Before xmas break we dissolved salt in water and showed how the polarity of water molecules makes it a great solvent. Today we continued those ideas but instead of salt, we dissolved sugar… the sugar coating of M&Ms to be exact. The labs can be found in the American Chemical Society’s Middle School Chemistry curriculum, Chapter 5, Lesson4: Why Does Water Dissolve Sugar? and Chapter 5, Lesson 6: Does Temperature Affect Dissolving?
The first lab involves placing a single M&M in a plastic cup and pouring just enough water in the cup to cover the candy… then observe for a minute or two. Almost immediately you see the colored coating dissolving and turning the water red (we used red M&Ms). I had printed out some large sugar molecules and put them on a white board and demonstrated how the water molecules attract the sugar molecules and separate them, but the sugar molecules STAY WHOLE, they don’t come apart like the salt (NaCl) does. Some of the kids actually said it before I demonstrated because they remembered the video I showed before with Tyler DeWitt.
The second part of the experiment is to determine if the sugar will dissolve as well in other liquids, so each group set up three cups with three different liquids, water, mineral oil and isopropyl alcohol. They swirled the cups for 30 seconds and then observed the candies. You can see in the photo below the liquid (water) in the middle cup has already turned pink showing that it dissolves sugar pretty easily. It took 5 to 10 minutes to see some difference with the alcohol and the mineral oil didn’t dissolve the sugar at all.
Students drew their results in their lab books. The candy in the water is black because all the sugar coating had dissolved leaving just the chocolate.
For the last exeriment the students had to design an experiment to determine if temperature affected the rate at which the sugar dissolved. We took three cups with very hot water, ice water and room temperature water and dropped in M&Ms.
You can see that the hot water, top cup, totally dissolved the sugar coating, while the room temperature cup on the right has dissolved all the red but the white coating is still covering the chocolate, and the cold water on the left still has plenty of red on the candy. So the students concluded that temperature is very important in determining how fast the sugar dissolves. I asked why that would be so, and many of them piped up that the molecules had more energy with higher temperature so they’re moving faster and can dissolve the sugar faster.
We still had some time left so I had them make a graph from data given in the lab handout on how much sugar and salt can dissolve in 100 ml of water. This activity had two goals, one – practice graphing data and two – see that the strong dependence on temperature for sugar is not necessarily the case for everything. The curve for salt is almost a straight line as it has a very little dependence on temperature while the curve for sugar increases pretty dramatically. I asked the students why we didn’t do the temperature lab with salt and they all answered that it wouldn’t have been as easy to see a difference.
When everyone finished their graph we ate the remaining M&Ms.