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Impurities on melting point and boiling point of water

  1. Sep 3, 2011 #1
    When impurities was added to the water, it tends to increase the boiling point of the water to 102 degree celcius and lower the melting point of the water to -2 degree celcius! Why this happen? Is it because the impurities tends to absorb the heat supplied to boil the water causing it to take in more heat energy in oder to vapourize?
     
  2. jcsd
  3. Sep 4, 2011 #2
    For starters, you can raise the boiling point much higher than that. I believe the boiling point for brine, or salty water, is 117 degrees Celsius.

    Despite that, yes, that is my understanding of it. On the molecular level, the impurities can "absorb heat" and thus increase the average kinetic energy of the body without increasing the water.
     
  4. Sep 4, 2011 #3

    Borek

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    Impurities don't absorb heat. Even if they did, it would not explain anything - absorbing heat would slow down the process of getting to the boiling point, but it would not change the temperature at which liquid starts to boil.

    In the case of boiling point, they decrease partial pressure of water over the solution (compare Raoult's law), thus you need higher temperature for the vapor pressure to meet external pressure (which is BP definition).

    Similar thinking can be used to explain lowering of the melting point.
     
  5. Sep 4, 2011 #4
    As Borek said, they decrease the partial vapor pressure of water in the solution. A substance boils when the saturated vapor pressure of the liquid for a particular temperature matches the environment pressure.

    However, what guarantees us that as we lower the external pressure, the temperature gets lower is actually the Clausius - Clapeyron relation and the fact that liquids absorb heat in order to vaporize and that the volume of the gas is many times bigger than the volume of the liquid.

    Also, he made use of Raoult's Law, which is derived using thermodynamics and the chemical potentials of each component. So, in some sense, you were right when you said that the atoms/molecules of the soluble substance absorb heat.
     
  6. Sep 4, 2011 #5

    Borek

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    I am not sure what you refer to - we are talking about constant pressure case.

    I am afraid this is misleading.

    Main problem here is that people quite often don't understand the difference between temperature and heat. The line of thinking behind "other substances absorbing heat" is that as these substances absorb heat, you need higher temperature to get to the point where something happens. That's not true - you have to add more heat (which can mean longer heating), but you don't need higher temperature for that.

    Your statement just enforces these misconceptions.
     
  7. Sep 4, 2011 #6
    What guarantees that the vapor pressure of a substance should increase as we increase temperature? Cause that is surely not a postulate in Thermodynamics.

    So, what does
    'substances absorb heat'
    have to do with not differentiating temperature vs. energy. Surely
    'substances absorb temperature'
    is not correct.

    Also, we are dealing with processes in equilibrium. How fast we get there is unimportant. For that you need the concept of power instead of energy.
     
  8. Sep 4, 2011 #7

    Borek

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    Dick, you are missing the point. This is a pedagogical problem, not a physical one. For you it is obvious what is correct, for students it is often not, and your explanation - physically correct - doesn't help them understand why they are wrong.

    In my experience students often think in terms of "more heat equals higher temperature", and they conclude if something absorbs heat (in this case those dissolved substances) you need a higher temperature. Yes, this is wrong, but they don't know it, and they try to build their understanding of thermodynamics on this misconception. Telling them that in a way they are right you don't help them, because they read your confirmation as "my thinking that more heat equals higher temperature is correct".
     
  9. Sep 4, 2011 #8
    But telling someone that its because partial vapor pressure drops is definitely not the whole answer as well. Please explain how higher temperature increases vapor pressure.
     
  10. Sep 4, 2011 #9

    Borek

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    We have already derailed the thread, it doesn't make sense to confuse OP more.
     
  11. Sep 4, 2011 #10

    Ygggdrasil

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    I think its useful to consider the problem in terms of entropy. The boiling point of a liquid is defined as the temperature at which the liquid and gas phases are at equilibrium. Mathematically, this means that the change in free energy from going to liquid to gas is zero (ΔG = 0).

    Now, recall that ΔG is composed of two components: enthalpy (ΔH) and entropy (ΔS). They are related by the equation ΔG = ΔH - TΔS. Since ΔG = 0 at the boiling point, we can solve for the boiling point: T = ΔH/ΔS.

    What happens to this value when we add an impurity into the liquid phase? The change in enthalpy (ΔH) describes the amount of heat required to break the interactions between liquid molecules in order to become a gas. This value does not change much when you add an impurity into the water. The change in entropy (ΔS) describes the entropy gained when going from the much more ordered liquid phase to the much less ordered gas phase. Adding a (non-volatile) impurity to the liquid phase increases the entropy of the liquid without affecting the entropy of the gas; the end result is that ΔS smaller for an impure liquid than a pure liquid.

    Adding an impurity to the liquid phase causes ΔS to decrease without changing ΔH. As you can see from the equation above, this situation must cause the boiling point (T) to increase. Therefore, adding an impurity to water, will cause it to boil at a higher temperature.

    In essence, vaporization is a process that is driven by the increase in entropy associated with going from the liquid phase to the gas phase. By making the liquid phase more disordered, this gain in entropy becomes smaller, and vaporization becomes slightly less favorable.
     
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