Question

A few crystals of copper-sulphate are heated in a dry boiling tube : (a) What is the colour before and after heating ?(b) What is the reason for the colour change?(c) Can its original colour be restored? How?

Answer 1

Before heating it was blue but after,it turned colourless (white).

This is because, CuSO4 crystals have water as part of their structure.OR, their actual formula is CuSO4.5H2O.When the blue crystals are heated the water of crystallisation is lost and what remains is anhydrous copper sulphate (CuSO4).

As this is a reversible reaction,it's colour can be restored by adding water to it

Answer 2

a) Colour before heating => blue

   Colour after heating=>white

b)Reason for colour change is as follow:-

1} The blue copper solution contain copper sulphate and 5 molecules of water.Hence it is known as copper sulphate pentahydrate.

2} Salts are generally as crystals with water molecules known as water of crystallization.

3}On heating CuSO4.5H2o hydrate solution , water of crystallization is looses due to evaporation thus CuSo4 tends to lose colour and shape leaving behind anhydrous copper sulphate acidic salt in form of white precipitate powder.

4} Therefore , we get white crystals of anhydrous copper sulphate on heating

c} By adding water its original colour can be restored.