Liquids and Solids

Inter-molecular forces in condensed phases

The cohesion of liquids and molecular solids comes from inter-molecular (van der Waals) forces, in increasing strength: London dispersion (instantaneous-dipole induced-dipole, present in everything), dipole-dipole (polar molecules), and hydrogen bonding (H bonded to N, O, or F). London forces scale with polarisability — bigger electron clouds give stronger forces, which is why I₂ is solid and F₂ is a gas at room temperature. Atkins Chapter 18 and FSc XI Chapter 4 give numerical scales: H-bonds are 5–40 kJ/mol, dipole-dipole 5–25 kJ/mol, dispersion 0.05–40 kJ/mol depending on size.

Hydrogen bonding — anomalies it explains

Water's boiling point of 100 °C is roughly 200 °C above the trend line set by H₂S, H₂Se, H₂Te. NH₃ (−33 °C) and HF (20 °C) show the same anomaly. Ice is less dense than liquid water because the open tetrahedral H-bond lattice has wide cavities; melting collapses it, raising density to a maximum at 4 °C. Hydrogen bonding also explains DNA base pairing (A=T two H-bonds, G≡C three) and the tertiary structure of proteins — Clayden Chapter 2 emphasises this for organic chemistry.

Vapour pressure, boiling, and the Clausius-Clapeyron equation

Vapour pressure rises exponentially with temperature: ln(P₂/P₁) = −(ΔH_vap/R)(1/T₂ − 1/T₁). Boiling occurs when vapour pressure equals external pressure. At a hill station with atmospheric pressure 0.7 atm, water boils at about 90 °C — why eggs cook slowly. ΔH_vap for water is 40.7 kJ/mol; Trouton's rule says ΔS_vap ≈ 88 J/mol·K for most non-associated liquids, but water's 109 J/mol·K reveals its hydrogen-bonded order in the liquid phase.

Crystal lattices and unit cells

Crystalline solids have long-range order. Seven crystal systems and 14 Bravais lattices cover all possibilities. Common cubic packings: simple cubic (Z = 1, 52% packing), body-centred cubic (Z = 2, 68%), face-centred cubic (Z = 4, 74%, equivalent to cubic close-packed). Hexagonal close-packed also achieves 74%. NaCl is fcc with Cl⁻ at corners and face centres, Na⁺ in octahedral holes; CsCl is simple cubic with Cs⁺ in the centre. Diamond is a covalent network with each C at the centre of a tetrahedron of carbons (sp³). Bragg's law nλ = 2d sin θ underpins X-ray crystallography.

Amorphous solids, liquid crystals, and phase diagrams

Amorphous solids (glass, rubber, plastics) lack long-range order; they soften over a temperature range rather than melting sharply. Liquid crystals — nematic, smectic, cholesteric — flow like liquids but retain partial molecular alignment, the basis of LCD displays. A phase diagram plots P vs T with regions of solid, liquid, and gas separated by coexistence curves; the triple point is where all three meet (water: 273.16 K, 4.58 mmHg) and the critical point ends the liquid-gas line. Above the critical point, supercritical fluids combine gas-like diffusivity with liquid-like density — a recurring MDCAT 'modern applications' question.