Chemistry · Aldehydes, ketones and carboxylic acids · chapter 8 of 10

Class 12 Chemistry Chapter 8: Aldehydes, Ketones and Carboxylic Acids Assertion-reason questions with answers

2 assertion-reason questions on Aldehydes, Ketones and Carboxylic Acids, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): Butanal boils at a higher temperature than pentane, of comparable molecular mass. Reason (R): The polar carbonyl group gives aldehydes dipole–dipole interactions between molecules.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both true: the C=O dipole makes aldehyde molecules associate by dipole–dipole forces, which a hydrocarbon lacks; that is exactly why the boiling point is higher.

Question 2
Assertion (A): Methanal undergoes the Cannizzaro reaction when heated with concentrated KOH. Reason (R): Formalin, a 40% aqueous solution of methanal, is used to preserve biological specimens.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both statements are true, but the Cannizzaro reaction happens because methanal has no α-hydrogen; its use as a preservative says nothing about that.

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