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Class VI - Science: Seperation of Substances
One Word Answer Questions:
Q) The method of separating seeds of paddy from its stalks is called?
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Q) Name the method used to separate cream from curd.
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Q) Which process is used to separate pebbles and stones from sand?
    
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Q) The process of conversion of water into its vapour is called?
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Q) Grain and husk can be separated from the process of decantation. True or False?
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Q) Which method is used to separate oranges from a mixture of oranges and apples?
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Short Answer Questions:
Q) What is Condensation?
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Q) Define the term handpicking.
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Q) Define Saturated Solution.
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Q) What do you mean by threshing? Where is it used?
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Long Answer Questions:
Q) Describe the method to obtain salt from sea water.
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Q) Explain any three methods of separation of substances in detail?
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Q) Explain the purification of water by Sedimentation, Decantation and Filtration processes?
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Q) Explain in detail the process of threshing.
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Q) Explain the method that can be used for separating the following mixture:
(i) Sand and husk
(ii) Wheat, sugar and stalk
(iii) Water and petrol
(iv) Rice and salt
(v) Sand and salt
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Introduction

In our day to day life we might have separate the useful substance from the mixture.
For example rice and pulses are cleaned before they cooked. We find tiny stones,small insects and husk among the grains.We need to separate them before they are cooked.

  • Substances are mostly available in mixture. A few substances are available in pure form. Hence, we need to separate the useful substance from the mixture by using some methods.
  • Before studying the various separation methods we should know the need to separate the components of a mixture.
Need for separation of substances

Substances need to be separated from each other to remove impurities or to get two or more useful products.

Methods Of Separation

Separation of substances can be carried out under two conditions

  • Dry conditions
  • Wet conditions
Separation of substances under dry conditions:

There are various methods for separating substances. They are

  • Hand picking
  • Threshing
  • Winnowing and
  • Sieving
Hand picking

It involves simply picking out substances by hand and separating them from others. The substances being separated may be impurities that have to be thrown away or it may be that both the substances being separated are useful -such as if you separate green grapes from black ones from a mixture of the two.

Separation Of Substances
Separating Green Grapes And Black Grapes
Threshing
  • This is done after harvesting of crops. Once wheat is harvested, the stalks are dried. Then, each grain of wheat is separated from the stalks to which it is attached,so that it can be ground into flour.
  • This method of separating grains from stalks is called threshing.
  • It is basically the beating of the dry stalks to shake off the dried grains
  • It can be done by hand, by cattle, or by using machines.Traditionally,threshing was done by hand, but cattle help to do this job quickly.

Nowadays threshing machines are also used to separate large quantities of grain at a time.One of these machines is a combine harvester.

Separation Of Substances
Threshing
Winnowing

Once the wheat has been threshed the grains should be cleared of the husk and chaff before grinding into wheat flour.

  • The process of separation of lighter and heavier particles from the mixture by wind or blowing air is called winnowing.
  • The chaff is separated from the grains by winnowing.In this method the farmer gently drops the mixture to the ground from a height.
  • The heavier grains are collected at a place while the lighter chaff blows away.
Separation Of Substances
Winnowing
Sieving

Sieving is used to separate substances that are of different sizes.In this method a sieve is used.The smaller components of the mixture can pass through the pores of the sieve whereas bigger components of the mixture are retained by the sieve.

For example:Wheat flour has some fine powdered wheat as well as some bigger impurities. When this flour is put through a sieve the fine powder falls through the small holes in the sieve, while the thicker impurities remain as they are too big for these holes, and hence the substances are separated.

Separation Of Substances
Sieving Flour

Pebbles and stones are separated from sand using sieve.

Separation Of Substances
Methods of separation of substances under wet conditions

Some solid particles are insoluble in water.

  • For example chalk powder,dust particles,sand,and tiny pieces of straw are insoluble in water.

To separate this type of particles generally we use the following methods.

Sedimentation and Decantation

"The process of separating insoluble substances which are heavier than liquid by allowing them to settle down on their own due to gravity is called sedimentation".
The particles which settle down during sedimentation are called sediments.

Separation Of Substances

Decantation follows sedimentation."In this process the clear upper liquid is poured out from the container without disturbing the sediment".
The upper clear liquid is called supernatant.

Separation Of Substances
Decantation

For example:Muddy water contains heavier particles like sand and soil.Leave this muddy water undisturbed for some time.The heavier soil and sand particles settle down.And the clear upper water is poured out from the container.

Filtration

Filtration is a process by which insoluble solids can be removed from a liquid by using a filter paper.

