In a modified form, this experiment was given as the science experiment for Connecticut’s mandatory test for all sophomores. The task was to compare the effectiveness of table salt relative to rock salt in defreezing roads or sidewalks. Students could devise experiments to test their hypotheses by either using thermometers, weighing scales or stop watches as ways of measuring rates of melting ice. Students were not told that the colligative properties of both should in theory be the same. In proctoring the examination I was struck by how conflicting students results were. The more finely ground table salt, because of its greater surface area, presumably would have increased the initial rate of melting, but it depended in part how the experiment was conducted as to whether this was a significant factor. Possibly there were impurities in the rock salt that led to unexpected results.
By setting this experiment on colligative properties into the problem raised by the State introduces debate and a more interesting context. It also gives the students the opportunity to wholly design their experiments for themselves and to defend their results where there might be a variety of good answers. As an extension, students could demonstrate how ice causes weathering of roads and how hot summers can accelerate the process.