On the first day of school, it is a good idea to show a demonstration that gets students thinking and sets the stage for what the class is going to look like over the course of the year. In this video, I'll tell you about five demonstrations that you might want to show to your students on the first day of class. The idea is to give a preview of what is coming in the school year. Some of these demos are puzzles. The idea is that the students have to guess not only the outcome but develop a model that explains the phenomena. The topics covered are waves, thermodynamics, electromagnetism, and the nature of science. This video should be helpful for anyone who teaches any physical science. Although I personally am a physics teacher, I would do some of these demos with my 8th-grade physical science class.
Salt vs. Fresh Water Melting
Which will melt ice faster, salt water or fresh water? Ice will be placed into cups of water, as often is done, and begin to melt. That the water will not be stirred is an important consideration. The result is surprising both for the outcome and for the mechanism. The result is that the freshwater melts the ice faster. As the ice is melted it becomes cold fresh water. Cold fresh water has a higher density than regular water and so the cold water sinks – but this will not happen in the salt water. Salt water has a higher density than cold fresh water and that causes the cold water to stay on top. This acts as a refrigerator for the ice, only being bathed in cold water and prevents further melting. I owe this demo to Bernard Cleyet a fellow Physics technician from UC Santa Cruz, although the food dye illustration is an original contribution.