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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: xangelofxdeathx on March 31, 2007, 01:42:17 AM

Title: What happens to the Boiling point when you mix 2 substances with different BP's?
Post by: xangelofxdeathx on March 31, 2007, 01:42:17 AM
Two pure organic compounds melt at 112 degrees Celsius and 114 Degrees Celsius, respectively. If equal quantities of them are mixed, at what temperature will the mixture begin to melt?

A) below 112 C
B) at 112 C
C) between 112 C and 114 C
D) above 114 C

Can someone briefly summarize what happens to the boiling point when you mix 2 substances with different boiling points? I looked for it in my AP Chem book but couldn't find anything about this process.

The answer is A by the way.

This is my theory:
Since 2 organic compounds would be soluble with each other when mixed, their intermolecular forces would decrease when they are mixed together to form a new, lower intermolecular force mixture. Since this new mixture's intermolecular force is less than either of the original intermolecular forces, the Boiling point should be below both of the original boiling points.

Can someone confirm if my theory is correct or not? Again, I couldn't find this in the book and I haven't learned about it yet so I just need someone who knows the answer to let me know if it's right  ;)

Title: Re: What happens to the Boiling point when you mix 2 substances with different B
Post by: allanf on March 31, 2007, 12:47:15 PM
The question says melting point not boiling point BTW.

It is actually very relevant to make the distinction between boiling points and melting points.  There are quite a lot of things that can happen with the boiling points of binary mixtures, which depend greatly on what the two compounds are and in what concentration.  For example the compounds could boil independently from one another, or form maximum or minimum boiling azeotropes.

As for melting point on the other hand, you're right that it will be depressed.  One explanation that I've heard is that because the two compounds are of different shapes (and size) the crystal structure of the mixture will not be as tightly packed as for each of the pure compounds.  They just don't stack together as well.  This irregular packing reduces the overall intermolecular forces keeping the solid together, and it melts at a lower melting point.  Which is basically what you said, but referring to solids instead of liquids.   ;)

Keep in mind, though, that his explanation hinges on the two substances being different enough in shape (and size).  When talking about metals, for example, this isn't always the case and things get more complicated.   Metals can form what are sometimes called substitution alloys where the melting point can actually be much higher (Copper-Nickel alloys can do this, for example).  They can also form eutectics where the melting point is much lower (an example of an eutectic would be solder, which is tin and lead I think)