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General ScienceMediumAsked in: NDA · NDA-I 2026, GAT

Superphosphate of lime—a phosphate fertilizer—is obtained by the reaction of

  1. A.calcium phosphate with sodium hydroxide
  2. B.calcium phosphate with sulphuric acid
  3. C.calcium phosphate with hydrogen chloride
  4. D.calcium phosphate with nitric acid

Correct answer

B. calcium phosphate with sulphuric acid

Explanation

The correct answer is B, calcium phosphate with sulphuric acid. Treating rock phosphate, which is insoluble calcium phosphate, with sulphuric acid gives superphosphate of lime, a fertiliser whose phosphorus plants can absorb.

In the reaction, Ca₃(PO₄)₂ reacts with H₂SO₄ to form calcium dihydrogen phosphate, Ca(H₂PO₄)₂, which dissolves in water, together with calcium sulphate (gypsum). The mixture of the two is sold as single superphosphate and contains about 16% P₂O₅ along with sulphur and calcium. When phosphoric acid is used in place of sulphuric acid, the product, with no gypsum, is triple superphosphate, which is richer in phosphorus.

A is wrong because sodium hydroxide is a base and does not convert calcium phosphate into a soluble phosphate.

C is wrong because hydrogen chloride is not used to make superphosphate.

D is wrong because treating rock phosphate with nitric acid gives nitrophosphate fertilisers, not superphosphate of lime.

Exam tip: single superphosphate = rock phosphate + H₂SO₄; triple superphosphate = rock phosphate + H₃PO₄.

Practice Questions

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Q1.General ScienceAsked in: UPSC Civil Services · CSE Prelims 2022, GS Paper IMedium

Which of the following are nitrogen-fixing plants?

1. Alfalfa

2. Amaranth

3. Chickpea

4. Clover

5. Purslane (Kulfa)

6. Spinach

Select the correct answer using the code given below:

  1. A.1, 3 and 4 only
  2. B.1, 3, 5 and 6 only
  3. C.2, 4, 5 and 6 only
  4. D.1, 2, 4, 5 and 6
Show answer
Correct answer: A. 1, 3 and 4 only

Alfalfa, chickpea and clover are legumes, and legumes fix nitrogen. Plants of the legume family (Fabaceae) carry root nodules that house Rhizobium bacteria.

Q2.General ScienceAsked in: UPSC CAPF · CAPF 2017, Paper IMedium

The mass number of an element is NOT changed when it emits

  1. A.Alpha and Beta radiations only
  2. B.Alpha and Gamma radiations only
  3. C.Beta and Gamma radiations only
  4. D.Alpha, Beta and Gamma radiations
Show answer
Correct answer: C. Beta and Gamma radiations only

Neither beta nor gamma emission changes the total number of protons and neutrons in a nucleus, so the mass number stays the same.

Q3.General ScienceAsked in: UPSC CAPF · CAPF 2025, Paper IMedium

Which one among the following statements about facilitated transport in plants is correct?

  1. A.All passive transports are types of facilitated transport.
  2. B.Facilitated transport is an example of active transport.
  3. C.Water moves across plasma membrane by active transport.
  4. D.Facilitated transport requires assistance of carrier proteins.
Show answer
Correct answer: D. Facilitated transport requires assistance of carrier proteins.

In facilitated transport, molecules cross the membrane with the help of membrane proteins, while still moving down their concentration gradient.

Q4.General ScienceAsked in: UPSC CAPF · CAPF 2017, Paper IMedium

Who among the following has coined the term 'Quark', the fundamental particles that make up protons and neutrons in an atomic nucleus ?

  1. A.Richard Feynman
  2. B.Murray Gell-Mann
  3. C.Albert Einstein
  4. D.Niels Bohr
Show answer
Correct answer: B. Murray Gell-Mann

The American physicist Murray Gell-Mann proposed quarks in 1964 and gave them their name. Gell-Mann, and independently George Zweig, suggested that protons, neutrons and other such particles are built from smaller units.