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Thread: existence of molecules

  1. #1 existence of molecules 
    DCC
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    Does anyone know how the existence of molecules has been proved? I read somewhere that their existence had been proved before we were able to see them, but not how they did that. Is it possible to detect them without perceiving them? Are we still unable to see them now? What's more, how do we distinguish between different kinds of molecules?


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    Brassica oleracea Strange's Avatar
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    It was based on chemistry initially. It was found that compounds combined in certain fixed ratios and this could best be explained by them being grouped together. It took hundreds of years to work out all the detail.

    There is a summary here: History of molecular theory - Wikipedia, the free encyclopedia (a bit brief and therefore it might be a bit confusing).

    The structure of many molecules was worked out using X-rays to determine the structure of crystals.

    Molecules can be visualized in a variety of ways. For example, this is an image of a pentacene molecule made with an atomic force microscope: Scientists Image the 'Anatomy' of a Molecule (w/ Video)

    Some molecules an be pretty large; for example, the human DNA molecule is about 3m long.


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  4. #3  
    DCC
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    Thanks, Strange.

    Avogadro's law surprises me, to be honest. The models of molecules that I've seen vary enormously in size and complexity. Some consist of single atoms while others consist of whole bulks of them. But according to his law, the size of the molecules involved is not a factor that determines the number of molecules you'll find in a specific volume of space. If the temperature and pressure is the same in two areas with equal volume, then they will contain the same amount of molecules, no matter the size of these molecules. Is this because the spaces between molecules are so much bigger than the molecules themselves, that the size of the molecules doesn't make a difference?

    Anyway, if that's what they found, then that's what they found. I do see that this allows you to start distinguishing between kinds of molecules without even seeing them. If you fill the same volume of space with different gases, and you keep the temperature and pressure at the same level, then differences in the weight of the volume show you differences in the weight of molecules making up the gases.
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