NDA (I) 2025 General Ability Test · Question 80 of 150

Correct — C, 'a compound is an impure substance' is the statement that is NOT correct. A compound is a pure substance: it has a single, fixed composition throughout and definite properties, unlike a mixture whose composition can vary. Because every sample of a given compound is identical in make-up, chemists classify it as pure, so calling it impure is wrong.

• (a)A compound is a substance composed of two or more elements, chemically combined in a fixed proportion — This is correct. A compound is exactly two or more elements chemically united in a fixed ratio by mass (for example water is always H:O = 1:8), so it is a true statement and not the exception.
• (b)Properties of a compound are different from its constituent elements — This is correct. A compound's properties differ sharply from its elements — sodium (reactive metal) and chlorine (poisonous gas) form harmless table salt — so the statement is true.
• (d)The constituents of a compound can be separated only by chemical or electrochemical reactions — This is correct. Because the elements are chemically bonded, they cannot be separated by physical means; only chemical or electrochemical reactions (such as electrolysis) break a compound apart.

Matter divides into pure substances and mixtures. Pure substances are either elements or compounds; a compound is a pure substance because its elements are chemically combined in one fixed proportion, giving it uniform, definite properties. A mixture, by contrast, has variable composition and keeps its components' properties.

Three statements describe genuine features of a compound; one mislabels it. The reasoning is to place a compound in the 'pure substance' box, which immediately shows (c) — 'impure' — as false. Fixed composition, new properties and separation only by chemical means all confirm the pure-substance status.

• A compound is a pure substance with a fixed, definite composition.
• Its elements are chemically bonded in a fixed ratio by mass.
• A compound's properties differ from those of its constituent elements.
• Its constituents can be separated only by chemical or electrochemical means, not physical ones.

• Confusing a compound (pure) with a mixture (impure/variable).
• Thinking a compound keeps its elements' properties.
• Believing a compound can be split by physical methods like filtration.

NDA/UPSC list statements about compounds/mixtures and ask which is NOT correct — the planted error is usually calling a compound impure or claiming physical separation works.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following is a pure substance?

  • (a)Air
  • (b)Sea water
  • (c)Distilled water
  • (d)Brass

  Answer(c) Distilled water — a compound of fixed composition; air, sea water and brass are mixtures.

• practice — not a real PYQ

  The components of a compound can be separated by

  • (a)Filtration
  • (b)Evaporation
  • (c)Chemical or electrochemical reaction
  • (d)Hand-picking

  Answer(c) Chemical or electrochemical reaction — the elements are chemically bonded, so physical methods cannot separate them.