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Question on the gas laws

  1. May 9, 2013 #1
    Hey guys. I'm new to the forum :). I'm a high school student who takes physical sciences as one of my subject. The thing is that I've been confused about something I read on chemistry. This might sound stupid but please try and understand that this is confusing me.

    In the gas laws, Gay Lussac's law states that: " The pressure of a fixed amount of gas at
    constant volume is proportional to its absolute temperature". Where in Boyle's law it states that: "The volume of a fixed amount of gas is proportional to its pressure at constant temperature".

    My question is that since the amount of pressure effects the amount of volume, How can the volume be kept at a constant when the temperature causes a change in pressure ( which in turn effects volume)?

    Also why is a direct relationship expressed as P/T and an inverse relationship expressed as PV=PV?
  2. jcsd
  3. May 9, 2013 #2


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    Staff: Mentor

    You can keep gas in a balloon (which will inflate or deflate) or in a rigid container, which will not change its volume.

    Please elaborate.
  4. May 9, 2013 #3
    Ah, I see. Thanks for the reply. I thought that it might be so, but I was confused by some other ideas. Like if the container has a hole in it, does that make it harder to increase the pressure inside the container itself?

    I think I'm beginning to get it now. Would this then be a logical explanation for why a balloon pops when exposed to a heat source: The gas molecules around the balloon have an increase in pressure due to increase in temperature, while the gas molecules inside the balloon have a lower pressure because of the lower temperature. Eventually the pressure outside is great enough to crush the inner pressure.

    I just read an explanation on wikipedia, so I think I'm fine on that. Thanks though.
  5. May 9, 2013 #4


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    Staff: Mentor

    If there is a hole, gas escapes, so you are no longer dealing with the same amount of gas.

    It is not clear to me what is the scenario you are trying to describe. If you heat the gas inside of the balloon, it expands till the balloon pops. If you heat the balloon surface locally, you make a hole and the balloon pops. If you put balloon in the warm air, it will slowly heat up and expand - till it pops.
  6. May 11, 2013 #5


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    Gold Member

    A relationship keeps the same ratio of variables. If one variable increases so does the other.

    An inverses relationship keeps the product of the variables constant, so if one variable goes up the other variable must decrease.

    This is basic mathematics.
    You should review your graphs of equations starting with y=mx +b ( direct relationship ), and continue on with inverse functions, logarithic, exponential, power, sine, etc. A specific "look" to a graph comes in handy when you are plotting data points from a chemistry or physics experiment, and you can then surmise the functioning equation between your variables of the experiment.
  7. May 12, 2013 #6
    *Sigh*, I hate the fact that my teachers are so crap. I have to pull through with self studying. Thanks for explaining this to me.

    Unfortunately, I wasn't really paying attention in class when we were discussing the straight line graph. Also the biggest problem that I have in school at the moment is that the teachers never explain properly, you can clearly see that some of them don't know their stuff. I'll have a look on it though, I found the internet to be bliss with multitudes of helpful tutorials.
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