Gravimetric Analysis: Is it worth the wait?

Waiting for the precipitate to settle!

One of the sixteen recommended labs in the AP Chemistry curriculum is a gravimetric analysis (GA) experiment using a metal carbonate to analyze the amount of calcium in hard water.  I read over the lab handout in the kit (Gravimetric Analysis of Calcium and Hard Water- Advanced Inquiry Laboratory Kit) and decided that I needed a lab that would be quicker than the guided inquiry version.  So I went to my "go-to" lab manual (Laboratory Experiments for Advanced Placement Chemistry, Guided-Inquiry Edition - Teacher Edition), which has all the classic AP Chemistry labs in the more traditional format, with detailed procedures and data charts to fill in.   I found the companion GA lab, but this time the metal carbonate is the unknown and the calcium carbonate product is used to determine the identity of the original alkali carbonate.  This version of the GA lab seemed like a nice and quick approach to teach my students the GA technique.

Waiting and more waiting! 

So we started the lab on a Tuesday, when we meet for a single block.  Each group received a sample of an alkali carbonate (either potassium carbonate or sodium carbonate) that they would identify using the GA technique.  The first step in the experiment was to dry the solid in a crucible because the alkali carbonates are hygroscopic.  Yes, you heard me, we dried the solid first.  So on the first day we heated and massed, then heated and massed some more, and then again.  By the end of the period everyone had dried, massed, and then dissolved their unknown alkali metals.  As the bell was ringing each group added the calcium chloride solution and barely had time to notice the precipitate that formed.

These two are very proud of their white precipitate.

On day two, the kids came in to see that their white calcium carbonate precipitate had settled nicely to the bottom of the beaker.  Now they had to decant and filter their solid.  Once again, the progress was slow and frustratingly tedious.  To get good results, we had to use quantitative filter paper, which by the way, filters really slowly.  Oh, did you remember to measure the mass of the filter paper before you started?!  Even AP students forget the basic steps, which is why they need to do these experiments so they can learn how to think like a chemist.  The kids decanted off the supernatant liquid into a waste beaker and started filtering the precipitate.  Everything was going along smoothly, but just very slow!  The bell rang on day two as the students rushed to get their wet filter paper into the drying oven and get out the door for their next class.

Here are the solids drying in the oven.

Day three started with a buzz in the air as the students opened the drying oven to see their products.  The white powdery solids were thoroughly dried and ready for the final mass.  The rush to get the solids out of the oven and onto the balance may have put us into a bit of a frenzy.  That's when tragedy happened:  one of my kids dropped his filter paper on the floor.  The calcium carbonate powder scattered into a fine dust all over the floor.  The boy was devastated.  He was on the verge of tears as he swept up the solid and loaded it into a weighing boat for a last-ditch effort to get an answer to their unknown identification.  His partner was incredibly kind even though he had just lost two days of lab work on the floor.  The excitement was replaced with a hush throughout the room and a "walking on eggshells" feeling.  None of the other groups wanted to lose their product on the floor; they were carrying the filter paper around like it held precious gems.  Day three ended with all the groups huddled together crunching the numbers, trying to figure out which solid they started with on Day One.

Is it over yet?  No!  On day four, the groups turned in their results and their identification of their

An analytical balance would be better, but we got good results.

alkali metal.  Everyone got it right, except for the group who dropped the product on the floor.  The experiment certainly wasn't much quicker than the guided inquiry version in the kit.  Was it worth the time?  My answer is yes.  How can you learn to think like a chemist if you don't do chemistry?  And, the lab accident taught us all to take good care of our experiments.  We all learned an important lesson about patience and careful lab technique along with the technique of GA.