2 Why a dissolved non-volatile solute lowers the vapour pressure
Point 5 of 22 — Module 4 — Aqueous Solutions and Their Properties (Colligative Properties and Osmosis)Question bank: 0 questions (0 with pictures)
05 2.2 Why a dissolved non-volatile solute lowers the vapour pressure
In pure water, the surface molecules can escape freely (held back only by hydrogen bonding with their neighbours).
When a non-volatile substance is dissolved:
Some molecules or ions of the solute occupy part of the liquid surface, so the number of water molecules at the surface that are free to evaporate is reduced.
There are strong attractions between water molecules and the solute particles (ion–dipole attraction for salts, hydrogen bonds for polar molecules), in addition to the attractions among water molecules. More energy is needed to free a water molecule.
Consequently fewer water molecules escape, and the vapour pressure of the solution is lower than that of pure water at the same temperature.
The decrease is directly proportional to the number of solute particles (molecules or ions) in the solution.
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