Continuing along in the Middle School Chemistry curriculum, today we did Chapter 6, Lesson 3: Forming a Precipitate. The lab starts out with a demonstration that forms a precipitate, which is a solid that forms from a chemical reaction. In this case, you mix some epson salts (magnesium sulfate) with water and some sodium carbonate with water in a separate cup. Stir both until the solute is dissolved then slowly pour the sodium
carbonate solution into the magnesium sulfate solution. A white solid forms, looking a lot like snow and it drifts down to the bottom of the cup (see photo above). In class we discussed how this was pretty clear sign that a chemical reaction had taken place.
For the lab the students got to make their own precipitate with calcium chloride (DampRid purchased from Lowes) and sodium bicarbonate (baking soda). They used a triple beam balance or the pocket scale to measure out 2 grams of each into small paper cups. Graduated cylinders were used to measure 20 ml of water into 2 clear plastic cups. All the cups were labeled with sharpies and the calcium chloride and baking soda were each poured into their own plastic cup and the students swirled them around until they were mostly dissolved. You can see there is still a little bit of baking soda in the bottom of that cup in the photo above, so when the students pour that cup into the calcium chloride cup they try to leave the residual baking soda behind. When they combined the liquids they observed bubbles and a white precipitate formed.
Students put the combined solution through a coffee filter to separate out the precipitate. The lab shows the students the chemical formula for this reaction and the products as calcium carbonate, sodium chloride, water and carbon dioxide. I asked the students which of these products was causing the bubbles and most guessed correctly that it was the carbon dioxide. Then I asked which product was the precipitate and that took a little more discussion because they know salt is a white powder but I reminded them that salt dissolves easily in water and there was no reason for it to precipatate out, which left the calcium carbonate (chalk!) as our precipitate.
This lab has the instructions for one more demonstration. Before class I made a copper II sulfate solution, which is a very pretty blue, and poured some of it into a test tube. To create a precipitate I used a disposable pipette to drop ammonia (10-20 drops), into the test tube. This formed a light blue precipitate which stayed on the top of the liquid and then a darker blue liquid layer formed above that. I also dropped in some hydrogen peroxide which made the dark blue layer even darker and occassionally brown, but I didn’t really see another precipitate which the lab said should form.
We had some time at the end of class so we watched these videos on youtube:
Crash Course Chemistry – this was way over their heads for some stuff but they asked to watch it.
We watched the first 10-15 minutes of the Royal Institute’s video on Fireworks. Its over an hour long but I figured if I got them hooked they would finish watching it at home.