Acetophenone when reacted with a base, $\mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa}$, yields a stable compound which has the structure

Key Idea : Aldehydes or ketones with $\alpha$-H atom, in presence of dilute base, undergoes aldol condensation to give $\beta$-hydroxy aldehyde or $\beta$-hydroxy ketone. On heating, aldols eliminate water molecule to form $\alpha, \beta$-unsaturated compound.
Note: When aldehydes condense with ketones, it is the $\alpha \cdot \mathrm{H}$ of ketone which takes part in condensation. This reaction is called crossed aldol condensation.