Which of the following decomposition reactions is NOT a redox reaction?

Correct — B. A reaction is redox only when some element's oxidation number changes. Apply that test to the keyed option:

CaCO₃ → CaO + CO₂
Ca: +2 → +2
C: +4 → +4
O: −2 → −2

Nothing is oxidised and nothing reduced, so this is a purely thermal decomposition.

In each of the other three an element leaves as a free element (Na, H₂ or O₂), and a free element has oxidation number 0 — proof that a change has taken place, which makes those three redox.

Why the others are wrong

• (a)
  Decomposition of sodium hydride — Sodium hydride decomposes as 2NaH → 2Na + H₂. Sodium goes +1 → 0 and hydride hydrogen goes −1 → 0, so both elements shift. Redox.
• (c)
  Decomposition of potassium chlorate — Potassium chlorate decomposes as 2KClO₃ → 2KCl + 3O₂. Chlorine falls +5 → −1 while oxygen rises −2 → 0 — the standard laboratory redox route to oxygen.
• (d)
  Decomposition of dihydrogen monoxide — Dihydrogen monoxide is water. In 2H₂O → 2H₂ + O₂ hydrogen goes +1 → 0 and oxygen −2 → 0, so splitting water is redox.

Assign oxidation numbers before you classify anything. The rules this question needs: a free element is 0; oxygen is normally −2; hydrogen is +1, except in metal hydrides where it is −1; and the numbers in a neutral compound sum to zero.

Decomposition then splits into two families. Reactions that release a free element — a metal, H₂, O₂, a halogen — are redox. Reactions that merely redistribute intact groups, a carbonate giving an oxide and CO₂ or a hydrated salt losing water, are not.

Key facts

• Calcium carbonate decomposes on heating into calcium oxide and carbon dioxide with no oxidation number changing.
• Hydrogen carries the oxidation number −1 in metal hydrides such as NaH.
• In the decomposition of potassium chlorate, chlorine goes from +5 to −1 and oxygen from −2 to 0.
• Any element released in its free form has oxidation number 0, which signals a redox change.

Common traps

• Assuming every decomposition must be redox because a compound is being pulled apart.
• Not recognising dihydrogen monoxide as water and guessing from an unfamiliar name.

No directly related past PYQ was found.