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impurities and vapor pressure & boiling point

Discussion in 'MCAT: Medical College Admissions Test' started by sendwich, May 4, 2004.

  1. sendwich

    sendwich you rock!
    10+ Year Member

    May 2, 2002
    Likes Received:
    why does the presence of impurities dissolved in solution lower the vapor pressure and increase the boiling point? i understand how you get this by using Raolt's law but i want to understand this more in the conceptual context.

    as for boiling point, i thought since you're adding things (ie disrupting the bonds molecules are making with each other), it becomes EASIER to break the bonds, hence the DECREASE in boiling point. For example, add salt into pure water, and boiling point goes down, right? is my thinking wacked?

  2. ASDIC

    ASDIC The 9th Flotilla
    15+ Year Member

    Apr 7, 2003
    Likes Received:
    Medical Student
  3. daffy

    daffy Member
    7+ Year Member

    Apr 16, 2002
    Likes Received:
    Hi GH,

    this is how I look at it ? to bring a soup to a boiling will take longer than just pure water. The soup represents different particles in the water (being the solvent). This tells us that the soup has higher boiling point than just pure water.
    On the other hand boiling point and vapor pressure are related ? in order for a liquid to get to boil the vapor pressure above it has to be equal to the atmospheric pressure. Because it is taking longer to bring to boiling that must mean that the vapor pressure above the liquid (in our example the soup) is lower than just pure water. Both vapor pressure and boiling point are dependent on number of particles (the more carrots you put in the soup the longer it will take to get boiling), also it is not dependent of the nature of the particles (the boiling point will change accordingly if you put the same number of carrots or potatoes). The interaction between the water molecules and the carrots will not allow that many water molecules to escape the liquid phase and contribute to the vapor phase, this respectively will lower the vapor pressure and raise the boiling point. Hope this helps. Best of luck
  4. ChemMN

    ChemMN Member
    5+ Year Member

    May 5, 2004
    Likes Received:

    Although the solvation interaction between the solvent and the solute molecules does contribute to elevating the boiling point of a solution, entropy is the main cause. When a pure liquid boils, the entropy of the system increases (liquid ---> gas). If you now introduce a nonvolatile solute into the solution, the solvent molecules still gain entropy upon escaping the solution, however now the solute molecules (salts for ex) become more concentrated and therefore lose some entropy. The overall increase in entropy for the boiling of the solution is therefore less than that for boiling the pure solvent. Using the equation dG=dH-TdS you can see that a higher temp is required to make the process spontaneous.

    The simple answer to your question is that boiling a solution of a nonvolatile solvent is less entropically favorable than for a pure liquid.

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