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Question
Explain why on addition of 1 mol of NaCl to 1 litre of water, the boiling point of water increases, while addition of 1 mol of methyl alcohol to one litre of water decreases its boiling point.
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Solution
• Boiling point of aliquid depends on the vapur pressure of the liquid as compared to amspheric pressure. Lesser the vapour pressure higher would be boiling point of a liquid or vice-versa, at a fixed atmosphere pressure.
• NaCl is a non-volatile solute, therefore, addition of NaCl to water lowers the vapour pressure of water. As a result, boiling point of water increases. Methyl alcohol on the other hand is more volatile than water, therefore its addition increases, the total vapour pressure over the solution and a decrease in boiling point of water results.
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