FSC Chemistry: Liquids, questions with answers

Answer: A. Osmosis

Water moves into the bone's protein/collagen matrix by osmosis, causing it to swell.

Answer: C. Ion dipole

An ion attracting the polar (dipolar) water molecules around it is an ion–dipole force, responsible for hydration of ions in solution.

Answer: A. CH₃OH

The liquid with the lowest boiling point has the weakest intermolecular forces and hence the highest vapour pressure at a given temperature — CH₃OH (65 °C).

C₂H₂: London dispersion forces (non-polar).
NH₃: hydrogen bonding (N-H).
HCl: dipole-dipole forces (polar covalent).
H₂O: hydrogen bonding (O-H, stronger than NH₃'s due to O's higher electronegativity).
Descending order of strength: H₂O > NH₃ > HCl > C₂H₂

1. Unusually high boiling and melting points compared to other molecules of similar molar mass (e.g. H₂S), because extra energy is needed to break the extensive hydrogen-bond network.
2. Ice is less dense than liquid water — hydrogen bonds hold water molecules in an open hexagonal lattice on freezing, so ice floats.
3. High surface tension and high specific heat capacity — the strong hydrogen-bonded network resists surface deformation and absorbs a lot of heat with little temperature rise.

(i) Petrol (a hydrocarbon mixture) molecules are held together only by weak London dispersion forces (no hydrogen bonding), so they escape into vapour much more easily than water molecules, which are held by strong hydrogen bonds — hence petrol evaporates first.
(ii) Water forms a more extensive hydrogen-bonded network (each molecule can form up to 4 H-bonds) than ethanol (whose bulky ethyl group limits hydrogen bonding), so fewer water molecules escape into the vapour phase at a given temperature — giving water a lower vapour pressure than ethanol.

Phenol and water are partially miscible liquids: below a certain temperature they form two separate layers — a phenol-rich layer and a water-rich layer — each saturated with a little of the other. As temperature rises, the mutual solubility of phenol and water increases and the two layers become more similar in composition. At a specific temperature, called the upper consolute (critical solution) temperature (~66°C for phenol–water), the two layers merge into a single homogeneous phase; above this temperature phenol and water are completely miscible in all proportions.

Crystalline solids have both long-range positional and orientational order (rigid, fixed lattice).
Pure (isotropic) liquids have no long-range positional or orientational order — molecules move and orient randomly.
Liquid crystals are an intermediate (mesophase) state: they retain some long-range orientational order like a solid (molecules point roughly the same way) but have no fixed positional order, so they can flow like a liquid while still showing solid-like optical properties (e.g. birefringence).