CDS (I) 2020 General Knowledge · Question 22 of 120

Correct — C, 5. Hydrated copper sulphate is CuSO₄·5H₂O, and NCERT puts it plainly — five water molecules are present in one formula unit of copper sulphate. Those five are the water of crystallisation, a fixed number of water molecules built into the crystal rather than moisture clinging to its surface, which is why the dry blue crystals do not feel wet. Heat them and the water is driven off, leaving anhydrous copper sulphate, which is white; add water back and the colour returns. That reversible change is itself the classic school test for the presence of water.

• (a)2 — Two is the count for gypsum, CaSO₄·2H₂O. It is the right answer to a different compound in the same section of the textbook.
• (b)4 — No standard hydrate of copper sulphate holds four waters. The number is a plausible-looking filler between the two that do appear in the syllabus.
• (d)6 — Six is a common hydration number elsewhere — cobalt chloride hexahydrate, for instance — but not for copper sulphate.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. It is written after a dot in the formula and is a genuine part of the crystal's structure, so the substance is a dry solid despite the water. Heating usually drives it off and gives the anhydrous salt, often with a colour change; the change reverses when water is added back.

The safe way through this family of questions is to memorise the three examples the syllabus actually uses and let them anchor everything else: copper sulphate five, gypsum two, washing soda ten. Each number is then attached to a use — copper sulphate to the water test, gypsum to plaster of Paris, washing soda to cleaning. The distractors here are the numbers from the neighbouring compounds, so a candidate who half-remembers the group can talk themselves into any of them. Note that plaster of Paris keeps a half water molecule per formula unit, written as CaSO₄ with one half H₂O, and setting it with water turns it back into gypsum.

• Hydrated copper sulphate is CuSO₄·5H₂O, with five water molecules in one formula unit.
• Water of crystallisation is a fixed number of water molecules present in one formula unit of a salt.
• Gypsum is CaSO₄·2H₂O and washing soda is Na₂CO₃·10H₂O.
• Heating gypsum at 373 K gives plaster of Paris, calcium sulphate hemihydrate, which sets back to gypsum on adding water.
• Anhydrous copper sulphate is white; taking up water restores the blue of the hydrate, which makes it a test for water.

• Borrowing gypsum's two or washing soda's ten for copper sulphate.
• Thinking the water of crystallisation makes the crystal damp; it is part of the solid structure.
• Writing plaster of Paris as CaSO₄·2H₂O; that is gypsum, and the hemihydrate carries only half a water.

As the number of waters in a named salt, as a formula-recognition item, or through the white-to-blue test for water.

• practice — not a real PYQ

  Blue crystals of copper sulphate are heated strongly in a dry test tube. The residue left behind is

  • (a)blue and hydrated
  • (b)white and anhydrous
  • (c)green and hydrated
  • (d)black and anhydrous

  Answer(b) white and anhydrous — heating drives off the five waters of crystallisation, and the colour returns if water is added back.

• practice — not a real PYQ

  The number of water molecules of crystallisation present in one formula unit of gypsum is

  • (a)One half
  • (b)Two
  • (c)Five
  • (d)Ten

  Answer(b) Two — gypsum is CaSO₄·2H₂O; the half-water form is plaster of Paris.