Question

When crystals of FeSO₄, are strongly heated the residue obtained is:

• A. red in colour
• B. blue in colour
• C. green in colour
• D. colourless

Answer 1

The correct option is A

red in colour

Decomposition reactions

Reactions in which one compound is broken down to two or more components are called decomposition reactions. Thermal decomposition involves the strong heating of the compound. As the temperature of the compound increases, the moisture content is removed first leaving the anhydrous compound. On further heating, the compound decomposes into simpler compounds or elemental state.

Decomposition of Ferrous sulphate

When Ferrous sulphate (FeSO₄) crystals are heated, the moisture content in the compound is removed forming anhydrous Ferrous sulphate crystals which is yellow in colour. When it is strongly heated, anhydrous Ferrous sulphate decomposes into Ferric oxide (Fe₂O₃), Sulphur dioxide (SO₂) and Sulphur trioxide (SO₃).

The chemical equation for the reaction is:

$FeS{O}_4\left(s\right)\stackrel{Heat}{\to }F{e}_2{O}_3\left(s\right)+S{O}_2\left(g\right)+S{O}_3\left(g\right)$

(Ferrous (Ferric (Sulphur (Sulphur

sulphate) oxide) dioxide) trioxide)

Of the products formed, Sulphur dioxide and Sulphur trioxide are gases. Ferric oxide is a solid with reddish brown colour remains as a residue. Therefore, the residue of decomposition is red in colour.

Therefore, the correct option is (A) red in colour.