Question:easy

Why does an old sample of benzaldehyde turn blue litmus red?

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Aldehydes slowly react with atmospheric oxygen on standing, and benzaldehyde forms acidic benzoic acid this way.
Updated On: Jul 3, 2026
  • Due to the conversion of benzaldehyde to phenol
  • Due to the presence of acid impurity with benzaldehyde
  • Due to aerial oxidation of benzaldehyde
  • Benzaldehyde is acidic in nature
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The Correct Option is C

Solution and Explanation

Alternative approach: rule out each option. "Conversion to phenol" is chemically unfounded here. "Benzaldehyde is acidic in nature" is also false, since freshly prepared, pure benzaldehyde does not turn litmus red at all. Between "acid impurity" and "aerial oxidation": the acid impurity, benzoic acid, does not appear in a freshly opened bottle. It builds up specifically because the sample has been exposed to atmospheric oxygen for a period of time. So "presence of acid impurity" is only a consequence, while "aerial oxidation" identifies the actual chemical event responsible.
\[\boxed{\text{Aerial oxidation of benzaldehyde to benzoic acid}}\]
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