Third graders have been practicing using the scientific method as a way to give structure to their questions and explorations. We have been working on asking testable questions, making predictions, finding answers through research or testing, and then recording and sharing results. On the previous lab day, we had tried cleaning pennies with different liquids. Finding that vinegar cleaned pennies well led to more questions. Is it because vinegar is an acid? What exactly is an acid, anyway?
In this pH lab, students used a natural indicator – cabbage juice – to test whether 4 liquids were acidic, basic, or neutral. Students used graduated cylinders to measure 25 mL of each liquid, poured them into test tubes, and then added cabbage juice with pipettes. The cabbage juice, which starts a dark purple color, turns a brilliant pink – indicating an acid – when added to the vinegar. Students were pleased to see their hypotheses validated by this result. Some were surprised to see that Sprite was a weak acid as well, and many didn’t expect saltwater to stay purple – indicating that it is neutral. The most unexpected change was when soapy water turned the purple cabbage juice green, showing that it is a base. In one final test, students were given a cup of mystery liquid. The liquid turned the cabbage juice green, so students knew it was a base. We then mixed the vinegar from the test tube with the mystery base. When the fizzing cup nearly overflowed, students guessed correctly that the mystery liquid was baking soda in water. They quickly noticed that the combined liquids turned purple, indicating that the combination of acid and base brought the liquid back to neutral.
Using a dedicated lab space for this experiment meant that we had access to two water sources in the room, we had the necessary measuring tools available, and I had the time and space to set up and dismantle equipment between classes. Students enter the lab knowing our expectations for handling tools and equipment, and can focus on the task at hand – careful observation, drawing conclusions, and generating more questions for future experiments!