A student says: "A solute makes the solvent boil at a higher temperature because the solute itself…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student says: "A solute makes the solvent boil at a higher temperature because the solute itself is harder to boil." Is she right? Use the vapour pressure to explain the rise of the boiling point, and to explain why the freezing point falls.
Need a reminder? The key ideaShow
A colligative property depends on the number of solute particles. For a dilute solution, and , where is in mol/kg. For water, and . The molar mass is , with the masses in grams.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The claim. The student is wrong. The solute does not need to be volatile, and the effect appears even when it is not. It also depends only on the number of particles.
Step 2. The boiling point. A liquid boils when its vapour pressure equals the outside pressure. The dissolved solute lowers the vapour pressure at every temperature. So at the usual boiling point the solution is below the outside pressure. We must heat it further, and the boiling point rises.
Step 3. The freezing point. The solid and the liquid are in balance when their vapour pressures are equal. Only the pure solvent freezes out, and its vapour pressure curve does not change. The solution curve is lower, so it meets the solid curve at a lower temperature. The freezing point falls.
Answer: both effects come from the lowering of the vapour pressure by the solute. The size of the effect depends on the number of solute particles.
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