Science Struck
Science Struck: A Comprehensive List of All the Physics Formulas
Provides a long list of physics formulas for easy reference.
Khan Academy
Khan Academy: What Is the Ideal Gas Law?
Learn how pressure, volume, temperature, and the amount of a gas are related to each other.
Wolfram Research
Wolfram Science World: Law of Combining Volumes
This site contains informaiton on the law of combining volumes. A formula is given and links to key terms.
Other
The Science House: Floating Candles
In this experiment students observe a combustion reaction and deduce the components necessary for the reaction to occur. They also see the relationship between pressure, volume, and number of molecules for gases.
Khan Academy
Khan Academy: What Is the Maxwell Boltzmann Distribution?
In a gas, there are lots of molecules traveling at lots of different speeds. Here's a framework for thinking about that.