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In the chapter 28.6 of book "Concepts in thermal physics "(2nd edition) written by Stephen J. Blundell, there is a sentence says "Adding a small quantity of solute to a solvent increases the entropy of the solvent because the solute atoms are randomly located in the solvent. This means that there is a weaker tendency to form a gas (which would increase the solvent’s entropy) because the entropy of the solvent has been increased anyway. This results in an elevation of the boiling point. Similarly, this additional entropy opposes the tendency to freeze and the freezing point is depressed."

However, it looks to me that if there is a tendency to form a gas, then raising smaller temperature than the pure substance situation is able to trigger the phase change, which means the boiling point is lowered. Which part is wrong in my thinking?

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A weaker tendency to form a gas is saying "less likely" or "more difficult" to form a gas. The liquid's entropy increased, it is now at a lower energy. This is all saying it is more stabilized in the liquid relative to being in the gas, which means the vapor pressure is lower. It also means the boiling point is higher.

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  • $\begingroup$ Thank you very much for the answer. But I still have confusion in two points: 1. Why increasing entropy means the liquid is at a lower energy? I think adding solute increase the energy of solute; 2. Why lower vapor pressure is related to higher boiling point? I think lower vapor pressure corresponds to the lower boiling point. Could you please favor me a few further hints? $\endgroup$
    – 8cold8hot
    Commented Jun 2, 2017 at 10:29

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