Page 27 - Understanding NCERT Science 09

A solution has a solvent and a solute as its proportion of the solute and solvent can be
components. The component of the solution varied. Depending upon the amount of solute
that dissolves the other component in it present in a solution, it can be called dilute,
(usually the component present in larger concentrated or saturated solution. Dilute
amount) is called the solvent. The component and concentrated are comparative terms. In
of the solution that is dissolved in the solvent activity 2.2, the solution obtained by group
(usually present in lesser quantity) is called A is dilute as compared to that obtained by
the solute. group B.

Examples: Activity ______________ 2.3
(i) A solution of sugar in water is a solid • Take approximately 50 mL of water
in liquid solution. In this solution, each in two separate beakers.
sugar is the solute and water is • Add salt in one beaker and sugar or
the solvent. barium chloride in the second beaker
with continuous stirring.
(ii) A solution of iodine in alcohol known
• When no more solute can be dissolved,
as ‘tincture of iodine’, has iodine (solid)
heat the contents of the beaker to raise
as the solute and alcohol (liquid) as the
o
the temperature by about 5 C.°
solvent.
• Start adding the solute again.
(iii) Aerated drinks like soda water, etc., are
gas in liquid solutions. These contain Is the amount of salt and sugar or barium
carbon dioxide (gas) as solute and water chloride, that can be dissolved in water at a
(liquid) as solvent. given temperature, the same?
(iv) Air is a mixture of gas in gas. Air is a At any particular temperature, a solution
homogeneous mixture of a number of that has dissolved as much solute as it is
capable of dissolving, is said to be a saturated
gases. Its two main constituents are:
oxygen (21%) and nitrogen (78%). The solution. In other words, when no more solute
can be dissolved in a solution at a given
other gases are present in very
small quantities. temperature, it is called a saturated solution.
The amount of the solute present in the
Properties of a Solution saturated solution at this temperature is called
its solubility.
• A solution is a homogeneous mixture. If the amount of solute contained in a
• The particles of a solution are smaller solution is less than the saturation level, it is
than 1 nm (10 metre) in diameter. So, called an unsaturated solution.
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they cannot be seen by naked eyes. What would happen if you were to take a
• Because of very small particle size, they saturated solution at a certain temperature
do not scatter a beam of light passing and cool it slowly.
through the solution. So, the path of We can infer from the above activity that
light is not visible in a solution. different substances in a given solvent have
• The solute particles cannot be separated different solubilities at the same temperature.
from the mixture by the process of The concentration of a solution is the amount
filtration. The solute particles do not (mass or volume) of solute present in a given
settle down when left undisturbed, that amount (mass or volume) of solution.
is, a solution is stable.
There are various ways of expressing the
concentration of a solution, but here we will
2.2.1 CONCENTRATION OF A SOLUTION
learn only three methods.
In activity 2.2, we observed that groups A and (i) Mass by mass percentage of a solution
B obtained different shades of solutions. So, Mass of solute
= ×100
we understand that in a solution the relative Mass of solution

16 SCIENCE

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