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Chemistry can be a lot of fun, but lots of really cool chemistry experiments require a whole lab and expensive (and sometimes dangerous) chemicals. Today, we’re going to do a bit of kitchen chemistry to test a quality of things from around your house: their pH level. pH is a measure of how much of an acid or a base (the opposite of an acid) something is: something with a low pH is an acid and something with a high pH is a base. And you don’t need any expensive lab equipment because you can measure it using cabbage juice! The juice from a red cabbage will change colors depending on the pH of the substance you mix with it, and each color will tell you how acidic or basic the substance is.
Materials:
- Red cabbage
- A kitchen knife
- An adult (to use the knife)
- A blender
- A strainer
- A large pitcher (for the strained juice)
- Clear plastic cups
- Spoons
Substances to test (use as many or as few as you like):
- Lemon juice
- Apple juice
- Baking soda
- Vinegar (white works best)
- Clear liquid soap (or shampoo)
- 7up, Sprite, or some other clear soda
- Antacid tablets (crush them into powder first)
- Tea
Process:
- Have your adult cut the head of red cabbage into chunks and then put it in the blender. Set to “puree” for at least 30 seconds, until it’s mostly liquid.
- Place the strainer on top of the large pitcher and pour the pureed cabbage into it. The juice will go through, and any leaf chunks that are left will stay in the strainer.
- Divide the juice up among some clear plastic cups.
- Stir a couple of spoonfuls of each of your test substances into different cups. Make sure you use a different spoon for each substance, so you don’t accidentally cross-contaminate!
- Use this color chart to determine the approximate of each of your substances:
- See if you can think of anything else in your house that you can try. Make sure it doesn’t have a lot of color of its own, so it doesn’t get in the way of the color change!
Summary
To get kind of technical, what pH measures is how much of something called a hydroxide ion, or an H+. The more H+ is in something, the higher the pH. The less H+ there is, the lower the pH. H+ is kind of the opposite of something called the hydroxide ion, or OH-, so when there’s a lot of H+, there isn’t a lot of OH- and vice versa. If you combine an H+ and an OH- you get HOH, or H2O, which is water. Since water has the same amount of H+ and OH-, it has a perfectly neutral pH of 7.
So how does this affect the cabbage juice? There is a pigment in the cabbage juice that gives it its purplish color. When you add substances that have more or less H+, that H+ changes the chemical structure of the pigment, which makes it reflect light differently. When it reflects light differently, the color of that light changes, and the color of the juice changes. Different amounts of H+ change the pigment in different ways, and you get a whole range of colors out of one single kind of juice!
