Role of Hydrogen Bonding in Chemical and Physical Phenomena

To determine which phenomena rely essentially on hydrogen bonding, we analyze each option:

1. Option (A): Ice floats on water In the solid state (ice), each water molecule is tetrahedrally bonded to four neighboring water molecules via intermolecular hydrogen bonds. This forms an open, hexagonal, cage-like crystal lattice containing empty interstitial spaces. Upon melting into liquid water, some of these hydrogen bonds break, allowing water molecules to collapse into the cavities and pack more closely together. Consequently, the density of ice is less than that of liquid water, causing ice to float. Thus, hydrogen bonding plays an essential role.

2. Option (B): Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions In aqueous solution, the basicity of amines depends not only on the electron-donating inductive ($+I$) effect of alkyl groups, but also on the hydration energy (solvation) of the resulting ammonium cations: $\text{RNH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{RNH}_3^+ + \text{OH}^-$ $\text{R}_3\text{N} + \text{H}_2\text{O} \rightleftharpoons \text{R}_3\text{NH}^+ + \text{OH}^-$ The conjugate acid of a primary amine, $\text{RNH}_3^+$, has three protons available on the positively charged nitrogen atom to form three strong intermolecular hydrogen bonds with water molecules. In contrast, the conjugate acid of a tertiary amine, $\text{R}_3\text{NH}^+$, has only one proton available for hydrogen bonding with water (along with greater steric hindrance around the nitrogen). The greater stabilization of $\text{RNH}_3^+$ via hydrogen bonding shifts the equilibrium to the right, which accounts for primary amines having higher basicity than tertiary amines in an aqueous medium. Thus, hydrogen bonding plays a central role.

3. Option (C): Formic acid is more acidic than acetic acid The difference in acidity between formic acid ($\text{HCOOH}$) and acetic acid ($\text{CH}_3\text{COOH}$) is primarily governed by electronic inductive effects. The methyl group ($-\text{CH}_3$) exerts an electron-donating $+I$ inductive effect, which destabilizes the conjugate base ($\text{CH}_3\text{COO}^-$) by increasing the negative charge density on the carboxylate group, making acetic acid a weaker acid than formic acid. Hydrogen bonding is not the determining factor for this difference.

4. Option (D): Dimerization of acetic acid in benzene In non-polar solvents such as benzene, acetic acid molecules associate to form a stable eight-membered cyclic dimer. This cyclic structure is formed and stabilized by two strong intermolecular hydrogen bonds between the carbonyl oxygen of one molecule and the hydroxyl group of the other: $2\,\text{CH}_3\text{COOH} \rightleftharpoons (\text{CH}_3\text{COOH})_2$ Thus, hydrogen bonding is responsible for the dimerization.

Therefore, hydrogen bonding plays an essential role in (A), (B), and (D).

Correct Answer: (A), (B), and (D)