The investigation of gases were what allowed scientists to first think about atoms and how atoms combine to make molecules as well as the determination of the mass of the atoms relative to each other. Scientists Amedeo Avogadro, Robert Boyle, and Jacques Charles were all deeply involved in helping to determine how gases behaved.
Materials:
• Large Syringe (no needle) – 60 mL is best but 35 mL syringe can be found at pet or farm supply stores
• Mini Marshmallows
Experiment:
Remove the plunger and place the marshmallow into the syringe. Notice the ease with which you can push the plunger into the syringe down to the marshmallow. Cap the end of the plunger with your finger and try to pull the plunger out. Is this difficult? What’s happening to the marshmallow? Remove your finger from the end of the syringe. What happened to the marshmallow? Pull the plunger out as far as it will go while still in the syringe, then cap the end of the syringe with your finger and push in on the plunger. Is this difficult? What’s happening to the marshmallow?
What’s happening?
The gas that we are referring to here is the air that is in the syringe and also in the marshmallow. When you first pushed the plunger in it was easy as the air could go out the tip of the syringe. Once you plugged the end and pulled on the syringe, you noticed that it became difficult to pull on the syringe because you were creating a vacuum (lower pressure) on the inside as the volume expanded with the same number of gas molecules. You also saw the marshmallow increase in size as the gas molecules that were on the inside of the marshmallow were pushing to get out of the marshmallow to escape.
This is because there was, in essence, a vacuum on the outside of the marshmallow. As you removed your finger from the tip, the marshmallow immediately shrunk in size as air rushed into the syringe alleviating the vacuum. The air molecules also returned to the inside of the marshmallow but the interior structure of the marshmallow had collapsed to a degree in the previous step when the air was being removed from the marshmallow. Finally, as the plunger was being pushed in with your finger over the tip, you noticed the marshmallow get smaller. The pressure inside the syringe was getting greater as the volume shrunk and the number of air molecules remained the same. With all the pressure on the outside of the marshmallow, the marshmallow decreased in size to take up less space. These manipulations show us that the pressure and volume of a gas are inversely related or that if the pressure of a gas goes up then the volume goes down. This is Boyle’s Law.
Extension:
Try blowing up a small water balloon so that it fits into the syringe and use the same procedures. Are there any changes? Try filling up the water balloon with water and do the same things. What’s different? Try filling the syringe with water instead of air and attempt the same manipulations. Can you explain these results?
References:
https://kids.nationalgeographic.com/books/article/marshmallow-madness
https://www.scientificamerican.com/article/in-and-out-demonstrating-boyles-law/
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
