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.
DWL