Complementary demonstrations
Variations on egg in the bottle — the gas laws
A. Egg in
Atmospheric pressure will push a hard-boiled egg through the neck of a 1 L Erlenmeyer flask. Show the class that the peeled egg is too large to fit through the neck of the flask. Soak a cotton ball in rubbing alcohol. Tell the students to watch the egg closely when you set it back on the Erlenmeyer. Then, holding the alcohol-soaked cotton ball with tongs in one hand and the egg in the other, light the cotton ball, drop it into the Erlenmeyer and quickly place the egg on the hole. The egg will jiggle several times as the heated air in the flask exerts a higher pressure, and some escapes, with the egg acting like a valve against the lip of the flask. The flame will go out from lack of oxygen, and as the air in the flask quickly cools back to room temperature, the reduced amount of air left in the flask will exert a lesser pressure and atmospheric pressure will push the egg into the flask.
B. Egg out
To get the egg out put a larger amount of air into the flask than was originally present, again using the egg as a one-way valve. Hold the flask upside down with the egg inside at the neck, put the lip of the flask up to your mouth as you would the mouthpiece of a tuba, and blow air into the flask. Simultaneously stop blowing and move the flask away from your mouth, and the egg will pop out due to the increased amount of air in the flask exerting a pressure greater than atmospheric pressure.
C. Egg in again
Place the egg on the lip of the flask again and place the flask in a container of ice water up to its neck. As the air is cooled the pressure of the constant amount of gas in the fixed volume decreases, and atmospheric pressure again pushes the egg into the flask. This is slower and easier to follow than the first method.
D. Egg out again
Suspend the flask upside down with the egg in the neck by setting it on a ring attached to a ring stand or a tripod. Using a high-temperature hair dryer or heat gun warm the body of the flask. The increased temperature increases the pressure exerted by the constant amount of gas in a constant volume. The egg will be pushed out.
E. Egg in once again
You will need a large mouth, (4 cm) 500 mL Nalgene CPE plastic wash bottle. If the bottle has a built-in squirt arm, block the nozzle with an eye dropper bulb. Squeeze the bottle to compress its volume. While holding it compressed, place a hard-boiled egg slightly larger than the mouth over the opening, holding it firmly in place until you release the sides. You may have to help the bottle resume its expanded shape, but when it does, the egg will be pushed into the bottle by atmospheric pressure because of the reduced internal pressure due to the expanded volume. An Ocean Spray cranberry juice or Gatorade plastic bottle (64 fluid ounces, 1.89 L) has a large enough mouth to be used for these pressure-volume experiments and may be compressed sufficiently even though they have ribs that tend to hold them somewhat rigidly in their expanded shape.
F. Egg out once again
Invert the wash bottle with the dropper bulb still blocking the squirt nozzle. Be sure the egg is in contact all around the inner mouth. Squeeze the bottle, and the decreased volume will increase the internal pressure to greater than atmospheric pressure and the egg will pop out.
Summation
Parts A and B illustrate the relationship between the amount (moles) of gas and the pressure it exerts in a constant volume at constant temperature (room temperature, after the system has cooled back down). Parts C and D illustrate the Boyle’s Law relationship between the volume and the pressure of a constant amount of gas at a constant temperature.
I wish to thank Rubin Battino and teachers who have attended our demonstration workshops at Wright State for their comments and discussions, which stimulated the development of the variations (C, D, E and F) on the standard presentation (A and B).
