Which one of the following is a reduction reaction ?

A reduction reaction is defined as a process where a substance loses oxygen or gains electrons, resulting in a decrease in oxidation number [4]. In the reaction 2 HgO (s) → 2 Hg (l) + O2 (g), mercuric oxide loses oxygen to form metallic mercury. This specific process is explicitly identified as the reduction of mercuric oxide to mercury [4]. In contrast, options 1, 2, and 4 represent oxidation processes where magnesium or sulfur combine with oxygen or sulfur, leading to an increase in oxidation state [3]. For instance, the burning of magnesium in oxygen (Option 1) is a classic oxidation reaction where magnesium gains oxygen to form magnesium oxide [3]. Therefore, the decomposition of mercuric oxide is the only reaction listed that fits the criteria for a reduction reaction based on the loss of oxygen [4].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > CuO +H Cu+H O 2 2 → Heat (1.29) > p. 12
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.4.3 Extracting Metals Low in the Activity Series > p. 51
3. [4] https://ncert.nic.in/textbook/pdf/kech201.pdf