Q.Give a reason to support that sulphuric acid is a dehydrating agent.

Step 1. Note sulphuric acid's affinity for water.

Concentrated sulphuric acid has an extremely strong affinity for water; when dissolved in water it forms distinct hydrates such as the monohydrate (H2SO4.H2O) and dihydrate (H2SO4.2H2O), and the dissolution process is strongly exothermic, showing how favourable the acid-water association is.

Step 2. Give the sucrose (dehydration) evidence.

When concentrated sulphuric acid is poured onto sucrose (table sugar, C12H22O11), it removes the hydrogen and oxygen atoms of the sugar in exactly the 2:1 ratio found in water, leaving behind a black mass of carbon: C12H22O11 + H2SO4 -> 12C + H2SO4.11H2O. This charring is a direct chemical dehydration, not simple physical drying, because the water removed is extracted from within the molecule's own atoms, not merely absorbed from the surroundings.

Step 3. Give the formic/oxalic acid evidence.

Similarly, concentrated sulphuric acid converts formic acid to carbon monoxide (HCOOH + H2SO4 -> CO + H2SO4.H2O) and oxalic acid to a mixture of carbon monoxide and carbon dioxide ((COOH)2 + H2SO4 -> CO + CO2 + H2SO4.H2O), in each case removing the elements of water from the organic molecule.

Step 4. Conclude the reasoning. …

Cite the exothermic hydrate formation and the classic sucrose-charring/formic-acid/oxalic-acid dehydration reactions as direct c …

Confusing 'dehydrating agent' (chemically removing water's elements from within a compound) with merely 'drying agent' (physically absorbing surrounding moisture) — sulphur …