NCERT Solutions for Class 9th Science Chapter 5 .2.3 Solubility of substances — Activity 5.2: Let us represent solubility graphically
Book page 775 Updated on2026-09-08
Q1.
Based on the information from the above graph, predict which of the two compounds, ‘A’ or ‘B’, will dissolve more in a given amount of water at a given temperature?
Answer
Compound ‘B’ — at every temperature on the graph its curve lies far above that of compound ‘A’.
Solubility curves of compounds ‘A’ and ‘B’ in water, redrawn from Fig. 5.6. Read a temperature on the x-axis, go up to the curve, then across to the y-axis.
At 20 °C: B ≈ 205 g per 100 g water, A ≈ 37 g per 100 g water At 60 °C: B = 287 g per 100 g water, A ≈ 56 g per 100 g water At 80 °C: B ≈ 360 g per 100 g water, A ≈ 66 g per 100 g water
How to read the graph: Pick a temperature on the x-axis, go straight up to a curve, then straight across to the y-axis — that reading is the maximum mass of solute which 100 g of water can hold at that temperature. Since B’s curve is above A’s everywhere, B always dissolves more in the same mass of water at the same temperature.
Q2.
(i) The solubility of compound ‘A’ in water at 20 °C is ______________ (less than/more than/similar to) its solubility at 60 °C.
Answer
Less than.
Solubility of A at 20 °C ≈ 37 g per 100 g of water Solubility of A at 60 °C ≈ 56 g per 100 g of water Increase ≈ 56 g − 37 g = 19 g per 100 g of water
Why it happens: Dissolving a solid pulls its particles apart from the crystal, and that costs energy. Heating supplies the energy and also makes the water molecules move faster, so they attack the crystal surface more often. Both effects raise the solubility of a solid solute as the temperature rises.
Q3.
(ii) The solubility of compound ‘B’ at 20 °C is ____________ (less than/more than/similar to) its solubility at 60 °C.
Answer
Less than.
Solubility of B at 20 °C ≈ 205 g per 100 g of water Solubility of B at 60 °C = 287 g per 100 g of water Increase ≈ 287 g − 205 g = 82 g per 100 g of water
Tip: B’s curve is not only higher than A’s, it is also much steeper. A steep solubility curve is exactly what makes a compound easy to purify by crystallization, because a small drop in temperature throws out a large mass of solid.
Q4.
(iii) The solubility of _____________ increases more than that of ______________ with an increase in the temperature.
Answer
The solubility of compound ‘B’ increases more than that of compound ‘A’ with an increase in the temperature.
From 20 °C to 60 °C: Compound B: 287 g − 205 g ≈ 82 g per 100 g of water Compound A: 56 g − 37 g ≈ 19 g per 100 g of water Compound B gains about 82 ÷ 19 ≈ 4 times as much
Why it happens: How steeply solubility rises is a property of the particular solute — it depends on how much energy is absorbed when that solid dissolves. Compound B absorbs much more heat on dissolving than compound A, so warming helps B far more. On the graph this shows up directly as the steeper slope of B’s curve.