Today we did the Molarity Calculation Lab from Ian Guch’s 24 Lessons That Rocked the World.  Students were given a beaker of 2.0 M (2.0 molar ) solution of NaCl (salt) and asked to produce 1.0 grams of NaCl.  They had to calculate how much of the solution to put in a small beaker, that would produce 1.0 gram of salt after boiling away the water.  They also needed to estimate how much they might lose in the process of heating (salt sputtering out of container during heating) and adjust their initial amount of solution.  One group asked if they could use an evaporating dish with lid to keep from loosing product and another group choose a bigger beaker after watching the first group lose some salt to sputtering.    The groups got pretty good results and this was a nice short lab for the last class before break.

I also set up a hot ice demonstration with a supersaturated solution of sodium acetate.  I prepared the supersaturated solution the night before, heating 160g of sodium acetate with 30 ml of water in a beaker on a hot plate.  A supersaturated solution contains more solute then it really wants to, so any little perturbation can cause it to crystallize and unfortunately my solution did this right before class.  I reheated it but it kept crystalizing in the beaker or was still too warm to crystallize instantly when poured.  But we still had fun watching it crystallize and got some cool crystals.

Here are some videos showing how the demo is supposed to work.

And here are the videos I asked the students to watch before class:

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