  • A filter paper is a special type of paper which has pores that are tiny enough to let only liquids pass through it.
  • If you pass a solution through filter paper any undissolved solid particles will get left behind on the paper whereas the liquid will filter through.
  • The liquid that passes through is called the filtrate and the undissolved solid particles are called residue.
    Example:A mixture of chalk powder and water can be separated by this method.
Separation Of Substances
Filtration

To separate the soluble solids form liquids we useevaporation and condensationmethods.

Evaporation

The process of conversion of water into vapour by heating it to its boiling point is called evaporation.
Example:The salt can be easily obtained from the salty water by the process of evaporation.If we boil this water the water evaporated completely and leaving behind only the salt.

Separation Of Substances
Process Of Evaporation
Condensation

The process of conversion of water vapour into its liquid form is called condensation.
Example:The water vapour condenses and we receive the same water in the form of rain.
If you pour cold water in a glass,you will observe the formation of water droplets on the outer surface of the glass.This is due to the condensation of water vapour present in atmosphere.

Separation Of Substances
Water Droplets Due To Condensation
Separation of substances using more than one method

A mixture of sand and salt can be separated by combination of methods.

  • The first method is sedimentation and decantation.This mixture is put in water and left for the sand to settle for some time. Then we will decant the salty water which will separate the sand from the mixture.
  • Now the salt can be separated from the water by evaporation. The water will boil away, leaving the salt behind.So the mixture of the sand,salt and water has been separated successfully using a combination of sedimentation,decantation,evaporation and condensation.
Solution and solubility
Solution

A solution is prepared by dissolving a substance in a liquid.

Solute + Solvent = Solution
Salt + water = Salt solution

Saturated solution

A solution is said to be saturated if it cannot dissolve more of the substance in it at a particular temperature.

Separation Of Substances

Unsaturated solution

A solution is said to be unsaturated if it can dissolve more of the substance in it at a particular temperature.


Separation Of Substances
Activity:1
Crystallization
  • Heat some salt water in a beaker, over a flame.
  • Stir the solution with a glass rod .
  • Continue heating till all the water in the beaker has evaporated.
  • What is left behind in the dish?
  • You will find salt crystals and powder in the dish.
ts_sos1.JPG

  • Do you Know?
  • Water is generally evaporated in sunlight.
  • We use this property while extracting salt from sea water.
  • Sea water is captured in wide pans and is exposed to air and sunlight.
  • Then water evaporates and the salt is left behind in the pans.
  • Distillation Before administering injections to patients, doctors mix injection powder with some liquid.
  • What is it? Is it water or any other liquid?
  • This is water and it is known as distilled water.
  • Where does this distilled water (pure water) come from?
  • Do you know the process of distilling water?

Activity:2
Get your own distilled water
  • Fill a conical flask with water, close it with a cork having a hole.
  • Insert a glass tube through the hole.
  • Take an another conical flask with a cork having a hole and insert another glass tube through it.
  • Connect both tubes with a plastic tube.
  • Now heat the flask containing water using a burner.
ts_sos2.JPG

  • After some time, water vapour goes into the second conical flask through the glass tube.
  • The water vapour will slowly turn to water.
  • The water in the second conical flask is called distilled water. It is free from impurities.

Activity:3
Sublimation of camphor
  • Take a mixture of camphor and powdered salt in a china dish and cover it with a funnel.
  • Close the tube of the funnel with cotton.
  • Place the dish on a stand and heat it with a burner .
  • ts_sos3.JPG

    • What do you observe in the dish? When camphor is heated, it transforms to gaseous form without changing into liquid.
    • Similarly, on cooling, the gaseous form of camphor changes directly into a solid without going to the liquid state.
    • The process in which a substance changes directly from solid to gaseous form and vice-versa is called sublimation.

Activity:4
A chalk with different colours
  • Take a whole stick of white chalk.
  • Around the curved surface of the chalk put an ink mark with blue or black ink.M
  • Now pour some water in a plate and keep the piece of chalk in the water.
  • Ensure that the water in the plate is very little and does not touch the ink .
ts_sos4.JPG

  • Now observe the colour patterns that form on the piece of chalk after some time.
  • Does chalk absorb water?
  • Can you find any change in ink mark on the chalk?
  • Remove the chalk before the water reaches its top.
  • Which colours do you see on the chalk from the bottom to top?
  • Draw a picture of the chalk in your notebook and the colours you have seen on the chalk.
  • From where did these colours come?
  • The ink appears to be made of a single colour but it is actually a mixture of many colours hidden in it.
  • This method is an example of chromotography.
  • Try to do chromatography with different inks and find out which colours they contain.
  • Where do we use the chromatography method?
  • We know that a leaf is green in colour.
  • Try to find whether the leaf consists of only one colour or more than one colour?
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