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The atmosphere and purity?

  1. May 24, 2009 #1
    Hi, I am not sure if this is the correct section for this question but it seems to involve both physics and chemistry so I will start here.

    In a recent conversation a friend stated that no substance within the earth's atmosphere can be 100 percent pure. It needs to be in space or some other kind of vacuum.

    Is this true? I don't know enough about physics or chemistry to refute the claim but it didn't sound correct. The context was in a discussion about deet bug spray. The aerosol spray can stated the contents were 99.9% deet. Then my friend stated the above as the reason why it could not have been 100% deet.

    Thanks for any help.
  2. jcsd
  3. May 24, 2009 #2


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    The bug spray presumably contained a propellant, so it could not be 100%. What is the meaning of pure in the general context? The atmosphere, at least in some places, is 100% atmosphere.
  4. May 25, 2009 #3
    Hey, thanks for the reply mathman.

    That's what I presumed regarding the spray as well, some sort of vehicle for the deet.

    But as far as the general statement, the way I understood his statement was that even an element such as gold or calcium, etc... cannot be "pure," that within our atmosphere their will be "contaminates", such as other elements or compounds, within or between the molecules of a substance, although not chemically bonded.
  5. May 25, 2009 #4


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    The DEET is probably the result of a nervous lawyer. To a scientist 100% DEET would mean between 100-99.5%, just as 100.0% would mean between 100.00-99.95%, but in court someone might claim that 100% DEET meant it contained DEET and nothing else. Since it's impossible that an entire factory full of the chemical didn't contain one molecule of some dirt or oil from a pipe or tank you can never claim it is 100%.
    There's a similar effect with alcohol, ethanol absorbs water from the air so you can never have 100% alcohol in an open container.

    There is also a problem with radioactive contamination, the atmosphere contains radioactivity left over from nuclear weapons tests in the 50-60. Any material exposed to the atmosphere will pick up some trace of radioactivity and any metal made today will have radioactivity mixed into it from being exposed to the air when it is melted. There is a thriving market recycling 100 year old lead coffins and even brass and copper salvaged from warships sunk in 1919 - this uncontaminated metal is then carefully stored in sterile conditions and used to make sensitive radiation detectors.
  6. May 25, 2009 #5
    The radiation contamination is an interesting point and example.

    It's interesting that the contamination is enough to make unexposed metal profitable enough for a market to develop for it.
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