[rookie]

ok i just started taking chemistry this year, and we're still at the fundamentals, so please don't laugh at my stupidity....but i did come across a question in my hwk, which i really need help on.

K so if a student checks the volume, melting point, and shape of two unlabeled samples of matter and finds that the measurements are identical, can he conclude that that the samples have the same chemical composition? And lets say he concludes that they have the same composition,  what other additional information might he collect to test this conclusion?
   thanks,
- rookie

[constant thinker]

In some cases a chromatgraph test could help.

[mike]

What does the volume and shape mean? Do you mane they have the same density?

Melting point is a good indicator that they could be the same. ALthough I am pretty sure that these tests you have described are in no way conclusive.

There are loads of tests you could do, if you mean without instruments, mixed melting point, solubility, pH, flame test, density. There would be a numbe rof chemical tests you could do depending on what you suspected the compound to be.

[mike]

If you wanted to know for certian and you have access to equipment, crystal structure, elemental analysis, NMR, chromatography etc are all good.

[rookie]

hmmm can u put that in like "easy terms" like i said, i just started and i don't know much about chemistry, i think that maybe though the person can't ultimately find out what the chemical composition is because he is just testing the physical properties, which maybe some chemicals do have in common, but not necassirly mean that they have the same chemical composition... am i going in the right direction?

[mike]

Yes two different compounds could have the same melting point (and other physical properties in common) so you would have to do lots of tests to narrow it down.

[rookie]

so  because the melting point, volume, and shape of these two samples of matter are the same, that is good evidence that they may have the same chemical composition, but further testing is needed to be sure... hmmm is that correct?

[mike]

Well I still don't know what you mean by shape, but yes there is a chance that they are the same, but you definetly could not be certain.

[rookie]

how would mixing melting points tell me if they are the same?

[jdurg]

Okay, volume and shape are two extensive properties of matter.  They cannot be used to identify what something is because those values change depending on the amount of a substance.  I can take some silly-putty and shape it into a knife, and the take some iron metal and shape that into a knife.  While the substances are completely different, their volume and shape would be the same.  You can't use extensive properties to identify a substance.  You can only use extensive properties to identify how MUCH of a substance you have.

The melting point of a substance is an INTENSIVE property.  Intensive properties don't change no matter how much of a substance you have.  You can have ten pounds of lead, or two grams of lead, and they would both have the same melting point.  If you have a bunch of different metals and are told which metals you may possibly have, then you can use the melting point to differentiate between the metals.  Now if you are given an unknown sample and told to identify it, then you need a bunch more INTENSIVE properties in order to conclusively identify it.  

Remember, INTENSIVE properties are the same no matter how much of a unique substance you have.  EXTENSIVE properties change depending on the amount of the unique substance you have.

[rookie]

oh ok that makes alot of sense    ok sorry im gonna bother u with one more question... let's say that these two samples have the same chemical composition...what other intensive properties can i use to further prove my conclusion?

[Donaldson Tan]

you might want to carry out a chemical test to see if they yield similar results.

if the substance in question is a metal, take equal mass of both sample and pour excess dilute HCl on it. Observe for effervescence of hydrogen gas. Measure the volume of hydrogen gas evolved.

If the volume of hydrogen gas is the same for both samples, and both sample exhibit the same melting point, the same density, there is a good chance they r really the same substance.

[pharm_chick]

hey guess who

[Mitch]

Carmen D. Craciun?