Have you ever made one of those vinegar and baking soda volcanoes or rockets? These are fun experiments that show the bubbling and foaming reaction that occurs when acids and bases are combined. But what does this mean exactly? Well, the terms “acid” and “base” refer to the concentration of H+ (higher= more acidic) and OH– (higher= more basic) ions present in a substance.
It’s a little complicated, and might make a bit more sense in a high school chemistry class. Today, we’re focused more on figuring out how to tell whether something is an acid or a base.
In chemistry, we use the pH scale to determine how acidic or basic a solution is. The range of this scale goes from 0 to 14, with lower numbers (0-6) indicating an acid, middle range numbers (6-8) meaning the solution is neutral, and higher numbers (8-14) representing a basic solution.
Again, it’s pretty complicated, but if there is nothing else you remember about the pH scale, the most important thing is that low=acid, high=base, and middle=neutral (the numbers don’t matter as much, you’ll learn more about those later).
So how can you tell if something is acidic or basic? Usually scientists use pH strips, or small pieces of paper that have different chemicals in them that react and change color based on the type of solution.
But you don’t need these fancy paper strips if you have a head of red cabbage at home! The video above shows you how to make your own pH indicator, and here’s what you’ll need:
1 head of red cabbage
Boiling hot water
2 large bowls or jars
Strainer or funnel
Several smaller jars or bowls (clear or translucent work best)- 1 per substance tested
Test substances from around the house (vinegar, lemon juice, bleach, baking soda dissolved in water, rubbing alcohol, salt water, tomato juice, coffee, tea, soapy water, sparkling water, or any other liquids you would like to test)
Adult supervision (Adult supervision at all times please)
- First, cut the head of red cabbage into 4 equal-sized pieces. To start this experiment, you only need one of those pieces, so you can set the rest of the cabbage aside for snacks later or to repeat this activity with additional test substances.
- Cut the quarter head of red cabbage into small pieces and place in the large bowl or jar.
- Cover the cabbage with boiling hot water and let sit for 20-30 minutes.
- When the time is up, you will need to strain the pieces of cabbage out of the bluish-purple water (cabbage juice). Using the strainer or funnel with the filter paper, pour the cabbage and water into the other large bowl or jar being careful that no pieces of cabbage sneak into the new jar. (You only want the liquid.)
- Dilute the cabbage juice with some room temperature water. Usually, you can add as much water as there is cabbage juice, but you don’t want to add so much that the color becomes very faint. Use the video above a good reference for how colorful the juice should be.
- You now have your pH indicator. Pour a small amount (1/2 to 1 cup) into each of your smaller jars. This diluted cabbage juice will change colors when it reacts with acids or bases. Acids will turn red, orange, or pink depending on how strong they are, while bases will turn blue, green, or yellow depending on their strength. Neutral substances will not change the color of the cabbage juice.
- In any science experiment, it is important to make predictions (or hypotheses) about what might happen. Before you get started, make some guesses about what colors the liquid will turn when you add each test substance.
- Be sure you put on your safety gloves and goggles before messing with any test substances. These could be strong acids or bases and can irritate your skin or eyes if they splash onto you.
- You are ready to begin testing your household substances! Let’s start with lemon juice (or another liquid if you are not using lemons). Pour 1-2 tablespoons of lemon juice into your first cabbage juice jar and observe how the color changes. Was it what you expected? Be sure to write down what happens so you can compare your observations to your hypotheses!
- Repeat step 9 for each of your test substances. If you run out of cabbage juice indicator, no worries! Just make some more by restarting with step 1.
*Bonus step: You can make your own pH strips by cutting filter paper into small strips, soaking these strips in the cabbage juice, and letting them dry. Once your pH strips are dried, store them in a Ziploc bag or small container. When you want to test the pH of a new substance, you will have your indicators ready and available!
Red cabbage gets its reddish-purple color from a natural blue pigment called anthocyanin. When the cabbage is soaked in the boiling water, this blue pigment is released giving the water the dark blue color you see. Anthocyanin is a natural pH indicator because it changes color when it encounters acidic or basic environments.