This is experiment is from Chapter 6, Lesson 8: pH and Color Change, in the American Chemical Society’s Middleschool Chemistry curriculum. I almost didn’t do this lab because when I tried it the night before class it didn’t really work that well but it worked great for the kids (I think they were being more careful than I was). The lab calls for universal indicator so I made some red cabbage indicator (you can find the recipe all over the internet) which is blue-purple around neutral, going to red for acids and green for basic solutions. Students filled 6 wells in a 24 well plate with the indicator and then made a weak citric acid solution – 5 ml of water with 1 toothpick (used as a scoop of sorts) of citric acid (True Lemon). They then took a pipette and dropped 1 or 2 drops of the citric acid in well 2, recorded the color and pH. Then they added another toothpick scoop of citric acid to their solution and so on.
As you can see in the photo above, the wells of indicator get steadily pinker as they increased the concentration of citric acid, indicating a decreasing pH. The second well of blue-green is created by doing the same experiment but increasing the concentration of sodium carbonate to make an increasingly basic (higher pH) solution.
This lab didn’t take very long so the kids played some more periodic table battleship.