18 March 2004

This is a scene from last Saturday's chemistry lab, in which the students quantitatively separated the three components of a salt, sand, and ammonium chloride mixture. First, they weigh the mixture, then heat it so that the ammonium chloride sublimes away. Weighing the residue allows for calculating the percentage of ammonium chloride in the mixture. Next, they add water to the residue and filter out the sand. After they dry out the filtered sand, they can determine the percentage of sand. If they heat the salt water filtrate, and weigh the salt that's left behind after the water evaporates, they can determine the percentage of salt. If they're really thinking about it, they can actually skip the last step, which is the most uncertain anyway, because the salt is prone to spatter out as the water is being evaporated; they can determine the percentage salt simply by subtracting the other two percentages from 100%. I'll be grading the labs later tonight to see how close they got to the percentages determined by the prep lab technicians, who made up the unknown mixtures.



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