Explain aldol condensation and the acidity of the $\alpha$-carbon of aldehydes and ketones. Provide the aldol condensation reactions for $ethanal$ and $propanone$.

(N/A) $1$. $\alpha$-Acidity: The $\alpha$-hydrogen atoms in aldehydes and ketones are acidic due to the strong electron-withdrawing effect of the carbonyl group and the resonance stabilization of the resulting conjugate base (enolate ion).
$2$. Aldol Condensation: Aldehydes and ketones containing at least one $\alpha$-hydrogen undergo a reaction in the presence of dilute alkali to form $\beta$-hydroxy aldehydes (aldol) or $\beta$-hydroxy ketones (ketol).
$3$. $Ethanal$ $(CH_3CHO)$: $2CH_3CHO \xrightarrow{dil. NaOH} CH_3-CH(OH)-CH_2-CHO$ ($3$-hydroxybutanal).
$4$. $Propanone$ $(CH_3COCH_3)$: $2CH_3COCH_3 \xrightarrow{Ba(OH)_2} CH_3-C(OH)(CH_3)-CH_2-COCH_3$ ($4$-hydroxy$-4-$methylpentan$-2-$one).