Q.Which of the following gases cannot be collected over water ?

A gas can be collected over water only if it is not appreciably soluble; SO2 dissolves readily in water, so it cannot be.

Collection over water works only for gases that are insoluble or only slightly soluble in water. H2, O2 and CH4 are all very sparingly soluble, so they can be collected over water. SO2, however, is very soluble and reacts with water to form sulphurous acid:

SO2 + H2O -> H2SO3

…

SO2 is polar because it is bent, so bond dipoles do not cancel — option (A).

Polarity depends on molecular shape: if the bond dipoles cancel by symmetry the molecule is non-polar; if they do not, it is polar.

• SO2 is bent/angular (a lone pair on S), so the two S–O bond dipoles do not cancel → net dipole moment → POLAR.
• SO3 is trigonal planar (symmetric) → dipoles cancel → non-polar. …

In SO2 sulphur is +4 (an intermediate oxidation state), so it can be oxidised (to +6) or reduced (to lower states) — it acts as both an oxidising and a reducing agent.

An element in an intermediate oxidation state can go up or down, so its compound can behave both ways.

• In SO2, S is +4. It can be oxidised to +6 (e.g. SO2 → SO4^2-), acting as a reducing agent, or reduced to a lower state (e.g. SO2 → S), acting as an oxidising agent. …