Mg$_{(s)}$  +   2HCl$_{(aq)}$ → MgCl$_2$$_{(aq)}$ +  H$_2$$_{(g)}$ In the reaction above, magnesium is a reducing agent because

Analyze the question: We're asked why magnesium acts as a reducing agent in the reaction Mg + 2HCl → MgCl₂ + H₂.

Concept: A reducing agent is a substance that causes another substance to be reduced. To do this, it donates (loses) electrons, so the reducing agent itself is oxidized.

Remember: oxidation is loss of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain).

Solution:

1. Assign oxidation numbers: • Mg (solid element): 0 • In MgCl₂: Mg is +2 • H in HCl: +1 • H in H₂: 0

2. Half-reactions: • Mg → Mg²⁺ + 2e⁻ (oxidation: oxidation number rises from 0 to +2) • 2H⁺ + 2e⁻ → H₂ (reduction: oxidation number falls from +1 to 0)

3. Magnesium loses electrons, so it is oxidized. In doing so, it gives electrons to H⁺ ions, causing them to be reduced. That is exactly what a reducing agent does.

Checking the options:

• A. oxidized ✔ (the reducing agent is the species that gets oxidized)
• B. reduced ✘ (that describes the oxidizing agent, here H⁺ in HCl)
• C. a metal ✘ (being a metal is not the reason; it's about electron loss)
• D. a solid ✘ (physical state has nothing to do with it)

Tip: The reducing agent is always the one that is oxidized, and the oxidizing agent is always the one that is reduced. Remember it as "the agent does the opposite to itself of what it does to the other."