Problem # 53 BP of water in a solution [Solved]

You are at sea level and you are boiling water in which a spoon of sugar has been dissolved. Question: Do you expect that solution to boil at: 100^oC, <100^oC, or >100^oC? Justify your answer.

Answer: Water in that solution, or any aqueous solution of a non-volatile solute, will boil at a higher temperature than 100^oC. As said in previous problems, the boiling temperature is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure. But we know that the vapor pressure of the liquid is formed by the molecules coming from the surface of the liquid. In this case, the surface of the liquid solution contains molecules of water and molecules or ions from the solute; in other words the number of water molecules, or fraction of water molecules, on the surface of the liquid solution is smaller than on the surface of pure water. That is why it will require more heating or higher temperature than 100^oC for the solution to form water vapor pressure equal to the atmospheric pressure.