[FiendishFrog]

Could someone please explain the concept of this to me?

An unknown compound is suspected to be an acetanilide (mp 113.5º C-114º C). What would happen to the melting point if this unknown were mixed with
a)   an equal quantity of pure acetanilide?
b)   an equal quantity of benzoic acid?

melting point of pure acetanilide= 113.5º C-114º C
melting point of pure benzoic acid= 121.5º C-122º C

How would we know if it goes up or down? Please help- thanks.

[Yggdrasil]

In general, the melting point of a mixture of two or more compounds will be lower than the melting point of its constituents.

[FiendishFrog]

So even if you added the pure acetanilide to the unknown compound, the melting point would go down a little?
And then if you added the benzoic acid, even though the melting point is higher for that one, it would still go down?

Sorry to be a bother.

[Yggdrasil]

A good way to test whether you have the compound that you think you have is though a mixed melting point test.  You think your compound is acetanilide.  If you mix your unknown compound with pure acetanilide, you still have a pure sample of acetanilide (assuming your unknown doesn't have many contaminants) that will melt at the same temperature as acetanilide.  However, if your unknown is not acetanilide, then the mixed melting point test will show that the melting point of the mixture decreases, indicating that the unknown and your test sample are not the same compound.

[FiendishFrog]

Ahh, thank you so much, it does make a little more sense. I am just curious about the benzoic acid, due to the fact that it has a higher melting point. From what I gathered from this conversation, the melting point of the mixture would still decrease if mixed with it- is this correct?

[Yggdrasil]

Yes.  Unless you have a mixture that is mostly benzoic acid, the melting point should be below the lowest melting point.