Chemistry · Solutions · NEET
Put a liquid in a closed bottle. Some molecules leave the liquid and become gas (vapour). After some time the number leaving equals the number coming back. At that point the vapour pushes on the walls with a fixed pressure. That fixed pressure is the vapour pressure. For a solution, it is the vapour pressure measured above the whole solution.
A non-volatile solute (it does not turn into vapour itself) sits on the surface of the liquid. It blocks some surface spots. So fewer solvent molecules can escape into the vapour. Fewer escaping molecules means less vapour, so the vapour pressure becomes lower than that of the pure solvent. This is why salt water evaporates slower than plain water.
It DECREASES it. Salt is a non-volatile solute. It lowers how many water molecules can leave the surface, so the vapour pressure drops. A common NEET mistake is to think it goes up. It always goes down for a non-volatile solute. Lower vapour pressure is also why the boiling point rises.
Now BOTH liquids give vapour. The total vapour pressure is the sum of each liquid's own partial pressure. Each partial pressure equals that pure liquid's vapour pressure times its mole fraction in the mix (Raoult's Law). So the total sits between the two pure values.
The more volatile liquid (higher pure vapour pressure) escapes more easily. So it makes up a bigger share of the vapour than of the liquid. Example: in a 1:1 benzene-toluene mix, benzene has higher vapour pressure, so the vapour has MORE benzene than toluene. NEET tests this exact idea.
A liquid boils when its vapour pressure equals the outside (atmospheric) pressure. If a solute lowers the vapour pressure, the solution must be heated more to reach atmospheric pressure. So lower vapour pressure means a HIGHER boiling point. This connects vapour pressure to elevation of boiling point, a colligative property NEET loves.
Which statement about the vapour over an ideal 1:1 molar mixture of benzene and toluene at 25 degrees C is correct? (Vapour pressures at 25 C: benzene C6H6 = 12.8 kPa, toluene C6H5CH3 = 3.85 kPa)
At 100 degrees C the vapour pressure of a solution of 6.5 g of a solute in 100 g water is 732 mm. If Kb = 0.52, the boiling point of this solution will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. A non-volatile solute cannot form vapour itself, and it blocks solvent molecules from leaving the surface. So the solution always has a lower vapour pressure than the pure solvent. The bigger the amount of solute, the bigger the drop.
A volatile solute (like benzene in toluene) adds its own vapour, so the total vapour pressure is a sum of two contributions. A non-volatile solute (like sugar or salt) adds no vapour and only lowers the solvent's vapour pressure.
Lowering of vapour pressure is the base for colligative properties like elevation of boiling point and depression of freezing point. NEET asks direct vapour-pressure questions and also questions that use this drop to find molar mass.
Study Raoult's Law next. It gives the exact formula that links vapour pressure to mole fraction, and it is used in almost every vapour pressure numerical in NEET.
