Today’s lab came out of the HomeScientist’s chemistry manual CK01A, VIII-1:Observe the Pressure-Volume Relationship of Gases (Boyle’s Law). I’ve tried to do this lab before with the Boyle’s Law apparatus from Home Science Tools and have never gotten very good results (and it actually broke today) so I decided to give this other version a try. I’m pretty sure the one from Home Science Tools just needed more oil on the syringe plunger. Anyway, the Home Scientist’s version of this experiment uses a much smaller syringe (10mL) and you just have to lightly hold a 2L soda bottle upside down on the plunger to exert the pressure on the gas. The syringes come with caps so you can contain the gas in the syringe. Using the soda bottle lets you easily increase its mass by 200 gram increments just by adding 200 ml of water (density of water is 1 g/ml).
The actual experiment is relatively easy, lubricate syringe plunger (very important to do a good job on this step), find mass of empty bottle and syringe plunger, add water to bottle to get a total of 200 grams to put pressure on the gas volume in syringe, record volume of gas off syringe. Then add 200 ml of water to bottle, find volume of gas and repeat until soda bottle is full.
The complicated part of this lab is the data analysis. We’re recording the mass exerting a force on the gas not the pressure, so we have to calculate the pressure and add the atmospheric pressure because its also pushing down on the syringe plunger. I helped the younger kids in the class with these calculations and showed the older kids how to set up a spread sheet to make it easier. Then the students plotted their data, volume as a function of pressure and found their data was pretty close to a straight line, as expected, showing that as pressure on a gas increases (at constant temperature) the volume will decrease.
I recommended the students watch the following videos before class: