Q.Formaldehyde polymerises to give :

Polymerisation of Aldehydes

Lower aldehydes readily undergo self-addition (polymerisation) because the polar carbonyl group, $\overset{\delta+}{C}=\overset{\delta-}{O}$, lets the carbon of one molecule be attacked by the oxygen of another, building up chains or rings of $-C-O-$ linkages. Ketones, having no carbonyl H and greater steric crowding, do not polymerise this way.

Formaldehyde ($HCHO$):

• With water/on evaporation it gives paraformaldehyde, a linear polymer $HO(CH_2O)_nH$ used as a disinfectant and a source of dry formaldehyde.
• On warming with dilute acid it forms the cyclic trimer trioxane, $(CH_2O)_3$.

Acetaldehyde ($CH_3CHO$):

• A trace of $H_2SO_4$ gives the cyclic trimer paraldehyde, $(CH_3CHO)_3$, a liquid once used as a hypnotic/sedative.
• At low temperature with acid it gives the cyclic tetramer metaldehyde, $(CH_3CHO)_4$, a solid sold as "meta" fuel and slug-killer. …