you are from a rural background then, have you ever seen farmers handpicking cotton from the cotton crop?

Or, have you ever seen milk or curd being churned to separate butter and buttermilk?

There are plenty of daily life examples where we separate one thing from another from a mixture and for that, we employ a variety of methods.

Before we use a substance, we need to separate harmful or non-useful substances that may be mixed with it. e.g. small stones need to be separated from the pulses before cooking.

Substances that need to be separated may vary in size, number, or even in a state of matter (solid, liquid, gas, plasma*, Bose-Einstein Condensate*)

Here is a small table of examples to explain why we need to separate substances.

Methods of Separation

We employ various methods to separate substances that are mixed together. Some of those common methods are:

1. Handpicking

As the name (handpicking = hand + picking) suggests we separate the substances from one another by picking them up with our hands.

Activity

1. Take a cupful of grains on a wide plate or on a white paper.
2. Observe, whether you find only one kind of grain or whether there are pieces of husk, stone, broken grains, or some other grains mixed in it.
3. Now, remove all the impurities from the grains by hand and collect them separately.

When to use the handpicking method of separation

• When the size of impurities is slightly larger to be able to be picked up by hand. e.g. husk, stones, etc. in grain.
• When the number of impurities are less.

2. Threshing

Have you ever seen the bundles of wheat or paddy stalks lying in fields after harvesting for drying under the sun before the grains are separated from them?

As each stalk or bundle bears thousands of grains. So, one can’t separate them with hands for sure.

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

Threshing can be done manually either by beating the stalk against a solid platform or using bullocks and it can also be done with the help of machines these days.

3. Winnowing

Doesn’t this name sound familiar with ‘wind’?

Well, Yes.

Because wind plays a huge role in this method of separation of substances.

Winnowing: method of separation is used to separate similar-sized heavier and lighter components of a mixture by wind or blowing air.

Activity

1. Make a mixture of sand and sawdust (powdered wood). Look at the mixture carefully, can you separate them by handpicking? (No, because both the components are very fine in size and similar in size as well).
2. Keep the mixture on a plate.
3. Now, take the plate to an open ground and raise the plate to your shoulder height.
4. Tilt the plate slightly, so that the mixture slides out slowly.
5. Observe where the components of the mixture fall.
6. Did Wind manage to separate the components?

Now that you have performed the activity, answer this very simple question.

Which component out of sand and sawdust was heavier? and which component falls near the separator and which one falls farther?

Application of Winnowing

Farmers use this method to separate lighter husk particles from the heavier grain seeds.

4. Sieving

This method of separation is used when components of the mixture have differences in size.

How does sieving work?

• In this method, we use a tool (sieve/mesh) and pour the mixture onto it.
• Then we either tilt the sieve or move it back and forth.
• The finer component of the mixture pass through the mesh or sieve whereas the larger component remain on the mesh.

Application of sieving

• For Separating wheat flour from bran and other impurities we use sieving.
• In flour mills, impurities like stones and husks are separated from grains using a sieve before grounding the grains.
• Sieving is the underlying principle behind the filtration method of separation.
• For construction purposes, sieving is used to separate pebbles and stones from the sand.

5. Sedimentation & Decantation

What if?

We have a mixture that can’t be separated from winnowing and handpicking.

e.g. such a mixture is rice and dust or soil mixture.

We add water to such a mixture. You definitely have seen this before at your home. Before we cook rice we always wash them with water to remove dirt.

when we add water to such a mixture the rice being heavier settles to the bottom of the container and dust along with some other light impurities starts to float.

Sedimentation: is the settling down of the heavier component of a mixture after the addition of water into the mixture.

Decantation: The removal of water and floating lighter impurities (dust or dirt).

Application of Sedimentation & Decantation

When we have a mixture of water and oil they form layers, with oil forming the top layer and water forming the bottom layer due to their difference in density.

Oil can be separated from water by decantation.

6. Filtration

Is a method of separation that involves the use of a filter medium to separate solid material from liquid.

