Q.What is the effect on the boiling point of water if 1 mole of methyl alcohol is added to 1 dm³ of water? Why?
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Start your 14-day free trial to unlock the full solution →Methyl alcohol is volatile, so instead of lowering the vapour pressure (as a nonvolatile solute would), it raises the total vapour pressure — and the boiling point of the water therefore falls, not rises.
Step 1. The whole boiling-point-elevation argument of section 2.8 rests on the solute being nonvolatile: such a solute contributes nothing to the vapour, lowers the solvent's vapour pressure, and thereby raises the boiling point.
Step 2. Methyl alcohol (methanol) is not such a solute — it is a volatile liquid, more volatile than water itself (boiling point about 65 ⁰C against water's 100 ⁰C), so it evaporates readily and contributes its own large partial pressure to the vapour above the mixture. …
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