I notice that my last entry ends with this ...
For reasons I'll explain in my next post, if my product had not been the same compound as the salicylic acid from benzene, even if the melting points were nearly identical, the tube with the mixture would melt at a lower temperature and over a broad range, as opposed to melting all at once. Since my product was pure, all three samples melted sharply at 160°C.What? Why did I say I'd explain that in my next post? I think I knew that I would go on hiatus with this project and put that there as a way of punishing myself or something. Well, I suppose I could explain this. Using pictures would help, but I'm going to try to do it without pictures because I like words so much and also because I'm lazy. Here we go...
The melting of a crystal involves the excitation of the particles forming the crystalline structure. In the case of organic compounds, the particles are organic molecules and the crystal is a specifically shaped arrangement of these molecules based on the intermolecular forces at work in the molecules. Now, the reason I say that I am punishing myself by explaining this now is that I haven't yet done a post on intermolecular forces yet. It's not a particularly difficult topic, but rather than going into the details, I'll just simplify and state that intermolecular forces cause molecules to interface with one another. This has a direct effect on the phases of matter (solid, liquid, gas, and the others). A crystal is solid because the intermolecular forces are strong enough to make the molecules stick together in the crystalline pattern. Adding heat excites these molecules and the intermolecular forces are no longer strong enough to hold them together, so the molecules slide against each other and bump around making a big mess or something.
The change in phase from solid crystal to liquid mess is, as you hopefully already know, referred to by the scientific term "melting." The precise temperature at which a pure crystal of a given compound melts is referred to as the compound's melting point. Different compounds, because they have different intermolecular forces based on their structures, have different melting points. So let's say we have Compound A with a melting point of 160°C (that's 320°F for those of you who still don't know the Celsius scale).
We have a sample that we think is Compound A. We can melt it and see that it melts at 160°C just like Compound A should. But now let's say that this sample is actually Compound B. Just by luck, Compound B, although made of completely different molecules than Compound A, happens to also have a melting point around 160°C. If we do a proper melting point analysis, we won't be fooled. Here's why.
We take some of Compound A and some of Compound B, mix them together, stir the powder up a bit and put some of our A/B mixture into a melting point sample tube (a thin tube made of glass, like I desribed in the last post), then use the melting point apparatus. As noted, this is a mixture. We don't have a pure, uniform crystal of one molecule, but pockets of two different molecules jammed up against each other. They interfere with each other's intermolecular forces and rather than a sharp melting point at 160°C, we'll probably see regions of softening and melting appear over a broad range of temperatures well below 160°C (because some areas will have a more even mixture meaning little crystalline structure while other regions might be isolated pockets of nearly pure A or B).
This technique is called a mixed melting point. Makes sense, right?
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