Today’s lab, Controlling the Amount of Products in a Chemical Reaction, is lesson 2 from Chapter 6 of the ACS Middle School Chemistry Curriculum. All you need is a graduated cylinder, some measuring spoons (or scale),vinegar, dish detergent and baking soda. At the beginning of the class I put 1/2 teaspoon of baking soda into a 50ml graduated cylinder and then pour in 10 ml (measured earlier) vinegar with 1 drop of detergent solution (1 tsp of detergent + 2 Tbsp water). If you’ve ever done the volcano labs you know this is going to react and make lots of bubbles/foam and indeed it bubbles up and out of the graduated cylinder. The detergent is just there to help form the bubbles/foam and is not part of the chemical reaction. The handouts I gave the students had the chemical reaction on it: acetic acid (vinegar) + sodium bicarbonate (baking soda) produces water, carbon dioxide and sodium acetate. The challenge I gave them was to find just the right combination of vinegar and baking soda to produce enough bubbles (carbon dioxide) to come just to the top of the graduated cylinder without overflowing. A couple of groups managed it quite quickly so I then gave them a 100 ml graduated cylinder and asked them to repeat the experiment for that cylinder. One girl immediately realized it was twice the volume so doubled their previous results and got it the first try. The handout says to use measuring spoons but I didn’t have any smaller than 1/2 tsp so I had the kids measure the mass of the baking soda in little paper cups.
For groups that finished early I had snatoms and zometools out so they could build the molecules involved in the reaction. After everyone was done I asked if we could just keep making larger quantities of carbon dioxide (bubbles) by just adding baking soda to the 10 ml of vinegar. Most of the students realized that no, you would need to add both baking soda and vinegar. I told them it was like when you go to make cookies and find you only have 1 egg and the recipe calls for 2 eggs. You can cut your recipe in half and make half a batch of cookies, but you can’t just use 1 egg and the same quantity of all the other ingredients. And if you want to make two batches of cookies, you’l need to double all the ingredients (and go buy more eggs).
We finished by watching the following videos: