Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are
- (a)5, 10 and 2 respectively
- (b)10, 2 and 5 respectively
- (c)5, 2 and 10 respectively
- (d)2, 5 and 10 respectively
Correct — A, 5, 10 and 2 respectively. Copper sulphate is CuSO4·5H2O (5 water molecules), sodium carbonate (washing soda) is Na2CO3·10H2O (10), and gypsum is CaSO4·2H2O (2). Taking them in the order asked — copper sulphate, sodium carbonate, gypsum — the numbers are 5, 10 and 2.
- (b)10, 2 and 5 respectively — Wrong order; these values do not match copper sulphate (5), sodium carbonate (10) and gypsum (2).
- (c)5, 2 and 10 respectively — Copper sulphate is right (5), but sodium carbonate has 10 (not 2) and gypsum has 2 (not 10).
- (d)2, 5 and 10 respectively — None of the three values matches in the order asked.
Water of crystallization is the fixed number of water molecules chemically combined in one formula unit of a hydrated salt. It gives many salts their colour and crystal shape, and it is driven off on heating to leave the anhydrous salt. Different salts hold different, characteristic numbers of these water molecules.
The four options are just permutations of 5, 10 and 2, so the whole question is about matching the right count to the right salt in the order asked.
- Copper sulphate pentahydrate, CuSO4·5H2O — 5 water molecules; the blue crystals turn white when heated.
- Washing soda, Na2CO3·10H2O — 10 water molecules.
- Gypsum, CaSO4·2H2O — 2 water molecules; on partial heating it gives Plaster of Paris, CaSO4·(1/2)H2O.
- Water of crystallization is lost on heating, changing the salt from hydrous to anhydrous.
In the order asked — copper sulphate, sodium carbonate, gypsum — the counts are 5, 10, 2, which is option (a).
- Getting the order wrong — read carefully which salt is asked first.
- Confusing gypsum (2 water molecules) with Plaster of Paris (half a water molecule).
Asked as the number of water-of-crystallization molecules in common hydrated salts, often as a match or a sequence.
No directly related past PYQ was found.
- practice — not a real PYQ
The number of water molecules of crystallization in washing soda is
- (a)2
- (b)5
- (c)7
- (d)10
Answer(d) 10 — washing soda is Na2CO3·10H2O.
- practice — not a real PYQ
Blue copper sulphate crystals turn white on heating because
- (a)they melt
- (b)they lose water of crystallization
- (c)copper is oxidised
- (d)the sulphate decomposes
Answer(b) they lose water of crystallization — CuSO4·5H2O becomes anhydrous CuSO4.