What is the function of concentrated H\(_2\)SO\(_4\)?

Concentrated sulphuric acid (H\(_2\)SO\(_4\)) has a very strong affinity for water. It is one of the most powerful dehydrating agents in chemistry, meaning it removes water (or the elements of water, H and O in a 2:1 ratio) from other substances.

This dehydrating ability is demonstrated in several ways:

• It chars sugar (sucrose, C\(_{12}\)H\(_{22}\)O\(_{11}\)) by removing water, leaving behind carbon.
• It is used in the laboratory to dry gases that do not react with it (e.g., drying chlorine, oxygen, or carbon dioxide).
• It dehydrates ethanol to produce ethene: \(\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{C}_2\text{H}_4 + \text{H}_2\text{O}\)

The other options describe properties that are not characteristic of concentrated H\(_2\)SO\(_4\). Reacting with metals to produce hydrogen and neutralising alkalis are properties of dilute acids, not concentrated sulphuric acid specifically. Producing a cryolite precipitate is not a recognised reaction of sulphuric acid.