Q.Explain why the solubility of alcohols in water decreases steadily from methanol to higher members such as pentan-1-ol.

An alcohol dissolves in water by forming hydrogen bonds with water molecules through its $-\text{OH}$ group, effectively replacing the hydrogen bonds it would otherwise form with itself. Every alcohol has exactly one such $-\text{OH}$ group, so the hydrogen-bonding contribution to solubility stays roughly constant across the series, but the hydrocarbon part of the molecule -- which cannot hydrogen-bond with water at all, and which water's own hydrogen-bonded structure must be locally disrupted to accommodate -- grows steadily from methanol to pentan-1-ol and beyond. Past a certain chain length the large, non-polar hydrocarbon portion dominates the molecule's overall behaviour, so the molecule increasingly resembles an alkane (essentially insoluble in water) rather …

Compare the fixed hydrogen-bonding contribution of one OH group against the steadily growing non-polar h …

Describing the fall in solubility as due to a change in the OH group itself, rather than to the growing, non-hydrogen-bond …