Identifying acids and bases in solutions

Acids and Bases

Juice for Sleuths": an experiment performed by a 3rd grade class

The 3rd grade class at the Osborn Elementary School used purple cabbage to test whether certain common household ingredients were either or acids or bases. This lesson plan was selected because learning the difference between an acid and a base is an important part of chemistry. It is something that students will have to know for all the chemistry classes they will have to take later on in their educational lives. It is also useful knowledge to have, as whether something is an acid or a base will affect one's choice of ingredients or products when cooking or cleaning.

The experiment makes use of common household and pantry items. This seemed to add to the level of the children's curiosity throughout the experiment. I was slightly nervous that the students might find the concept uninteresting or too difficult to grasp, given their relatively young age, but I hoped that the tactile nature of the experiment, which involved squishing cabbage leaves as well as just testing solvents, would make it fun. I thought it would be more interesting than when I had performed a similar experiment as a slightly older grade school student with a ph-testing strip.

The purpose of the experiment was to show students how to easily demonstrate the difference between acids and bases. Red cabbage juice can be used to show if a substance is acidic or basic. The students made cabbage juice by putting purple cabbage in a bag, covering it with water, sealing the bag, and squeezing it until it turned a deep shade of purple. This got the students involved in a hands-on way immediately. (Asking students if they liked to eat cabbage might be a great way to introduce this part of the activity!).

Red cabbage juice turns redder in the presence of an acid and green when it is mixed with a basic solution. The darker the shade of the cabbage juice, the higher the level of acidity or alkalinity of the substance. The students were given a reaction tray and a pipets to put vinegar into one well of the reaction tray and baking soda into the other side of the reaction tray. Then, they added cabbage juice with another pipet to both substances. This was useful to get students accustomed to manipulating simple chemistry apparatus. It also was an opportunity to ask students when they used baking soda and vinegar, or saw them used, to show how chemistry is a part of their daily life. The students did the same with lemon juice and milk of magnesia, after cleaning out their pipets and the wells. The students then recorded the color changes on their worksheet.

It was rewarding to see students carefully test these substances with great curiosity and care, as they could not guess what the result would be. The more challenging part came when the students had to mix acidic and basic solutions, including vinegar and baking soda, and lemon juice and milk of magnesia together and record the results. This was to show how when an acid is added to a base, its acidity level falls as it becomes neutralized, changing the color of the red cabbage water. The opposite occurs when a base is added to an acid. The students were encouraged to try their own mixtures of different substances on hand.

A left the classroom feeling that the students had enjoyed the activity, had learned which substances were acidic and basic and gained greater confidence using laboratory tools. This was a good introduction to the scientific method and process of discovering something, rather than being taught it by rote. I am not sure they understood why it was important to know certain things were acidic and basic. I might have talked more about how and when certain acidic and basic substances are used, like how milk of magnesia is used to treat an acid stomach. I would also bring more ordinary household substances for the students to use in their own experiments.