Hydrogen bonding plays a central role in the following phenomena:

a) Ice floats in water:
- When water freezes, the hydrogen bonds between water molecules become more ordered and stable.
- The water molecules arrange themselves in a hexagonal lattice structure, with each water molecule forming four hydrogen bonds with its neighboring molecules.
- These hydrogen bonds create an open, hexagonal structure, which causes the solid ice to be less dense than the liquid water.
- As a result, ice floats in water because it is less dense.

b) Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions:
- Primary amines contain a nitrogen atom bonded to two hydrogen atoms and one alkyl group, while tertiary amines have three alkyl groups bonded to the nitrogen atom.
- In aqueous solutions, primary amines can form hydrogen bonds with water molecules through their lone pair of electrons on the nitrogen atom.
- These hydrogen bonds enhance the solubility of primary amines in water and increase their Lewis basicity.
- On the other hand, tertiary amines do not have a hydrogen atom directly bonded to the nitrogen atom, so they cannot form as many hydrogen bonds with water molecules.
- Therefore, the Lewis basicity of primary amines is higher than that of tertiary amines in aqueous solutions.

c) Formic acid is more acidic than acetic acid:
- Both formic acid and acetic acid contain a carboxylic acid group (-COOH) which can donate a proton (H+) in an acid-base reaction.
- However, formic acid is more acidic than acetic acid due to the presence of the hydrogen atom directly bonded to the carbonyl carbon in formic acid.
- The hydrogen atom in formic acid is more acidic because it is more easily ionizable due to the inductive effect of the electronegative oxygen atom in the carboxylic acid group.
- This inductive effect stabilizes the conjugate base of formic acid, resulting in a stronger acid compared to acetic acid.

d) Dimerization of acetic acid in benzene:
- Acetic acid can form dimers in non-polar solvents such as benzene due to hydrogen bonding between two acetic acid molecules.
- In the dimer, the oxygen atom of one acetic acid molecule forms a hydrogen bond with the hydrogen atom of the other acetic acid molecule.
- This hydrogen bonding interaction stabilizes the dimer and allows acetic acid to exist as dimers in benzene.
- The formation of these dimers is driven by the favorable energy associated with hydrogen bonding.
- Hydrogen bonding plays a central role in the dimerization of acetic acid in benzene.