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GATE 2025 ES – Question 14

Environmental Chemistry · Fundamentals of Environmental Chemistry · 1 mark · Multiple choice

Which reaction is a redox reaction?

  1. $H_2CO_3\rightleftharpoons H^++HCO_3^-$
  2. $Hg^{2+}+2OH^-\rightleftharpoons Hg(OH)_2$
  3. $C_6H_{12}O_6+6O_2\rightleftharpoons6CO_2+6H_2O$
  4. $CaCO_3(s)\rightleftharpoons Ca^{2+}+CO_3^{2-}$

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Correct answer: (C) $C_6H_{12}O_6+6O_2\rightleftharpoons6CO_2+6H_2O$

Explanation

In glucose oxidation, carbon’s average oxidation state rises from 0 to +4 in CO₂ while oxygen falls from 0 in O₂ to −2. Electrons are transferred, so C is redox. The other reactions are acid dissociation, complexation/precipitation or dissolution without oxidation-state changes. Hence **C**.

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