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NCERT Solutions for Class 9th Science Chapter 5 .2 Solutions; 5.2.1 Concentration of a solution — In-text Questions

Book page 745 Updated on2026-09-08

Q1.
In what proportion are a solute and a solvent present in a solution? Can these be expressed quantitatively?
Answer

There is no single fixed proportion — a solution can be dilute or concentrated. But the proportion actually used can be stated exactly, and that number is called the concentration of the solution.

Concentration = amount of solute present in a given amount of solvent or of solution

At Class 9 level it is written as a percentage in one of three ways:

% m/m = (mass of solute ÷ mass of solution) × 100
% m/v = (mass of solute ÷ volume of solution) × 100
% v/v = (volume of solute ÷ volume of solution) × 100
Why it matters: Saying “salt in water” is not enough for ORS, for a saline drip or for a pesticide spray — each needs a definite proportion. A hospital saline drip must be 0.9 % m/v sodium chloride, that is 0.9 g of salt in every 100 mL of solution, because that is the concentration at which the solution matches blood and does not damage the blood cells.
Q2.
If they do not do so, what is likely to happen?
Answer

The spray will fail in one of two ways.

  • Too little pesticide: the concentration is below what the pest can withstand, so the insects survive and the crop is not protected. Repeated weak doses can even help resistant pests to multiply.
  • Too much pesticide: the excess scorches the crop, builds up in the soil, is washed into ponds and rivers, and leaves residues on the grain that reach the people who eat it.
Why it happens: A pesticide acts through the number of its molecules that reach each pest. That number depends on concentration, not on the total volume sprayed. Diluting the same 1 mL of pesticide into 100 mL of water instead of 10 mL cuts the concentration to one-tenth, and the effect falls with it. This is exactly the reasoning behind Example 5.3, where 1 mL of pesticide in 100 mL of spray is a 1 % v/v solution.
Q3.
Can you think of other such examples?
Answer

Yes — almost every prepared solution has a proportion that must be respected.

SituationWhy the exact proportion matters
ORS for a child with diarrhoeaThe salt and sugar must match the concentration of body fluids, or water will not be absorbed properly
Saline drip in a hospital0.9 % m/v NaCl matches blood; stronger or weaker damages the blood cells
Bleaching solution for cleaning a floorToo weak does not disinfect; too strong corrodes the surface and irritates the skin
Sugar syrup for jam or a cold drinkDecides taste and how long the food can be preserved
Fluoride in toothpaste, chlorine in drinking waterA useful amount protects; more than that is harmful
Sugar and milk in a cup of teaSame ingredients, different proportion, completely different taste
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