How many words starting with E?
Every instructor will have one or two favourite demonstrations that they can adapt to a variety of student ages. In this article I would like to describe my favourite demonstration,1 which I have used many times for students in elementary grades, high school and all the way up to university engineering. I briefly described the demonstration a few years ago2 but would now like to expand upon that teaching note. Depending upon the grade level, the purpose is for the students to come up with as many words starting with the letter “E” to describe what they have observed during the course of the demonstration.
The necessary equipment includes a hot plate, two Florence flasks, two balloons, and some water. If you have a class of younger students, and ask them what is in the Florence flask before you add any water, the likely first response will be EMPTY. With luck, some of the students will disagree, and say that there is actually air in the container. This particular observation will become very important later.
You can now add a small amount of water to the first flask and ask what would happen if it was left on a table for a week. If the students respond by saying that there will be less water in the flask, then EVAPORATION must have taken place. You may now place a balloon on the top of the flask, and place it on the hot plate at a low setting. The balloon will slowly inflate, indicating that some of the water has moved from the liquid phase to the gas phase, resulting in the EXPANSION of the balloon. This EXCITING result will continue, and if the heating continues for too long, the balloon will likely EXPLODE. If you now remove the source of heating, the balloon will deflate; placing the flask back on the hot plate will re-inflate the balloon, and this process can be repeated many times. Younger students never tire of seeing the process repeated several times.
If you are dealing with older students in an introductory chemistry course, they will likely be able to describe the above process as being an example of an EQUILIBRIUM system. In order for the balloon to inflate, heat must be provided to the liquid water, meaning that the process can be labeled as ENDOTHERMIC. When the source of heat is removed, the process is reversed and must therefore be EXOTHERMIC. One may also assign the ENTHALPY changes for the equilibrium system, positive for the conversion of liquid water to steam, and negative for the conversion of steam back to liquid water. This release of a large amount of heat is the reason why a “steam burn” is so painful.
The signs for the ENTROPY changes for the equilibrium system may also be assigned; positive for the conversion of liquid water to steam, and negative for the conversion of steam back to liquid water. Many years ago,3 there was a proposal to extend this sign convention to a consideration of the work component. When heat is being applied to the liquid water the resulting steam causes the balloon to inflate; therefore, one could consider the process to be EXOWORKIC, and the sign for the work component would be negative. In this direction, work is flowing out of the system to inflate the balloon. The reverse process would then be labeled as ENDOWORKIC, with the sign of the work component being positive as work is flowing into the system.
You are now ready to repeat the demonstration with the second Florence flask. This time, allow the water to boil for a few minutes before placing the balloon on top of the flask. The balloon will again inflate, but this time when you remove the source of heat, the balloon will deflate and then re-inflate inside the Florence flask. This observation will cause most of the students to look very bewildered, until someone remembers that the flask originally contained air before any water was added. Boiling the water removes most of the air from the flask, and when the heat source is removed, the resulting partial vacuum allows the atmosphere to “push” the balloon into the flask. You can then put the flask back on the hot plate to show the students that the balloon can be inflated and deflated many times. The balloon will remain inside the Florence flask for several weeks until the balloon develops a tear. I would keep the flask on the side of my desk, a wonderful conversation starter when colleagues dropped by as they would ask, “how did you do that?”
Students will be amazed when the balloon goes into the flask.
If you are teaching the second half of a general chemistry course, you may now want to extend the discussion to include a consideration of Gibbs free ENERGY for the system.
ΔGo = ΔHo – TΔSo
For this phase change:
H2O(l) ⇌ H2O(g)
liquid water is favoured by ENTHALPY considerations, while steam is favoured by ENTROPY considerations. Whether the process as written is spontaneous or not will depend upon the temperature. At room temperature the liquid phase is favoured; therefore, the sign for ΔGo will be positive, and the process as written will be ENDERGONIC (non-spontaneous). As the temperature increases, eventually the gas phase becomes favoured; at this point the sign for ΔGo will be negative, and the process as written will be EXERGONIC (spontaneous). At 100 oC (the normal boiling point of water) the value of ΔGo will be zero.
I hope that instructors will find this demonstration useful. There are a total of 15 different words starting with the letter “E” that can be used to describe this phase change involving water in the liquid phase and the gas phase.
References
- Presented at the 42nd College Chemistry Conference, Halifax, NS, 2015.
- R.R. Perkins, Chem 13 News, 223, 26, 1993.
- B.D. Joshi, Journal of Chemical Education, 60, 1983,
page 895.
