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Homework Help: Determining State of Matter for a Compound

  1. Jun 24, 2006 #1
    Recently, I have been studying for a chemistry placement exam for college. On the sample exam (which I have been studying with), one of the questions asks, "Which of the following substances is a gas at 20 degrees Celsius and one atmosphere of pressure?

    A.) O3

    B.) C

    C.) I2

    D.) Ca

    The answer to the question was O3, but I really don't know why. Is this some kind of given that needs to be memorized, or do I need to do some work to get to the answer? (I can reason out Ca though.)

    Thanks for your help,

  2. jcsd
  3. Jun 24, 2006 #2


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    This is what used to be called "descriptive chemistry" --- knowing a few things --- has fallen by the wayside in the textbook business. You'll get a little in Greenwood & Earnshaw, or Cotton & Wilkinson (upper level inorganic), but pretty much left to fend for yourself otherwise.
  4. Jun 24, 2006 #3
    So I'll have to "guess" if I see another question like this that I don't know on the real placement exam? :frown:
    Last edited: Jun 24, 2006
  5. Jun 24, 2006 #4
    You just need to show a little knowlage about what each substance is and a logical deduction of the answer, it wouldnt supprise me if the real marks in such a question are for explaining why you chose your answer rather than the answer itself.

    O3 is of course Ozone, and you can state a little about what it is and where its found ( including at photocopying machines, where it can be smelled as a strange "fresh air" type smell, which is one of those facts you pick up and dont seem to forget ). You can deduce that its likley to be a gas at thos conditions because its found in the atmosphere as a gas.

    Then you could eplain away the others by contradiction, solids or liquids under the stated conditions, giving examples.

    In most tests a periodic table is given, and you could also say a little about what information you can get from that to help add weight to your argument.

    Its entirley dependent on how much effort/reward there is for the question imo.
  6. Jun 24, 2006 #5


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    The question is basically asking which is a gas at near standard conditions (e.g. room temperature and pressure), you need to read into the question; that is think things through. Ozone should stick out right away since you know it is an actual earth gas. Niether carbon nor calcium makes sense, calcium gas consist of its ion constituents, while carbon conforms itself to tetravalent structures. I would have questioned I2 though it's not like chlorine gas in the sense that it is higher in molecular weight, and thus it may be a liquid at STP. However, I believe that it's an exception mentioned in almost all chemistry texts.
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