Question

On heating hydrated copper sulphate, it turns to ________ colour?

Answer 1

Hint: Copper sulphate crystals exist in two forms, one is hydrated form and the other is anhydrous form. The chemical formula of hydrated copper sulphate is \[CuS{O_4}.5{H_2}O\] . It contains five molecules of water. It appears as blue coloured crystals. The copper will be in \[ + 2\] oxidation state. The chemical formula of anhydrous copper sulphate is \[CuS{O_4}\]. It appears as colourless crystals.

Complete answer:
Hydrated copper sulphate i.e., \[CuS{O_4}.5{H_2}O\] a pentahydrate crystal which means that five water molecules are loosely attached to copper sulphate crystals. The dot between the copper sulphate and water molecules in the formula denotes that the water molecules are loosely bounded. The hydrated copper sulphate crystals appear blue because of these loosely bound water molecules and they are known to be water of crystallisation.
The water of crystallisation is the fixed number of water molecules present in one formula unit of salt. The crystals which have water of crystallisation are known as hydrated.
When hydrated copper sulphate is heated, it decomposes and loses five molecules of water that is water of crystallisation. Losing five molecules of water, hydrated copper sulphate becomes anhydrous copper sulphate. Since the water of crystallisation is lost the blue coloured hydrated copper sulphate crystals become colourless(anhydrous). Therefore on heating hydrated copper sulphate i.e., \[CuS{O_4}.5{H_2}O\] turns to colourless anhydrous copper sulphate i.e., \[CuS{O_4}\] salt.
The chemical reaction occurring on heating \[CuS{O_4}.5{H_2}O\] is given as
\[CuS{O_4}.5{H_2}O(s)\xrightarrow{{heat}}CuS{O_4}(s) + 5{H_2}O(g)\]
On adding water to the anhydrous \[CuS{O_4}\] salt, we again get back the hydrated copper sulphate i.e., \[CuS{O_4}.5{H_2}O\] with the liberation of heat.
\[CuS{O_4}(s) + 5{H_2}O(g)\xrightarrow{{cold}}CuS{O_4}.5{H_2}O(s)\]
 And hence On heating hydrated copper sulphate, it turns to blue colour.

Note:
If the heating continues, then the anhydrous copper sulphate undergoes further decomposition to form copper (II)oxide (cupric oxide) i.e., \[CuO\] which is black in colour. Oxygen and sulphur dioxide are also released.
The chemical reaction for the decomposition of \[CuS{O_4}\] on heating is given as
\[2CuS{O_4} \to 2CuO + {O_2} + 2S{O_2}\].
The copper sulphate finds its uses more in agriculture and also in industrial settings.