Separation of Substances
Why do we need to separate different components of a mixture? Give two examples.
We need to separate different substances present in a mixture for the following purposes :
(1) To remove the unuseful or harmful component.
For example – removing tea leaves after making tea.
(2) To obtain the useful component – For getting wheat/rice grains after separating husk, dirt, etc.
(3) To remove impurities for getting a pure sample,
For example – obtaining sugar crystals from jaggery (gur) or salt from sea water.
What is winnowing ? Where is it used ?
Winnowing is a method of separation which is used to separate lighter and heavier components of a mixture. It is used by farmers to separate husk from grains in a field. The farmer allows the mixture of grain and the husk to fall from a height.
The grains which are heavier fall vertically down on the ground. The husk which is lighter, is carried away by the wind and forms a separate heap at a short distance from the heap of grain.
How will you separate husk or dirt particles from a given sample of pulses before cooking ?
Husk or dirt particles are separated from pulses before cooking by hand-picking.
What is sieving ? Where is it used ?
Sieving is a method of separation used when the components of a mixture are of different sizes. Sieving the wheat flour through a sieve allows fine particles of the flour to pass through the holes of the sieve while the bigger husk particles remain on the sieve.
How will you separate sand and water from their mixture ?
This can be done by taking a beaker or a glass tumbler half-filled with the given sample of water. Water is stirred thoroughly before keeping it undisturbed for some time. The sand being heavy settles down at the bottom of the beaker.
Now, the clear water is poured into another beaker without disturbing the layer of sand with the help of a glass rod. [Fig. 5.13 (a) and (b)].
Is it possible to separate sugar mixed with wheat flour ? If yes, how it is possible ?
Yes, it is possible to separate sugar mixed with wheat flour by sieving.
How would you obtain clean water from a sample of muddy water ?
Muddy water is cleared by the process of loading. Take a piece of alum, tied with a thread, it is swirrled in water; then after sometime, the water becomes clear on standing.
The water becomes clear because the mud particles get loaded by the alum particles and become heavy. The loaded mud particles settle down at the bottom of the container. The water, therefore, becomes clear and can be decanted for further use.
Fill in the blanks :
(a) The method of separating seeds of paddy from its stalks is called _________.
(b) When milk is poured onto a piece of cloth the cream (malai) is left behind on it. This process of separating cream from milk is an example of _________.
(c) Salt is obtained from seawater by the process of _________.
(d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this example is called _________.
Sedimentation and Decantation
True or False :
(a) A mixture of milk and Water can be separated by filtration.
(b) A mixture of powdered salt and sugar can be separated by the process of winnowing.
(c) Separation of sugar from tea can be done with filtration.
(d) Grain and husk can be separated with the process of decantation.
Lemonade is prepared by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar ? In which case would it be possible to dissolve more sugar ?
We should add ice to the lemonade after dissolving sugar. It would be possible to dissolve more sugar before adding ice.