The labs we did this week all came from Chapter 5 of the American Chemical Society’s free Middle School Chemistry curriculum. The first activity involved cutting out sodium (Na) and chloride (Cl) ions and water molecules. The curriculum has a page of them to print, but before I printed them out for the students I went a head and colored them on the computer to save some time. Students cut them out and arranged the Na and Cl ions to form a salt crystal. I had used this activity a few years ago and still had a set that I had taped to little magnets so I could arrange the ions/molecules on my white board, so the kids could follow along with them. Once we had the NaCl crystal we ‘dissolved’ it in water by bringing the water molecules up to the crystal and the surrounding the different ions. The positive side (H+) of the water molecules are attracted to the Cl- ion and the negative (O-) side of the water molecules are attracted to the Na+ ions. I challenged the students to take the paper models home and make stop motion movies of the water molecules dissolving the NaCl. Some of the students taped the ‘dissolved’ Na and Cl ions surrounded by the polar water molecules into their notebooks.
The next activity in the curriculum asks the students to find out whether isopropyl alcohol or water would be better for dissolving salt. So we talked about what experiment we could do to figure that out. Should we use a lot of salt or a little? Should we use a liter of water or a smaller amount? Should we use the same amount of alcohol and water? Should we stir for the same amount of time? I wrote down what we came up with on a marker board and then the students did the experiment. They got two cups, labeled one water and one iso, then measured 5 grams of salt for each cup. They started a timer and poured 50 ml of water into one of the cups and started stirring. When all the salt was dissolved they wrote down the time. Then they did the same with the alcohol, but found salt would dissolve in the alcohol. I asked them what they thought about that and with some prodding realized the alcohol molecules are not as polar as water so they aren’t strong enough to pull apart the Na and Cl ions. I had them write down their procedures and results in their books.
We still had some time (guess I should have let them color the molecules themselves) so we did the next activity which was dissolving baking soda in water – does it dissolve as well as salt? They basically repeated the above experiment, 5 g of baking soda in 50 ml of water and found it didn’t dissolve as easily as the salt.
We then watched a couple of movies. I’m pretty sure I had already showed the Dogs teaching chemistry but since we’ve been talking about covalent and ionic bonds a lot I decided to show it again.
Then I showed Tyler Dewitt’s video on “What Happens When Stuff Dissolves?”