Experiences

Not being able to do the classes in the lab

R.T.

Romania

Chemistry

Teaching in class, Laboratory

Direct

The most frequent difficulty I have to face is not being able to do the classes in the lab. There is only one laboratory in the school and we are four teachers. It is true that sometimes the experiments are simple and can be done in a common classroom but the laboratory represents a different setting suitable for experiments, where sometimes teachers feel like a magician. For the following experiment you need a hard-boiled egg, a glass bottle, whose mouth is smaller than the eggs, a match and a piece of newspaper. Ask students to remove the shell from the egg, then set the egg on the mouth of the bottle; the egg does not fit through the mouth. Ask them to fold the piece of newspaper into a strip that can be dropped into the bottle and then light the match and use it to light the folded strip of paper. Ask students to remove the egg from the mouth of the bottle and drop the burning strip of paper into the bottle. Before the fire goes out, the egg is set back onto the mouth of the bottle. Within a few seconds the egg will squeeze through the mouth and into the bottle.
Why does the egg slide into the bottle, even though no one is pushing it? Because the pressure of the air is pushing it. Before the burning paper was put into the bottle, the pressure of the air inside the bottle was the same as outside the bottle. The burning paper, however, heats the air inside the bottle. This causes the air inside to expand. When the egg is placed on top of the bottle, it seals the bottle, and the fire eventually goes out. When the fire goes out, the air inside the bottle cools. As it cools, the air contracts, and the pressure of the air inside the bottle becomes less than the pressure outside. Then, the higher outside pressure pushes the egg into the bottle!