Application of Filtration method of separation

• Tea leaves are separated from liquid tea using a strainer.
• Muslin or a fine cloth used for separating dirt from water coming out of a tap.
• RO water purifier.
• Fruit and vegetable juice is filtered before drinking to separate the seeds and pulp.
• In the preparation of cottage cheese (paneer), a piece of cloth is used as a filter to separate the paneer from the liquid.

How does filtration work?

We use a filter medium whose pore size is smaller than the impurities so that only liquid passes through the pores, not the solid impurities.

Filtration using filter paper

If we have very very fine impurities then we use a special filter i.e. Whatman filter paper that has a pore size that is significantly very small.

This filter paper is mainly used in laboratory practices.

How to use a Whatman Filter Paper

• It is mainly circular in shape.
• We fold it in a cone shape and fix it into a funnel.
• Then we pour the mixture on the filter paper.
• After some time, the liquid passes through the fine pores of the filter paper and solid impurities remain on it.

7. Vaporisation, Evaporation & Condensation

Before learning about these terms and phenomena.

Try to think of various methods that you can employ for separating salt and water from a salt solution prepared by mixing a spoonful of salt in 100 mL of water.

Vaporisation: This is the conversion of water (liquid) into vapours or steam (gas) by boiling.

Evaporation: This is the conversion of the top layer of water (liquid) into vapours or steam (gas) by not boiling but slowly heating by sunlight.

Condensation: This is the conversion of water vapours (gas) into water (liquid) with a decrease in temperature.

Fun Fact: The process of evaporation and condensation is the reason behind rainfall.

Activity:

• Transfer the liquid into a kettle and close its lid.
• Heat the kettle until steam starts to come out from the spout of the kettle.
• Hold a metallic plate onto the spout with steam hitting the plate and put ice onto the plate.
• Steam when it comes in contact with the cold metallic plate will condense and form liquid water.
• Once all the water has been boiled off only salt will remain in the kettle and water has been recovered by condensation of the steam.

We generally employ more than one method of separation for the separation of various components of a mixture as we’ve observed in the last example.

Fun Fact: Sea water is dried in pits to obtain salts and then those salts are processed to get the common table salt.

Saturated Solution

Before talking about a solution. There are two terms that we need to be familiar with.

I) Solute: A substance that is being dissolved. A solute is always in less quantity.

II) Solvent: A substance into which a solute is being dissolved. A solvent is always in larger quantity.

Hence,

Solute + Solvent = Solution

e.g. sugar (solute) + water (solvent) = sugar solution

Activity:

1. Take 100 mL water into a beaker or cup and label it as ‘A’
2. Add a tablespoon of salt to it and keep on stirring it constantly.
3. Once the salt is completely dissolved, add one more tablespoon of salt to the solution.
4. Keep doing it till the last tablespoon of salt that you added doesn’t dissolve anymore even with constant stirring.
5. Note down the number of a tablespoon of salt that you have added into the water.
6. Try to use a smaller-sized table spoon.
7. Now take 100 mL water in another beaker or cup and label it as ‘B’.
8. Repeat the same steps but with sugar this time.

What did you observe?

You observed that:

• After a certain amount, the solution saturates and no more salt or sugar can be added to it making a saturated solution.
• The amount of salt and sugar needed to make a saturated solution is different.

Saturated Solution: a solution that contains a maximum amount of solute dissolved in a given amount of solvent, and no more solute can be dissolved in it.

Effect of Temperature or Heating on Saturated Solution

Now take the solutions labelled as ‘A’ and ‘B’ and try to heat them and observe them carefully.

You will observe that the undissolved salt has dissolved with an increase in temperature.

Conclusion

The separation of components of a mixture has direct importance in our daily life. We employ various methods or should I say an integrated approach in separating the components of a mixture.

References & Credits

• Class 6 Science NCERT Textbook
• Image by brgfx on Freepik
• Image by brgfx on Freepik

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