In this lesson, students observe a demonstration of Gay-Lussac’s law. A hard-boiled egg is forced into an Erlenmeyer flask by cooling the air inside to alter the air pressure inside the flask. You will discuss the effects of flexible and rigid containers on the pressure, volume, and temperature of a gas sample.
Prepare hard-boiled eggs the night before class. You need only one egg per class if all goes well, but it is useful to have a few extra in case one breaks. Place a bit of water in the flask and the shelled egg on top of the opening before students walk in.
Before students enter the class, have the egg and bottle set up in front and the following prompt on the board,
"Examine the egg and bottle setup. How could you use gas pressure to get the egg into the bottle? What variables would you change: pressure, volume, and/or temperature? Draw an image of the egg/bottle set up and annotate your drawing to support your ideas."
Give students 8-10 minutes think, write/draw and then call on students to share their ideas on how to get the egg in the bottle. Some questions you may ask to push student thinking could be:
Demonstrate how differences in gas pressure can be used to force an egg into a bottle.
Add 5ml of water to the flask and heat the flask on the hot plate for several minutes. Be sure to add enough water so it does not all evaporate. Take the flask off of the hot plate with beaker tongs when you see steam coming out of the flask.
Place the hard-boiled egg on the opening of the flask so it makes a seal. Observe what happens as the air inside the flask cools (the egg gets pushed into the flask).
You can speed up the process by placing the flask in cool water or ice water.
Ask students to explain what they think happened.
Ask students how they could use gas pressure to get the egg back out of the flask. Listen to their suggestions. Then turn the flask upside down so that the egg falls into the opening. Hold the flask so that it is tipped sideways, and reheat the flask on the bottom until the egg is pushed back out of the flask. A Bunsen burner works best for this last procedure, but you can use a hot plate or a blow dryer.
Ask your students to explain what is happening. Have them explain what you think caused the egg to go into the flask. The egg was pushed, not sucked, into the flask. Explain what this means.
As the gas inside the flask cools, the pressure of the gas inside the flask decreases. Because the air pressure decreases inside the flask, the force on the egg exerted by the air outside the flask is greater than the force exerted on the egg by the air inside the flask. When the difference becomes great enough, the egg is pushed into the flask. The change in gas pressure happens both because gas pressure is proportional to temperature and because some of the water vapor in the flask becomes a liquid. Notice that the egg is not “sucked” into the flask. It is pushed into the flask by the higher air pressure outside the flask. Gaseous molecules cannot “suck.”
A key point to stress here is that pressure is proportional to temperature if the volume and amount of gas are not changed and if the temperature is expressed in kelvins. This is Gay-Lussac's law.
Give students some an opportunity to practice working with the formula for Gay-Lussac's Law.
Hand out the student worksheet and work together to solve the first few problems as a class before turing them loose tot work independently.
Below is a great video that explains the steps in working through the formula that you can share with your students.