How can you distinguish between sulphuric acid and aqueous sodium sulphate?

• A. There are three tests. A metal carbonate will produce carbon dioxide with sulphuric acid but not with sodium sulphate.
• B. They have identical reactions with all common reagents.
• C. Only pH testing can distinguish them.
• D. A metal carbonate will produce hydrogen gas with sulphuric acid but not with sodium sulphate.

Answer: (A)

The key idea is using how a solution reacts with carbonate to tell if it’s an acid or a neutral salt. An acid like sulfuric acid has H+ ions, so when a metal carbonate is added, the carbonate is protonated and releases carbon dioxide, giving visible fizz. The reaction also forms a salt and water. For example, calcium carbonate reacting with sulfuric acid makes calcium sulfate, carbon dioxide, and water. This fizzing is the telltale sign that an acid is present.

Aqueous sodium sulfate, on the other hand, is a salt solution and does not supply H+ ions. Adding a metal carbonate to it won’t produce carbon dioxide, so there’s no fizz.

That’s why this test is effective: it relies on the acid–carbonate reaction that releases CO2, which won’t occur with a salt solution. Other approaches like pH testing alone aren’t as definitive, and hydrogen gas isn’t produced in this carbonate–acid reaction.