Understanding Pure Substances vs. Mixtures for Chemistry Students

[Larrytsai]

This might be a stupid question... but can anyone tell me the difference between Pure Substances and Mixtures?

I read the definition both in the book but when I look at my hwk problems they don't come to my head what's what. For example A metal that melts not fully at 300 degrees and fully melts at 440 degrees. I thought that it would be Pure substance because a compound is a pure substance. But its actually Homogeneous mixture.

Any help to help me understand the difference is appreciated

 

[MichaelXY]

Well let's take a look at brass, which is a metal. But it is homogenous mixture because it is a mixture of copper Cu and Zinc Zn.
Now if you had a chunk of copper Cu which is also a metal, then you would have a pure substance, ignoring any oxide.

 

[Larrytsai]

Well let's take a look at brass, which is a metal. But it is homogenous mixture because it is a mixture of copper Cu and Zinc Zn.
Now if you had a chunk of copper Cu which is also a metal, then you would have a pure substance, ignoring any oxide.

but isn't CuZn a compound. and a compound is a pure substance?

 

[MichaelXY]

Good question. Brass is not a compound, it is an alloy.
http://en.wikipedia.org/wiki/Alloy

I was trying to think of a better way to explain this, and the best I can come up with is that the Cu does not bond with the Zn, rather when both elements are melted the Cu atoms and the Zn atoms blend together, sort of like NaCl and H2O.
You don't end up with HCl or NaO, you still have H2O with Na and Cl ions, but if you evaporate the H20 the remaining component is NaCl.
I don't know if I said that well, but I hope it helps.