A student perfomed the experiment of heating ferrous sulphate crystals in a boiling tube. He smelt fumes of a pungent gas and saw colours of ferrous sulphate disappear.
(i) Write the chemical formula of the pungent gas.
(ii) Why does the colour of crystal disappear ?
(iii) Identify the nature of this chemical reaction.

Explanation

(i) `SO_(2) rarr` Sulphur dioxide
`SO_(3) rarr `Sulphur trioxide
(ii) Ferrous sulphate crystal contain 7molecules water of crystallization `(FeSO_(4). 7 H_(2) O)`. These crystals are green in colour. When these crystals are heated, they first lose 7 molecules of water od crystallization to form anhydrous ferrous sulphate `(FeSO_(4))` whis is white in colour.
(ii) It is a thermal decomposition reaction
`underset("Green")(2FeSO_(4)) underset("(decompositon)")overset("Heat")(rarr)Fe_(2)O_(3)(s)+SO_(2)(g) +SO_(3)(g)`
because in this reaction one substance is splitting up into three substances on heating.