Comprehensive notes on Liquids, Intermolecular Forces (Dipole-Dipole, London Dispersion, H-Bonding), Physical Properties (Evaporation, Vapour Pressure, Boiling Point), Surface Tension, and Viscosity.
Forces of attraction between molecules. Dipole-Dipole Forces: Attraction between positive end of one polar molecule and negative end of another (e.g., HCl). London Dispersion Forces (LDF): Instantaneous dipole-induced dipole forces. Significant in non-polar molecules (e.g., He, Ne). Strength depends on Polarizability and Surface Area.
Electrostatic attraction between a highly electronegative atom (N, O, F) and a Hydrogen atom covalently bonded to another electronegative atom. Applications: High boiling point of H2O, Structure of DNA (Double Helix), Proteins, Floating of Ice (Open Cage Structure leads to lower density).
Evaporation: Spontaneous conversion of liquid to vapour at all temperatures. It causes cooling. Depends on Surface Area, Temperature, and IMF.
Vapour Pressure: Pressure exerted by vapours in equilibrium with liquid at a given temperature. Increases with Temperature. Weaker IMF → Higher Vapour Pressure.
Temperature at which Vapour Pressure equals External Atmospheric Pressure. External Pressure Effect: High altitude (lower pressure) → Lower B.P (Water boils at 98°C at Murree). Pressure Cooker (higher pressure) → Higher B.P (>100°C) cooks food faster.
Molar Heat of Vaporization (ΔHvap): Heat required to vaporize 1 mole of liquid at its B.P. (Water: 40.7 kJ/mol).
Molar Heat of Fusion (ΔHfus): Heat required to melt 1 mole of solid.
Molar Heat of Sublimation (ΔHsub): Heat absorbed when 1 mole of solid sublimes directly to vapour.
Surface Tension (γ): Force acting per unit length causing the surface to contract like a stretched membrane. Unit: N m-1. Decreases with Temperature.
Viscosity (η): Internal resistance to flow. Unit: Poise (P) or N s m-2. Stronger IMF → Higher Viscosity (Honey > Water). Temperature rise decreases viscosity of liquids.