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Homework Help: Acetone and Petrol- Evaporation Rate

  1. Jun 7, 2016 #1
    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

    Need a bit of help here: Which has higher rate of evaporation :- Acetone or Petrol?
    Last edited by a moderator: Jun 9, 2016
  2. jcsd
  3. Jun 7, 2016 #2


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    Staff: Mentor

    Welcome to the PF.

    When I Google the quoted phrase, this PF thread is the first hit on the list, but the other hits would seem to get you to an answer. Is this for schoolwork?
  4. Jun 9, 2016 #3
    Yeah this is for schoolwork. By the way I already Googled it but didn't got an answer anywhere.
  5. Jun 9, 2016 #4


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    Staff: Mentor

    We can't provide answers to schoolwork questions here (it's against the PF Rules).

    What have you found so far in your searching? You must have found information on evaporation rates of liquids...
  6. Jun 9, 2016 #5


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    There's plenty to find on Google about this! I'm not claiming that your assertion that you were unable to find anything is false, but the answers are there if you know how and where to look. You don't appear to know what petrol is. Assuming you DO know what the chemical structure is of acetone, the first step is to find out what petrol is. This should be obvious. At least it should have been obvious as soon as your first attempt to 'look it up' failed. Since I don't know what science/chemistry knowledge level you are at (and I don't know what you know about chemical compounds and chemical materials), I won't be more specific in my help.

    As background, the reason why a material is liquid at any given temperature is that there are molecule-to-molecule interactions which attract molecules (or atoms) to each other. You might have ionic interactions, but often the attractions are non-ionic (almost always true for pure organic liquids, almost). The non-ionic attractions between molecules (including single atoms, if they are not chemically (covalently) bonded to some other atom) are secondary forces such as hydrogen bonding, dipole-dipole interactions, and similar.

    To evaporate a molecule, you need to provide it with enough energy to break any of these secondary forces and to give it velocity to move away from the liquid. Giving a molecule velocity requires energy directly proportional to the molecule's mass. IOW, higher MW molecules boil (evaporate) at higher temperature (if the secondary bonding is equal). So, comparing something like acetone and methyethylketone, its virtually a sure bet that acetone, being lighter, evaporates faster. Or comparing acetone with tert-butane, you should know that since the difference is one has a O= bonded to the central C atom, and the other has a H3C- that acetone will be slower. (two reasons: one is that it's a bit higher MW, the other is that O is much more electronegative than C, so that the intermolecular (secondary) forces would be predicted to be a lot higher. (Generally alkane H-C bonds aren't considered to be polar).

    One last fact, technically a pure compound under specified conditions (surface area, gas/air flow across surface, temperature, humidity, gas composition) will have a single evaporation rate, while a mixture of compounds will have a mixture of evaporation rates, so that any measure of "the" evaporation rate of a mixture will depend critically on the exact process used to measure it. (Meaning it can vary between the quickest component and the slowest comonent of the mixture, depending on how you define it, even when the same 'specified conditions' are always used.) Realistically, 'evaporation rate' is the number you get from testing/measuring it, its very difficult for two labs using different techniques to get the same number.
    Last edited by a moderator: Jun 9, 2016
  7. Jun 9, 2016 #6


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    Homework Helper
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    You have no direct experience whatever of these substances?
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