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NCERT Solutions for Class 9th Science Chapter 5 .3.1 Crystallization — Activity 5.3: Let us prepare

Book page 785 Updated on2026-09-08

Q1.
Leave it for some time. What do you observe?
Answer

The water evaporates from the drop of saturated copper sulfate solution on the glass plate, and small blue crystals of copper sulfate are left behind, spread over the plate.

Why it happens: Here the solution is not being cooled — the solvent is being removed. As water molecules escape from the surface, the amount of water falls while the amount of copper sulfate stays the same, so the solution passes its saturation point and the excess solute separates as solid. The crystals are blue because copper sulfate crystallises with water in its structure, as hydrated copper sulfate.
Q2.
Did you get crystals? If yes, is this a good way to experiment? Explain.
Answer

Yes, crystals do form — but this is not a good method if you want good crystals or a pure product.

  • The crystals are tiny and badly shaped. Evaporation from a thin film is fast, so many crystals start growing at the same moment and none of them gets time or material to grow large.
  • The product is impure. Everything dissolved in the drop — including the impurities — is left behind on the plate when the water goes. Nothing is removed.
  • No control. The rate depends on the room temperature, the breeze and the humidity, so the result cannot be repeated reliably.

The proper method is the one in steps 1 – 6: filter the hot saturated solution to take out insoluble impurities, cover it, and let it cool slowly and undisturbed.

Why slow cooling is better: When cooling is slow, the solution stays only slightly supersaturated at any moment. Few crystal nuclei form, and each of them has time to collect particles from the solution and grow into a large, shiny, well-shaped crystal. The soluble impurities remain in the liquid left over, which is poured off — that is what makes crystallization a purification technique, not just a drying technique.
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