Acids and Bases in the Kitchen: Using Red Cabbage as a pH indicator

This set of experiments explores the chemistry of acids and bases. Acids and bases are very interesting because they have the potential to be very dangerous, but we use many in our everyday lives. One place where you are guaranteed to find many acids and bases is in the kitchen. Today, we need to make something called an “indicator” to use in our experiment so let’s get started on that first. Then we can discuss what we are doing in more depth while our solution is brewing.

1. Wear goggles for the whole lesson, as acid and base solutions are used.
2. Identify locations of nearby sinks so you can flush thoroughly with water if any chemicals get on skin or in eyes. If any student thinks a chemical may have gotten on him or her, they should immediately begin flushing skin with water as a precaution. We aren’t using anything stronger than household cleaners, but some of these can irritate the skin if they aren’t washed off.
3. Hotplates can burn. Students should not touch them when they are on.

Hot plate
Large beaker of hot water (preheat before students arrive) for stewing
2 cups copped red cabbage (about half a head)
knife for chopping (please chop and put the knife away before the student arrive)
Spoon for stirring
Cutting board
Demo test tubes of color indicators
Hot mitts

12 beakers with a few mL of each sample (you may want to dilute some with water)
1 flask with cabbage juice indicator
Plastic pipettes
Test tubes
Cardboard test tube holder / test tube rack

Milk
Water – from sink
Water – bottled
Clear cola (not dark colored) Look for one with phosphoric acid in the ingredient list, which is what gives it a sharper flavor.
Windex/Lysol/household cleaner
Lemon juice or lime juice
Vinegar
Kool-aid
Baking soda solution
Baking powder solution
Salt in water
Soap solution
Hand sanitizer
Mouthwash
Anything else you want to test!

Heat the water and chop the cabbage into strips/pieces (increase the surface area and amount of cut surfaces). If doing this experiment as a demonstration, know that the cabbage juice indicator needs to cook for about 10 minutes. Before the class begins you should begin heating the water and have the cabbage chopped and ready to be added to the boiling water. Wait to add the cabbage until the students are there and you introduce yourself, but get the cabbage into the water before you start talking about acids and bases, so that is has time to cook.

Add the chopped cabbage to a beaker and with roughly 500 mL boiling water (you need enough water to cover all the cabbage). Allow color to leach out of plant for 10 minutes in the boiling water. (During this time you can move on to the discussion.) When the cabbage is ready, decant off the bluish-purple solution leaving behind the excess plant matter in the original beaker. This is a little stinky, but not at all harmful. When the plant matter is cooled down it should be discarded in the trash. Students should wear goggles for the intro discussion if a safety shield isn’t available.

Take a beaker or flask with some of the indicator and test small samples of various kitchen “chemicals” using clear test tubes or small glass beakers. Have the students first predict acid or base for each item. Then, add a few drops of the indicator to the each of the various liquids/solutions.

It’s important not to make solutions and dilutions in advance because we want to demystify the science and show the students that the liquids they are studying are just common household items. All solids should be made into a water solution. Most of the liquids listed below do not need to be diluted and should produce a nice visible color change as appropriate. Since Coke doesn’t work (the color is too dark—you can ask the students to think about why we don’t test it) we’re using clear cola.

Assess general pH using the following color scheme:
Acid or base? – Color
Very acidic – Red
Slightly acidic – Purple/Pink
Neutral (neither)- Blue (no change in indicator color)
Slightly basic – Blue-green
Very basic – Greenish yellow

Have you heard of acids and bases? Can you name an acid? A base? It’s likely you have head of acids before, like citric acid (in fruits), hydrochloric acid (which is in the stomach), ascorbic acid (vitamin C) and others. Have you ever had lemon juice get into a cut in your finger? It probably stung, and that stinging feeling came from the acid in your cut. One of the properties of acids is that they are usually sour in taste.

Let’c compare to bases now. These are a litte bit harder to think of examples. Some common bases are ammonia and baking soda. Bases are usually bitter in taste and are slippery. Can you think of something that is slippery? Yes—soap is another base that we use every day.

In order to test different substances to see if they are acids or bases, we’ll use the cabbage indicator and tell based on the color change if each thing is an acid or a base. You can test anything you like; be creative!

Q: Why do some acids turn a dark red color, while others stay more of a purple color?
A: There are different strengths of acids and bases. It is not simply a label, but more of a spectrum. Things which are strong acids turn a very dark red color, while things which are weak acids only had a slight color change.
Q: Why didn’t the salt solution and kool-aid solution change colors?
A: They are not acids or bases.

An acid is something that has (or that can release) a HYDRONIUM ion (H3O+). A base can release a HYDROXIDE ion (OH-). Think of the acid and the base as being opposites. Now, chemists use special sentences to describe what’s going on in a CHEMICAL REACTION, sort of similar to how you might write out a math problem. We write two things that can react with each other on the left, and what happens after they have reacted on the right, like so:.
H3O+ + OH- —> H2O + H2O
(note: writing 2 H2O is correct but may be confusing at first to some.)

The hydronium ion and hydroxide ion can REACT to make something else—namely, water! When that happens, the solution becomes neutral—neither an acid or a base.

All household cleaners and cabbage indicator can do down the drain. Collect solid cabbage waste (and any other solid waste—pipettes, plastic test tubes) and discard in the trash. Be sure to discard glass or broken glass in a proper glass disposal container, if one is available.