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Unit 12

Phases and solutions

Why does ice water sit at 0 °C however hard you heat it? Why does water boil below 90 °C on the Tibetan plateau, why does dry ice never melt, and why does salt melt snow? This unit links heating curves, vapour pressure, phase diagrams and the colligative properties of solutions into one picture — every curve computed from measured data and thermodynamics.

7 lessons ≈ 195 min 0 of 7 lessons complete

  1. 1 Heating curves and the heat of phase change The burner keeps burning, yet ice water stays at 0 °C. Heat a block of ice all the way to steam and see where the energy goes: warming stretches, the melting plateau, and the long boiling plateau. 25 min
  2. 2 Vapour pressure and boiling Molecules leave and return across every liquid surface. Vapour pressure depends only on temperature; when it catches up with the outside pressure, the liquid boils — which is why water boils at 88 °C in Lhasa. 25 min
  3. 3 Phase diagrams: water and carbon dioxide One pressure–temperature map tells you the state of a substance under any conditions. Water’s melting line leans left, dry ice can only sublime at 1 atm, and beyond the critical point liquid and gas become one. 25 min
  4. 4 Forming solutions and solubility “Like dissolves like” is a trade of intermolecular forces. Solid solubility changes with temperature, and gas solubility follows Henry’s law — opening a soda bottle is a Henry’s-law experiment. 30 min
  5. 5 Colligative properties: freezing, boiling and osmosis The number of dissolved particles, not their kind, sets freezing-point depression, boiling-point elevation and osmotic pressure. See how antifreeze, road salt and seawater desalination use them — and why ions fall short of their full count. 30 min
  6. 6 Binary phase diagrams Why does distillation never give 100 % alcohol, and why does solder melt more easily than tin or lead? Put composition across: Raoult’s law draws the liquid–vapour lens, the lever rule gives the amounts of two phases, and azeotropes and eutectics explain the limits of distillation and the arrests of cooling curves. 30 min
  7. 7 Surfactants and micelles Soap and detergent molecules have a water-loving head and a water-shunning tail: they cover the surface first, then huddle into micelles above the critical micelle concentration. With the measurements compiled by the US National Bureau of Standards in 1971, see the CMC halve with each CH₂ and fall with added salt. 30 min