Answer: Lower vapour pressure than expected from Raoult's law, due to stronger attractive forces between unlike molecules.
- A Vapour pressure that varies mainly with temperature and barely changes with composition
- B Lower vapour pressure than expected from Raoult's law, due to stronger attractive forces between unlike molecules
- C Vapour pressure close to that calculated from Raoult's law, with a small deviation either way
- D Higher vapour pressure than expected from Raoult's law, due to weaker attractive forces between unlike molecules
Correct answer: B. Lower vapour pressure than expected from Raoult's law, due to stronger attractive forces between unlike molecules
Explanation: Negative deviation occurs when solute-solvent attractive forces are stronger than the corresponding forces in the pure components, lowering the escaping tendency of molecules and hence the vapour pressure.
Adding a non-volatile solute lowers the solvent's vapour pressure at every temperature, which is the root cause behind all four colligative properties: it shifts the freezing point down and the boiling point up.
Concept context
Learn how substances dissolve and form mixtures. Covers concentration terms, colligative properties (boiling point elevation, freezing point depression, osmotic pressure), and ideal vs non-ideal solution behaviour.