MadSci Network: Physics

Re: What happens to gases as they are heated and cooled?

Date: Thu Nov 4 19:08:07 2004
Posted By: Jeff Yap, Physics Teacher
Area of science: Physics
ID: 1097590979.Ph

Dear Jelly,  (Cool name!)

Great question!  This concept is very important in chemistry and physics, 
and you will see it appear in almost every science class you will take.

The temperature of an object is basically a measure of the movement of 
the individual molecules.  When you heat something up, you're adding 
energy to the molecules, making them bounce around faster and faster.  If 
you imagine a room full of dancers, gently swaying to something slow and 
mellow, they won't bump up against the walls very often, and if they do, 
they won't hit very hard.  But if you make the music faster and more 
energetic, they'll start dancing faster, bouncing off each other more, 
and running into the walls more often and harder.  The dancers are the 
gas molecules, and the energy of the music is the temperature of the 
gas.  Heating up a gas will make it push against the sides of its 
container harder.  This will do one of two things.  It will either make 
the container expand and get larger (like a balloon) or it will increase 
the pressure inside the container (like a kettle).  These are known 
respectively as Charles's Law and Gay-Lussac's Law.

Cooling does the exact opposite of heating.  It makes the gas molecules 
move slower, so they don't hit the sides as often or as hard.  The 
pressure will go down and/or the volume will go down.  The 
pressure/volume/temperature/amount of gas relationship can be summed up 
in a single rule, known as the Ideal Gas Law.  This law combines Charles' 
Law, Gay-Lussac's Law, and Boyle's Law (Pressure vs. Volume).  You can 
look this law up on your own, but I like to call it the "piv-nert" law.

I hope this helps!  Keep asking good questions!

Jeff Yap
Mad Scientist


Cal State Sacramento 
Chemistry Department

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