EPFO EO/AO 2023 General Ability Test · Question 88 of 120

Correct — B, (b) white colour salt. Copper sulphate pentahydrate is the familiar blue crystal, sold as blue vitriol, and its formula carries five molecules of water for every unit of copper sulphate. Those five are the water of crystallisation: they are not moisture clinging to the surface but part of the crystal's own structure, and they are the reason the salt is blue. Heating drives that water off. The crystals lose their water in stages as the temperature rises, crumbling from clear blue crystals to a dull powder, and droplets of water condense on the cooler upper part of the tube — the visible proof that the water came out of the solid. What is left when the last of it has gone is anhydrous copper sulphate, and anhydrous copper sulphate is white. That is exactly what the stem asks for. The colour is worth understanding rather than memorising. In the hydrated crystal the copper ion is surrounded by water molecules, and it is that arrangement of copper with water around it that absorbs red light and leaves the salt looking blue. Remove the water and the arrangement is gone, so the absorption goes with it and the solid appears white. The reaction is easily reversed, and the reversal is more useful than the forward direction. Add a few drops of water to the white powder and the blue returns at once, with noticeable warmth. Because the colour change is sharp and needs only a trace of water to appear, anhydrous copper sulphate is the standard laboratory test for the presence of water — in a liquid suspected of being wet alcohol, for instance, or in a gas being dried. A candidate who remembers the test remembers the answer to this question automatically, because the test only makes sense if the anhydrous salt is white and the hydrate is blue. One boundary to note: this is the ordinary heating of the crystals. Driven to a much higher temperature the anhydrous salt itself breaks down, leaving a black residue of copper oxide, but that is a different and far more severe treatment than the one the question describes.

• (a)blue colour salt — Blue is the colour the crystals have before they are heated, not after, so this option answers the wrong end of the change. It is also the colour the residue returns to if water is added back, which makes it the most tempting wrong choice for a candidate who remembers the demonstration as a whole but not its order. Hold the sequence: blue hydrated crystal, heat, white anhydrous powder, add water, blue again.
• (c)green colour salt — Green belongs to a different salt in the same family of school demonstrations. Ferrous sulphate heptahydrate is the pale green crystal, and it is the one whose heating is used to illustrate decomposition; copper sulphate is blue in its hydrated form and white in its anhydrous form, and it passes through no green stage. A candidate who has revised the two demonstrations together and merged them will reach for this option.
• (d)brown colour salt — Brown is again borrowed from the ferrous sulphate demonstration, where strong heating of the green crystals leaves a reddish-brown residue of ferric oxide along with pungent gases. Copper sulphate leaves nothing brown: gentle heating gives the white anhydrous salt, and only much stronger heating decomposes it further, to a black oxide rather than a brown one. The option tests whether the candidate has kept the copper and iron demonstrations distinct.

Water of crystallisation is a fixed number of water molecules built into the crystal lattice of a salt as it forms from solution. It is written in the formula after a dot — five for copper sulphate, seven for ferrous sulphate and for magnesium sulphate, ten for washing soda, two for gypsum — and because the number is fixed, hydrated and anhydrous forms are distinct substances with different colours, densities and shapes. A salt that contains water of crystallisation is still dry to the touch, which is what makes the idea counter-intuitive at first meeting. Heating removes the water, and the change is usually reversible by adding water back. Several familiar materials turn on this. Plaster of Paris is made by heating gypsum until it has lost three-quarters of its water, and it sets hard when mixed with water because it takes that water back and returns to gypsum. Washing soda left in the open loses water to the air of its own accord and crumbles, which is efflorescence, while common salt and calcium chloride do the opposite and take water from the air, which is deliquescence. Anhydrous salts that absorb water strongly are used as drying agents in the laboratory for exactly this reason.

The chemistry items in the EO/AO science block are drawn from school laboratory demonstrations and everyday materials, and this is one of the two or three most demonstrated reactions in the syllabus. The paper is testing recall of what the candidate has seen or read about rather than any calculation. The habit rewarded is fixing each classic demonstration as a coloured sequence — what it looked like before, what it looked like after, and what came off — because most questions in this family name one point in that sequence and ask for the next.

• Copper sulphate pentahydrate carries five molecules of water of crystallisation and is blue; it is commonly called blue vitriol.
• Heating drives off the water of crystallisation and leaves anhydrous copper sulphate, which is white.
• Adding water to the white anhydrous salt restores the blue colour, with the release of heat.
• Anhydrous copper sulphate turning blue is the standard laboratory test for the presence of water.
• Water of crystallisation is part of the crystal structure, not surface moisture, and its quantity is fixed for each salt.
• Ferrous sulphate heptahydrate is green, and strong heating of it leaves a reddish-brown residue of ferric oxide.
• Gypsum is calcium sulphate dihydrate; heating it to drive off part of that water gives plaster of Paris, which sets by taking the water back.
• Washing soda is sodium carbonate decahydrate, and it effloresces in dry air by losing water of crystallisation.

• Giving the colour before heating rather than after; the demonstration must be remembered in order.
• Merging the copper sulphate demonstration with the ferrous sulphate one, which supplies the green and the brown.
• Believing water of crystallisation is dampness that can be shaken off; it is part of the lattice and its amount is fixed.
• Extending the answer to strong heating, where the anhydrous salt decomposes further to a black oxide.

Hydrated salts appear in EO/AO papers as a colour question like this one, as a formula question about how many molecules of water a named salt carries, or as an application question about a laboratory test or about plaster of Paris. Build a short table of the half-dozen common hydrates with their popular names, their water content and their colour before and after heating, and the whole group becomes routine.

No directly related past PYQ was found.

• practice — not a real PYQ

  Anhydrous copper sulphate is used in the laboratory to test for the presence of :

  • (a)Carbon dioxide
  • (b)Water
  • (c)Oxygen
  • (d)Ammonia

  Answer(b) Water

• practice — not a real PYQ

  The number of molecules of water of crystallisation present in washing soda is :

  • (a)Two
  • (b)Five
  • (c)Seven
  • (d)Ten

  Answer(d) Ten