Making Sense of the Gas Laws

         
          You're sitting in front of the computer at the testing center.  You've been studying for weeks.  Your nerves are frayed.  You click Begin and the first question appears on the screen.  "Which of the following gas laws can be calculated only if temperature is constant?"  Your heartbeat quickens.  You know the answer is Boyle's law.  Wait!  Or is it Charles' law?  Gay-Lussac's?  The words swirl together in your mind as the back of your neck begins to feel hot and prickly.  You know the answer!  Which is it? 

          This is where memory aids can help.  There's no replacement for a solid understanding of the material, but when things get hairy, a few easy-to-remember mnemonics can get you out of scrape and back on track.

             Boyle's law, Charles' law, and Gay-Lussac's law are closely related rules that predict the relationship between the volume, pressure, and temperature of a gas.  Boyle's law states that at a constant temperature, the volume of a gas varies inversely with pressure.  According to Charles' law, the volume of a gas varies directly with the absolute temperature if the pressure is constant.  Gay-Lussac's law states that the pressure of a gas is directly proportional to the temperature at a fixed volume.  Is it all starting to sound the same yet?

          Let's start sorting out the facts we know.  All three gas laws involve exactly two of three possible variables:  volume, pressure, and temperature.  So, how do you remember which law uses which two variables?  Easy.  Just remember, 'Boyle's law doesn't boil and Charles in Charge was on TV'.  I use these mnemonics to remind myself that Boyle's law doesn't have temperature in the equation.  It only involves pressure and volume.  Remember the sitcom Charles in Charge?  That's right, with Scott Baio.  I didn't say that the show was any good, just the mnemonic.  The TV part of the mnemonic reminds me that Charles' law only involves temperature and pressure.

          Now, what about Gay-Lussac's law?  If Boyle's law uses pressure and volume and Charles' law uses temperature and volume, then what's left?  The only remaining combination is temperature and pressure, and that's exactly the two variables found in Gay-Lussac's equation.

If you own PACES version 2.1 and would like 10 FREE CCNA practice questions, you may download The Gas Laws expansion pack here.

 

"...Charles in Charge was on TV..."

Maybe all those sitcoms you watched growing up weren't wasted time after all.

         
         So, now that we can remember the variables in each equation, how do we remember each formula?  The key is to remember that we are calculating a change in the values.  That means each variable must appear twice--once on each side of the equation.  For example, Boyle's law is

P1V1 = P2V2

Charles law is

V1/T1 = V2/T2

and Gay-Lussac's law is

P1/T1 = P2/T2

          You may wonder why the formula for Boyle's law requires you to multiple the two values whereas Charles' and Gay-Lussac's law have you divide the values to get the answer.  That's because the gas laws state that pressure and volume are inversely related (Boyle's law) and that temperature and volume (Charles' law) and pressure and temperature (Gay-Lussac's law) are directly related.

          So, now you know that Boyle's law doesn't boil, Charles in Charge was on TV, Boyle's law is the only inverse (non-fraction) relationship, and direct relationships (Charles' and Gay-Lussac's laws) sit directly on top of each other.  All you need now is some practice using the mnemonics to identify the correct formula and calculate some values.  The Gas Law Expansion Pack is available absolutely FREE for users of PACES 2.1 and will give you several CCNA exam style questions to practice with.  Don't forget to check out the other free expansion packs available on our website while you're there!

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