The last couple of classes involved dissolving solids (salt and sugar) in liquids, but today we dissolved liquids in water. All the activities we did today are from Chapter 5, lesson 7 in the American Chemistry Society’s Middle School Chemistry curriculum. First I asked the students how did we know when the salt or sugar dissolved in water? How do we know when something doesn’t dissolve? They came up with the answers pretty quick – things that don’t dissolve stay separate, float on top or sink to the bottom of the cup, while dissolved things spread out uniformly in the solution. I demonstrated this with food
coloring in a glass of water to show that liquids (food coloring) can also dissolve in water. Then we discussed how we should go about testing if other liquids would dissolve in water. We tested mineral oil, isopropyl alcohol and corn syrup so we needed three labeled plastic cups and filled them half way with room temperature water. Students used graduated cylinders to measure 15 ml of isopropyl alcohol and then poured it into a cup of water to see what happened. They wrote down their observations then stirred it to see if they could get it to dissolve, then watch to see if they separate. They did this for all three liquids and filled out a chart with their observations.
For the second activity I put some water and a few drops of blue food coloring in one cup and isopropyl alcohol (91%) with yellow food coloring in another cup. Students took disposable pipettes and put 5 drops of blue water on a paper plate (one with wax coating) to form one big blue drop and 5 drops of yellow alcohol next to it but not touching. Then they took a toothpick and dragged the yellow alcohol til it touched the blue water and watched it wiggle and jiggle til the whole thing turns green. This is very cool to watch and I let the kids do it over and over again. The lab handout says they do not have a clear understanding of what is causing the wiggling interaction, it might be the different densities of the two liquids, the differences in surface tension, or maybe even an increase in molecular motion because the solution gets a bit warmer when the two mix. Anyway, here’s a slow motion video I took of the experiment.
Lastly I filled two graduated cylinders, one with 50 ml of water and one with 50 ml of isopropyl alcohol. I asked the students what would happen if I poured one into the other… how much liquid should I have? Everyone says 100 ml, but when you pour one into the other you actually end up with only 95 ml total. So where did the missing liquid go? I asked them what would happen if I had 50ml of marbles in one cylinder and 50ml of sand in the second and then mixed them and they quickly understood that the sand would fill in the gaps between the marbles. I told them the alcohol and water did the same thing. They are very different sized molecules and they can mix like the marbles and sand, taking up less total space.
With the time we had left we watched the following youtube videos: