Teaching students to make colorful solutions
In my AP Chem class, I was teaching about solutions. My students learned about solutions, molarity and dilutions (M1V1 = M2V2) during the first year of Chemistry. In AP Chemistry, we review the calculations at the beginning of the unit, before working on solubility rules, net ionic equations, and redox reactions. I had used a PhET simulation as an introductory inquiry activity for this unit and I had a net ionic lab planned for the end of the unit. I just keep thinking that I wanted something else. Something besides working practice problems on paper.
So, I had an idea. I wanted my students to make solutions. Normally, I make solutions for labs before class, to save on time. I had been thinking that it would good for my students to know how to properly make a solution and be able to describe the process. Not only for possibly having to write about on the AP exam, but I also want them to be prepared for college. I felt that this could be a great skill to have for college labs.
My idea was to give the students an ionic compound, with a specific molarity and volume, and have them make the solution. Then, they could make a dilution of their solution. In class, I had gone through the basic steps of making a solution, but I did not give them a set procedure. Instead, I wanted them to write out the procedure. I would be available for questions and clarification, but I wanted them to think through the process and carry out their steps. Sounds great, right?
But…I had a dilemma. After thinking about and writing out the lab, I had that little voice in my head that decided to bring up all the things that could go wrong. I have roughly 40 students and if they were in groups of 2 or 3, that is 13-20 lab groups. That would be 13-20 solutions being made, each one requiring a certain amount of ionic solid. Those chemicals cost money. I did not have anything planned for the solutions, so basically, they would just go to waste. Not all ionic compounds can go do the sink, so then I had to think about disposal. For a lab that I wanted to be a quick activity for ½ a class period, there was now a lot of prep involved. Finding ionic compounds that cheap and easy to dispose, not to mention making sure I had all the equipment ready to go.
As I started pulling chemicals from our chemical storage area, I noticed something. All the compounds I gathered were white crystalline solids and they were going to make colorless boring solutions. Now, I know that I don’t teach 1st graders, but even my 16, 17, and 18-year-old students like things that are colorful and pretty. (I mean, that’s one of the reasons why we color the periodic table and love doing the cabbage juice acid-base lab.) That little voice started again… ugh, just another boring lab and there’s not even a cool chemical reaction happening. OK…how to make it colorful???
Colorful, cheap, easy to dispose…
When I was in middle school (about 1994), I was helping at my church’s vacation Bible school. My job was to help in arts and crafts. I lived in a small town and the nearest craft store (Hobby Lobby) was about 40 miles away. We also had a limited budget, so when the craft activity was to make sand art, the lady in charge of arts and crafts had to be creative. She needed about 8 different colors of sand but didn’t really have the money to buy colored sand, just for kids to spill it as they tried to put colorful layers into a baby food jar. Her solution to the problem came to my mind as I was standing in our chemical storage room.
Iodized salt, food coloring and a microwave
I went and purchased 2 containers of salt (54 cents each) and I already had food coloring. I poured about ½ cup of salt in a glass bowl and placed it in the microwave for approximately 20 seconds. Then, I put 5-6 drops of food coloring in the salt and started stirring. The salt is warm, so it makes it easier, but you do have to use a spoon to break up the clumps of color. Keep stirring and add more color if you want it darker. I placed the colored salt in some screw top tall glass jars and labeled them with names of soluble ionic compounds. I know that the colors aren’t correct, but I knew my students didn’t know what color these compounds should be. The only color they probably remember is copper (II) sulfate, because every time we use it in class, I talk about how it’s my favorite!
I had to “SELL IT”… I knew if the lab was going to be something they would remember and help them make connections, I had to make sure to make it “authentic.” They know that many of the ionic compounds we use in class are crystals. I told them that to make sure it didn’t take too long, I had used a mortar and pestle to crush the samples. I even picked one or two of the groups and told them that they had to wear gloves while making the solution 😊
Results: My students really enjoyed this activity. Overall, they did a great job of making their solutions. They did have specific questions about procedure and a few of them made some mistakes, but I really believe they learned something in the end. (One group placed their solid into a graduated cylinder, then tried to add 50 mL of water. They came to me with a very puzzled look and said, “but now the solution is above the 50 mL mark.” I don’t think they will make that mistake again!) Each group put their solution and dilution in a screw top glass bottle (for some reason my school has a ton of these) and I took this opportunity to talk about the importance of labeling and what to include on a label.
Did the color really make a difference? I think so. It really didn’t take me that long to make the colored salt. In the end, I loved hearing my students talk about how cool it was to see the correlation between color and concentration. That seems like such an easy concept and something that should be common sense, but it is not for every student. Hearing students discuss and make connections on their own is so worth every minute of prep on my part.
Adjustments: You could tell your students that it is salt and that this is just a practice, that is obviously your preference. Also, I had my students use graduated cylinders to make the solutions, because we do not have a ton of volumetric flasks. I discussed volumetric flasks with my students and explained the accuracy of them, but due to limited quantities and sizes I chose to give them different equipment.
If you would like to see the sheet I gave my students, Click to download pdf