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Why is melting point of an impure substance lower while its boiling point is higher compared to a pure one?

Daniel - 2008-10-17 04:05:47 - Chemistry

So, yeah why is the melting point of an impure substance lower than a pure substance while the boiling point of an impure substance higher than a pure substance? Please don't use too complicated terms, because I'm only in secondary 4, that would be about grade 10 in America. Thanks.

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Best Answer:

If you add an impurity to a solid, the impurity disrupts the formation of the lattice of the solid. It makes it weaker, so it is easier to break. Therefore, less energy is required to break the lattice and this leads to a lower melting point. Now, if you melt the mixture and then heat it until near the boiling point...for a liquid to boil, the molecules of the liquid must gain enough energy to escape into the vapour phase, and enough of them must do so to make the pressure of the vapour equal to the pressure of the surrounding atmosphere. An impurity acts as a hindrance to these molecules that are trying to escape. It's a bit like you trying to run at a certain speed. If some pain in the neck is holding you back, you need to exert yourself more to reach that certain speed. Well, for an impure liquid to boil, the molecules need more energy than is normal in order to escape from the liquid and become vapour. Hence, more heat is needed and this explains why the boiling point of an impure liquid is higher than the pure liquid.

Answers:

(As far as I can gather) Your problem is that an impure substance is exactly that--impure. You can only judge the boiling point of a substance if that substance is pure, any impurities would skew your results.

If you add an impurity to a solid, the impurity disrupts the formation of the lattice of the solid. It makes it weaker, so it is easier to break. Therefore, less energy is required to break the lattice and this leads to a lower melting point. Now, if you melt the mixture and then heat it until near the boiling point...for a liquid to boil, the molecules of the liquid must gain enough energy to escape into the vapour phase, and enough of them must do so to make the pressure of the vapour equal to the pressure of the surrounding atmosphere. An impurity acts as a hindrance to these molecules that are trying to escape. It's a bit like you trying to run at a certain speed. If some pain in the neck is holding you back, you need to exert yourself more to reach that certain speed. Well, for an impure liquid to boil, the molecules need more energy than is normal in order to escape from the liquid and become vapour. Hence, more heat is needed and this explains why the boiling point of an impure liquid is higher than the pure liquid.

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