Question

Which of the following is an example of a redox reaction?

• A. NaCl dissolving in water
• B. 2H$_2$O$_2$ (aq) $\to$ 2H$_2$O (l) + O$_2$ (g)
• C. NaOH dissolving in water
• D. CaCO$_3$ (s) $\to$ CaO (s) + CO$_2$ (g)

Hint:

In a redox reaction, one species is reduced (gains electrons) and another is oxidized (loses electrons).

Answer 1

The Correct Option is B

Definition: A redox reaction involves a change in oxidation states of elements — one species gets oxidized (loses electrons), and another gets reduced (gains electrons).

Option 1: NaCl dissolving in water

\( \text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^- \)

This is a physical change, not a redox reaction. No change in oxidation numbers.

Option 2: \( 2H_2O_2 \rightarrow 2H_2O + O_2 \)

Oxidation number of O in \( H_2O_2 \) is -1. In \( H_2O \), it's -2, and in \( O_2 \), it's 0.

• Some oxygen atoms are reduced: -1 → -2
• Others are oxidized: -1 → 0

✔ This is a **disproportionation redox reaction**.

Option 3: NaOH dissolving in water

\( \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \)

Again, only dissociation. No redox — no oxidation number change.

Option 4: \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)

This is a thermal decomposition reaction, but oxidation states remain:

• Ca: +2 → +2
• C: +4 → +4
• O: -2 in all species

❌ Not a redox reaction.

✅ Correct Answer:

Option 2: \( \boxed{2H_2O_2 \rightarrow 2H_2O + O_2} \)