NCERT Solutions for Class 9th Science Chapter 5 .3.1 Crystallization — Pause and Ponder

Book page 795 Updated on2026-09-08

Q4.

Refer to the solubility curves given in Activity 5.2. If equal masses of hot, saturated solutions of compounds ‘A’ and ‘B’ are cooled from 80 °C to 60 °C, which solution is likely to deposit more solid?

Answer

The solution of compound ‘B’ will deposit far more solid.

Step 1 — read the solubilities off Fig. 5.6 (in g per 100 g of water):

Compound B: at 80 °C ≈ 360 g at 60 °C = 287 g
Compound A: at 80 °C ≈ 66 g at 60 °C ≈ 56 g

Step 2 — solid deposited per 100 g of water

Compound B: 360 g − 287 g ≈ 73 g
Compound A: 66 g − 56 g ≈ 10 g

Step 3 — the question says equal masses of solution, so scale to 100 g of solution

B: 100 g water + 360 g solute = 460 g of solution
deposit = (73 g ÷ 460 g) × 100 g ≈ 15.9 g per 100 g of solution
A: 100 g water + 66 g solute = 166 g of solution
deposit = (10 g ÷ 166 g) × 100 g ≈ 6.0 g per 100 g of solution

Either way of counting gives the same verdict: B deposits more — about 2.6 times as much for the same mass of solution.

Why it happens: What decides the yield of crystals is not how much solute a compound dissolves, but how steeply its solubility falls as the solution cools. B’s curve drops sharply between 80 °C and 60 °C, so a large excess is thrown out. A’s curve is almost flat, so hardly anything separates. This is why compounds with steep solubility curves, such as potassium nitrate, are purified by crystallization, while nearly-flat ones such as sodium chloride are obtained by evaporating the solvent instead.

Tip: Values read off a graph are approximate. Quote them as “about 360 g”, not 360.0 g — only 287 g and 241 g are printed exactly on Fig. 5.6.

Q5.

Will there be any change in the size of common salt crystals if the rate of evaporation is increased or decreased? Explain.

Answer

Yes. Slow evaporation gives large crystals; fast evaporation gives many small ones.

Rate of evaporation	What happens in the solution	Crystals obtained
Slow (shallow pan, mild sun, still air)	The solution creeps past saturation gently, so only a few nuclei form and each keeps growing	Few, large, well-shaped crystals
Fast (strong heating, dry wind, thin film)	The solution shoots far past saturation, so a huge number of nuclei appear together	Many tiny, poorly formed crystals

Why it happens: The total mass of salt obtained is fixed by how much was dissolved — evaporation rate cannot change that. What it changes is how that fixed mass is shared out. Fast evaporation creates a large number of growth centres, so each gets only a small share; slow evaporation creates few, so each grows big. The particles also need time to line up in the regular geometric pattern of a crystal, and rapid evaporation does not give them that time.

Did you know? Indian salt makers used exactly this idea long before the science was written down: karkatch salt was made by the slow evaporation of seawater in shallow pans, while panga salt was made by boiling concentrated brine — and the two methods gave crystals of clearly different sizes.

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