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Boiling and melting point of impure substances

  1. Feb 9, 2012 #1
    They say that an impure substance has an increased boiling point and reduced melting point (i know that they will melt/boil at a range of temperature). But does impurity mean that its melting and boiling point is higher than the substance itself or it doesn't matter?

    I think that the impurity has to have a higher boiling. When we dissolve salt and water, the boiling point increases to about 117 degree celcius. when we say the boiling point increases, we take the water's 'perspective'. meaning we see it in water's point of view. however, if we look at salt's perspective, the boiling point has decreased. So i think that the impurity has to have a greater boiling point. Also, is this assumption correct: We take the perspective of the substance with the lower boiling point when an impurity is present. And the boiling point of that substance will increase. So using this assumption, when we pour ethanol and water into a beaker and heat it up, the boiling point will be around 85 degrees? As, i take the perspective of ethanol being the lower boiling point substance.

    I also think that the impurity has to have a higher melting point. When salt is dissolved in water and frozen then melted, the melting point of the substance decreases to around -5 degrees. Again, we take the perspective of the substance with the lower melting point. Water's melting point is 0 degrees while salt has a large boiling point. so when they say impurities decrease melting point, they are taking the perspective of water. So is this assumption right: We take the perspective of the substance with the lower melting point when an impurity is present. And the melting point will further decrease from this. So, using this assumption when alloys are made the impurity will be the metal/non-metal with a higher melting point and thus the other metal will have a lower melting point. So, i look at the lower melting point metal and decrease it further. This seems true since alloys have a lower melting point than the metals in it.

    thanks for the help!
     
    Last edited: Feb 10, 2012
  2. jcsd
  3. Feb 10, 2012 #2

    Borek

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    Staff: Mentor

    It can be even more complicated than that. Statement "impure substance has a higher/lower boiling/melting point" is only an approximation, and it works exactly only when the impurity has much, much lower vapor pressure than the main component of the mixture. Hence it works OK for a salt in water, but in the case of water/ethanol mixture it becomes much more problematic.
     
  4. Feb 10, 2012 #3
    oh ok. i also read somewhere that if a volatile substance is placed into water (for example) then the boiling point will drop again taking into perspective or the water. this makes sense as in all my impurities case, i was taking the impurity to be non volatile with a high boiling point.

    so are impurities in these case considered to be non volatile with high boiling and melting point?

    thanks for the help!
     
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