Question

Superphosphate of lime is a mixture of:
A. Primary calcium phosphate and Epsom salt
B. Primary magnesium phosphate and Epsom salt
C. Primary calcium phosphate
D. Primary calcium phosphate and gypsum

Answer 1

Superphosphate of lime is an important fertilizer, obtained from the action of sulphuric acid on phosphate rock. By knowing the products of the reaction of sulphuric acid and phosphate rock, we can determine the composition of superphosphate. As it is a mixture, it will contain more than one compound.

Complete Solution :
The mineral of phosphate rock which reacts with sulphuric acid is calcium phosphate,$\text{C}{{\text{a}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}$
The reaction of pulverized calcium phosphate with sulphuric acid is given as:
$\text{C}{{\text{a}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\text{+ 2}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{+ 4}{{\text{H}}_{\text{2}}}\text{O}\to \text{Ca(}{{\text{H}}_{\text{2}}}\text{P}{{\text{O}}_{\text{4}}}{{\text{)}}_{\text{2}}}\text{+2 CaS}{{\text{O}}_{\text{4}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O}$
The word equation is given as:
Calcium phosphate + Sulphuric acid $\to$Calcium dihydrogen phosphate + Calcium sulphate

- Thus, the product of the above reaction is calcium dihydrogen phosphate and calcium sulphate, commonly known as gypsum. The mixture of calcium dihydrogen phosphate and calcium sulphate (gypsum) is called superphosphate of lime.
So, the correct answer is “Option D”.

Note: Superphosphate contains higher percentage of phosphorus than calcium phosphate. It is the principal carrier of phosphate, that too in soluble form.
- Superphosphate, due to its high calcium sulphate content, is suitable and beneficial for crops that have high calcium demands. It is also suitable for saline soils.
- Superphosphate is ideal for encouraging strong root development as it contains and also helps move sugars and proteins around the plant so that fruits and vegetables ripen properly.