Today’s lab involved a lot of stirring, stirring and more stirring as the students tried to take data to make a solubility curve for sodium carbonate in water. The lab, Session II-1: Solubility as a Function of Temperature is from the homescientist chemistry manual CK01A. There are three parts to the lab – 1) making the sodium carbonate by heating baking soda in the oven for 45 minutes, 2) calibrating a thermometer and 3) dissolving sodium carbonate in a beaker of water until saturated, change the temperature and add more sodium carbonate. Since we only have two hours for class I made the sodium carbonate ahead of time by spreading baking soda on a cookie sheet and heating it at 450F for 45 minutes (stirring it every 15 minutes). The students all checked their thermometers in ice water and boiling water and found the thermometers were off a degree or two but it wasn’t quite linear so they couldn’t just had a degree to all there measurements, but it did show them that they can’t always assume their equipment is accurate.
The students started with 200 ml of water near zero degrees celsius in a beaker and had another beaker full of sodium carbonate. They measured the mass of the beaker and sodium carbonate so that after they had dissolved some in the water they could mass it again to find how much they had used. Even knowning they had to put in at least 10 grams to start this took forever to dissolve and involved a lot of stirring. It took so long that the water temperature rose almost 10 degrees so we put the beakers in ice baths to try to keep the temperature down. Some of the solute formed a hard crust on the bottom of the beakers and was very hard to break up and dissolve. We’re not sure why that happened. We did NOT use distilled water, which we should have but I don’t think that would have caused that. When they finally got a saturated solution they would put the beaker of water on a burner and raise the temperature roughly 10 degrees and then add more sodium carbonate til it was saturated again. I haven’t seen the graph of the results yet since the students were still taking data when class ended, trying to get at least one point past 35 degrees celsius where the curve should start leveling off. If I was to do this again with a class, I might have each group do a different temperature and then share the data instead of each group trying to take all the data. Though if they did that they might not realize just how much the solubility depends on temperature – not having seen it themselves. It was pretty impressive how much sodium carbonate that you can dissolve in 200 ml of water, especially at higher temperatures and the lab shows that solubility depends strongly on temperature.