If I had to pick a favorite unit of study in high school chemistry, it’s definitely Solutions. I find that the math isn’t so bad. The students are sort of used to these equations and can use them with ease by this point in the year. So it’s a lot less of a battle, and we can really dive into and enjoy the chemistry. There are a lot of fun labs in chemistry, including the very famous Freezing Point Depression Lab. But this molarity lab activity is a ton of fun!
Molarity Background
When a chemist makes a solution, it’s important for them to know “how much stuff is in their other stuff.” Or really, how much solute is in the solvent (usually water). But terms like “dilute” and “concentrated” are relative terms. They can change depending on what other solutions show up to the lab bench. A solution that looks concentrated can all of a sudden seem dilute if a deeper, richer in color solution shows up.
Chemists make a point to quantify what is dissolved in the water of their solutions. Sometimes they do it as a percentage by mass or volume. Other times, especially when the concentration is small, they work in parts per million. But the most common is molarity – the amount of solute in moles, dissolved in liters of solution.
The reason for using molarity is that it is the fastest way to compare the concentrations of solutions where the solutes have vastly different masses. For example, 1M NaCl vs, 1M CuSO4. The molar mass of NaCl is 58 grams and CuSO4 is 155 grams. But if you have one mole of each of them in one liter of solution, the number of ions in each solution is equal (since they appear in a 1:1 ratio).
Set Up
In this molarity lab activity, students are going to make solutions of Koolaid with different conentrations. They’ll have to do some calculations to figure out how much sugar is used to make each solution. I like to allow my students to drink the solutions they make, so in that case, they would need to have food safe measuring tools. I have my students use my food safe balances, fresh weigh boats and paper cups.
Really if everything is food safe, it’s a lot of fun and a really simple lab. Plus it’s delicious so it tricks your students into doing chemistry and molarity calculations they don’t really want to do!
My Tips
Using a Different Solute
If your intention is to use the materials in your stockroom, I suggest using CuSO4 as the salt. (Obviously in this case, they can’t taste the chemistry.) That’s because it’ll create a colored solution (thanks to the transition metal). Plus it’s not tooooo ugly of a heavy metal. I also like having this on hand for precipitation reactions, since sulfates make pretty good precipitates.
Importance of the Visual
I think the really important part about this lab is the concentration of the color of the solution. The visual is really going to help your students commit the importance of concentration to memory. If you were to do this lab with sodium chloride, for example, it wouldn’t give the same visual effect. I mean, if you did that food safe, I guess you could have your students drink it to try to understand, but that’s super gross.
Cutting Down on Materials
If you really wanted to cut down on materials, instead of dissolving in 250 mL of water (1 – 8oz cup) you could instead cut down the water to 25 mL (just short of 1 oz, which is about 30 mL) and put the solutions into a spot plate to observe differences. The white ceramic spot plate is my favorite, but the plastic with a sheet of white paper underneath works well too. It’ll be up to you if you want to arrange this lab to be food safe for the taste test.
Get the Lab
You can get this molarity lab activity individually from my TPT store. Or upgrade to the full year chemistry lab manual. You can read more about it here.
