CAN WATER DISSOLVE ANY AMOUNT OF A SUBSTANCE
Many substances dissolve in water and form a solution.The solution made in water are called aqueous solutions.

Activity : Pour half a cup of water in the beaker. Add one teaspoonful of salt and stir it well, until the salt dissolve completely. Again add a teaspoonful of salt and stir well. Go on adding salt, one teaspoonful at a time and stir.

After adding a few spoons of salt, we find that some salt remains undissolved and settles at the bottom of the beaker. This means that no more salt can be dissolved in the amount of water we have taken. The solution is now said to be saturated.

Now, add a small quantity of salt to this saturated solution and heat it. 

We find that now salt dissolves in it. Such solution which contains more amount of substances dissolved in it than required to form a saturated solution is called super saturated solution. Let this hot solution cool. We notice that salt appear to settle at the bottom of the beaker again.

A solution that contains less amount of solute than required to form a saturated solution is called an unsaturated solution.

KEY WORDS

1.    Churning (or centrifugation) : It is the process of separation of the lighter particles of a suspended solid  from a liquid. For example, to obtain butter from the curd or milk.
2.    Condensation : The process of conversion of water vapour into its liquid form is called condensation.
3.    Crystallisation : The process of crystallisation is used for obtaining pure crystalline substance from impure sample.
4.    Decantation : It is the transfer of clean liquid from one vessel to the other without disturbing the settled (sedimented) particles.
5.    Evaporation : It is the process of removing water (or moisture) from a mixture either by heating on flame or direct sunlight. For eample, salt from sea water is obtained by this method.
6.    Filtration : Filtration is used to separate solid particles from liquid by passing the mixture through a filter paper.
7.    Handpicking : This method is used for separating small particles of dirt, stone, husk, etc., from the grains of wheat, rice, pulses, etc.
8.    Loading : It is the process of faster sedimentation by suspending alum to a liquid.
9.    Sedimentation : It is the process of settling of heavy solid particles in a mixture at the bottom of the vessel.
10.    Sieving
        (i)   Sieving is used when two components of a mixture have different particle sizes.
       (ii)  Sieving allows the fine particles to pass through the holes of she sieve, while the bigger particles remain on the sieve. For example, sieving of wheat flour, sieving of sand at construction sites.
11.    Saturated solution : A solution in which no more soluble substance can be dissolved at room temperature is called saturated solution.
12.    Solution : When a soluble substance is dissolved completely in a liquid (say sugar in water), a homogeneous mixture is formed. It is known as a solution.
13.    Threshing : The process that is used to separate grain from stalks is threshing.
14.    Winnowing : Winnowing can be used to separate lighter and heavier components of mixture. For example, to separate husk from grain with the help of air.

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