JEE Advance - Chemistry (1985 - No. 20)

Reduction is a chemical process by which a substance gains electrons. Hydrogen gas is commonly used as a reducing agent because it can easily donate electrons to other substances. However, the ability of hydrogen to reduce an oxide depends on the reactivity of the metal in the oxide.

Let's consider each option:

Option A: Heated Cupric Oxide (CuO)

Copper(II) oxide can be reduced by hydrogen gas to form metallic copper and water. The reaction is as follows:

$$\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$$

Option B: Heated Ferric Oxide (Fe_2O_3)

Ferric oxide can also be reduced by hydrogen gas to form metallic iron and water. The reaction is as follows:

$$\text{Fe}_2\text{O}_3 + 3\text{H}_2 \rightarrow 2\text{Fe} + 3\text{H}_2\text{O}$$

Option C: Heated Stannic Oxide (SnO_2)

Stannic oxide can be reduced by hydrogen gas to form metallic tin and water. The reaction is as follows:

$$\text{SnO}_2 + 2\text{H}_2 \rightarrow \text{Sn} + 2\text{H}_2\text{O}$$

Option D: Heated Aluminium Oxide (Al_2O_3)

Aluminium oxide cannot be reduced by hydrogen gas. Aluminium has a very high affinity for oxygen, making the reduction of aluminium oxide (Al_2O_3) with hydrogen gas not feasible under normal conditions. This is because aluminium is much more reactive and forms a very stable oxide.

Therefore, the correct answer is Option D: heated aluminium oxide.