What is the function of concentrated H$_2$SO$_4$?

Analysis of the Question

The question asks for the primary chemical function of concentrated tetraoxosulphate(VI) acid ($H_2SO_4$). We need to evaluate each option based on the chemical properties of concentrated $H_2SO_4$.

Concept Explanation

Concentrated $H_2SO_4$ is a versatile chemical reagent in the laboratory. Its most distinct properties include:

1. Dehydrating Property: It has a very strong affinity for water. It can remove chemically combined water (hydrogen and oxygen in the ratio 2:1) from organic compounds like sugar ($C_{12}H_{22}O_{11}$), ethanol, and oxalic acid.
2. Oxidizing Property: Concentrated $H_2SO_4$ is a strong oxidizing agent, especially when hot.
3. Acidic Property: It reacts with bases to form salts and water (neutralization).

Step-by-Step Evaluation

• Option A: While dilute $H_2SO_4$ reacts with reactive metals to produce hydrogen gas, concentrated $H_2SO_4$ acts as an oxidizing agent. Instead of hydrogen gas, it typically produces sulphur(IV) oxide ($SO_2$), water, and a salt. Therefore, this is not its primary defining function.
• Option B: Concentrated $H_2SO_4$ is widely used in chemistry as a dehydrating agent. For example, when added to sugar, it removes water molecules, leaving behind a black carbon residue. This is one of its most famous laboratory roles.
• Option C: While it does neutralize alkalis, this is a property of acids in general (including dilute acids). It is not the unique or primary function that distinguishes concentrated $H_2SO_4$ from other acids.
• Option D: This statement is chemically incorrect; concentrated $H_2SO_4$ does not produce a cryolite precipitate.

Conclusion

The most significant and characteristic function of concentrated $H_2SO_4$ among the given choices is its ability to act as a dehydrating agent.

Exam Tip

When you see "Concentrated $H_2SO_4$" in a JAMB question, immediately think of two keywords: Dehydration (removing water) and Oxidation. If you see a question about removing water from a gas or an organic compound, $H_2SO_4$ is almost always the answer!