When we study acids, many people immediately consider things like lemons and limes. Citrus fruit as a relatively high acid content compared to other things like milk, for instance. Stomach acid is a stronger example – it takes a powerful acid to help break down our food for digesting. If the pH balance of your stomach was less acidic, you would have a difficult time digesting solid food. Did you know that water – the same stuff that comes out of your tap, the local lake, and even the ocean – can all contain acid at varying levels? Scientists have discovered ways of measuring levels of acidity, or pH balance, using materials that change color based on a substance’s chemistry. Acids can be present naturally (like with citrus) or through chemical engineering.
The pH scale is a number that falls from one to fourteen. Zero to seven indicates an acid, with zero being the strongest acid. Hydrogen ions are numerous in acids. Eight to fourteen indicates a base, with fourteen being the strongest base. Hydroxide ions are numerous in bases. While acids are certainly dangerous at extreme levels, bases can also be dangerous at high levels. The body needs a neutral (in the middle) pH and controls the amount of acids and bases through processes involving the kidneys, lungs and blood so that they can function properly. What does pH indicate if it falls somewhere between the extremes? Does it take expensive equipment to test for acids? How do we know if an unknown substance is acidic or not? In today’s experiment, you will function as a chemical engineer and differentiate between acidic and basic materials.
You should see the indicator change color depending on what substance was added to it. If a substance is acidic, it will make the indicator turn pink or red. If a substance is basic, the indicator will turn lighter blue or green. Cabbage juice works well as an indicator because it contains a pigment called anthocyanin, which dissolves in water, and looks different when exposed to acids and bases. If the acid or base is strong, more of it will interact with the anthocyanin and the color will change more dramatically. If the acid or base is weaker (close to neutral), then the pigment will change less.
Don’t forget to clean up when you are done. Any of the suggested testing substances can be washed down the drain or thrown away. Any leftover indicator can be poured down the drain. Wash any containers that you used, as well as the blender and strainer. Put any leftover substances you tested back where you found them.
One fun thing to explore with pH indicators is the fact that liquids can become more acidic or more basic based on gases that get mixed into them. One way to mix a gas into a liquid is to seal it into a bottle and shake it. That’s not the only way though, and in fact blowing bubbles can work really well. If you blow bubbles into water with your cabbage indicator, does it change color? Whether it does or doesn’t, what does that say about the effect of the gasses you exhale? Do they make the water more acidic or basic, or have no effect? If there is a change, think about what you know about the gasses that we exhale, and see if you can come to any conclusions about their effect on a liquid. Of course, you can do some research and see if your conclusions match what other scientists have found.
One other thing to know is that cabbage isn’t the only plant with anthocyanins. In fact, they’re very common in lots of plants, and even a lot of different foods. Fruits and seeds that are purple or purple-ish are a good place to look, such as various berries, grapes, purple corn, or black beans. Some red and blue foods also have them, like raspberries, cherries, and blue corn. Another place to look is leaves that turn red or purple-ish in the fall, since those colors are a sign of anthocyanins. Similarly, flowers with these colors are also a good place to look. Some of these plants might not work as well as cabbage for making a pH indicator, especially if the plant itself makes the liquid acidic or basic, but you can use the same process you used here to try them out.
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.