Cabbage and Chemistry

Have you ever licked a lemon and wondered what makes it so sour? What about eating a piece of dark chocolate and being surprised by its bitterness? As it turns out, flavors like sour and bitter come from the food’s pH level. But we don’t want to test the pH of everything by tasting it— many substances are too dangerous to eat! Instead, we’ll learn how to determine a solution's pH level using a common vegetable: red cabbage. 

 MATERIALS:

  • Red cabbage

  • Isopropyl alcohol (70% or higher)

  • Distilled or deionized water (different from bottled water) 

  • Pot (for boiling)

  • Strainer 

  • Bowl

  • Measuring tools (preferably a graduated cylinder, but traditional kitchen tools also work)

  • 3+ paper coffee filter

  • 3+ empty cups

  • Spoon or stirring rod

  • 1 tbsp of baking soda

  • ½ tsp of white vinegar (or lemon juice)

  • pH test strips

TIME ESTIMATED:

30 minutes

DIRECTIONS: 

For the cabbage and alcohol solution:

  1. First, cut or tear the red cabbage into small pieces. We’re going to use it to make an extract. 

  2. Fill one of the cups with the distilled water. It should be around 150 milliliters, or a little over half a cup. 

  3. Pour the distilled water and cabbage into the pot. With an adult’s permission and/or help, turn the stovetop to medium or medium high heat and bring it to a boil (you can also try this in the microwave with a microwave-safe container, it's only important that it boils). 

  4. Once it reaches a boil, turn the heat down and let it simmer for around 10 minutes, or until the water turns purple. 

  5. Let it cool and mix in isopropyl alcohol to keep it from spoiling, adding around 15-20 milliliters (a little over 1 tablespoon). The amount should be around 1/8th the volume of the cabbage mixture. 

  6. Strain the mixture into a bowl so we catch all the liquid. Be careful not to spill it, this can stain your clothes and countertops!

  7. Place the coffee filters over the top of each cup and pour an equal amount of the cabbage extract into each. Remove the filters after it’s done straining. You can also pour some of the mixture into additional cups if you’d like to test the pH of other liquids. 

  8. If you prepare the cabbage and alcohol solution beforehand, keep it in a sealed bottle inside the refrigerator— it lasts up to one week. 

For the pH tests:

  1. Now it is time to do some tests! Set one cup to the side and don’t touch it for the rest of the experiment. This is our control. 

  2. Next, pour the white vinegar into the second cup and stir. 

  3. Add the baking soda to the third cup and stir. 

  4. Test the pH of each cup using the pH test strips.

  5. Observe any changes in color to the solutions and analyze the differences in their pH balances.

THINK LIKE A SCIENTIST!

  1. Why do the solutions with baking soda and vinegar change colors? Why are these colors different? 

  2. Why do we use red cabbage for this experiment? 

HOW DOES IT WORK? 

In this experiment, we used our red cabbage and alcohol solution to test whether vinegar and baking soda are acids or bases. This is determined by their pH level, which stands for “power of Hydrogen” (or potential of Hydrogen) and measures the amount of hydrogen ions in a solution. For these types of tests, we can only measure the pH of aqueous solutions, or solutions that use water to dissolve the other substances. This is because the pH scale uses distilled water as a starting point, calling it neutral. It has a pH level of 7, and solutions with less hydrogen ions than water have a higher pH level. We call these bases, and they can taste bitter or soapy when eaten. Acids have more hydrogen ions than water and have a lower pH level. A common acid is the citric acid found in lemons and oranges, which tastes sour. Notice that pH level has an inverse relationship with the amount of hydrogen ions— when one number increases, the other decreases. We should also avoid eating acids or bases unless we know they are safe (like lemon juice). Our bodies work hard to keep their pH levels at around 7.4, and changes in this affect the ways it functions! 

Now you might be wondering how red cabbage helps us determine any of this. This amazing vegetable has a chemical called anthocyanin, which is a natural pH indicator. A pH indicator is a substance that changes colors when mixed with an acid or base. Red cabbage turns bases a greenish/blue color and turns acids a pinkish/purple color. The pH scale ranges from 0-14, and the more acidic or basic a substance is, the more intensely our indicator changes colors! Now, can you tell which of our solutions contains an acid and which contains a base? Answer: the pH level of white vinegar is around 2.5 and the pH of baking soda is around 8.3. Recall that numbers above 7 are basic, and those below it are acidic.

FURTHER EXPLORATION: 

Anywhere there is water, there is a measurable pH. Not surprisingly, scientists spend lots of time studying the pH level of the ocean. One of their biggest concerns is ocean acidification. As we release more carbon dioxide into the atmosphere, more gets dissolved into the ocean. CO2 lowers the pH of ocean water, so it makes it more acidic. Since the industrial revolution, ocean pH levels have dropped from 8.2 to 8.1. A change of 0.1 might not seem like much, but for certain animals, this can mean life or death. 

Malacologists study mollusks (animals like clams, squids and snails). Acids are corrosive— they have the tendency to dissolve things. Because of this, many mollusk shells are dissolving in the acidic water. They also rely on a substance called calcium carbonate to build these shells. It is a slightly basic substance, so it mixes with the acidic CO2 to try and neutralize the water (you can mix the baking soda and vinegar and test their pH to see a similar result). This means mollusks have a harder time finding the material to build their shells, and when they do find it, the shells they build aren’t as strong and last a shorter amount of time! This makes them very vulnerable to predators and decreases their lifespan. 

Anticipated COncerns:

The red cabbage solution stains very easily, so don’t do this above/near anything you don’t want stained. You can put a tray under all the cups so if spills do happen, they will be contained there. Also, be sure to get an adult’s permission/help when boiling the water!

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