Red cabbage juice is a safe, natural, easy-to-make acid/base indicator that allows you to see the carbon dioxide in your breath. The trick is to use a very small volume of cabbage juice, since it's not very sensitive.

CO2 red cabbage experimentCourtesy Liz Heinecke
You'll need red cabbage, drinking straws, and very small cups (sample cups or the ones for measuring liquid medicine with work well.) Chop a head of red cabbage, cover it with water in a pan, and boil for about 10 minutes. Then, let it cool and collect the juice. The juice will be purple, but it turns blue when exposed to a base or pink when exposed to an acid. Pigments in the cabbage, called flavanoids, change color when they come in contact with acids and bases.

Pour an equal volume- a teaspoon or two (5 to 10 ml)- of the (cooled) juice into each of two small cups. Take a straw, put it all the way against the bottom of one cup and blow through the straw repeatedly for a few minutes until you see the cabbage juice you're blowing into turn noticeably pinker than the juice in the control cup.

red cabbage CO2 test photo 2Courtesy Liz Heinecke
What happens? The carbon dioxide in your breath combines with the water in the cabbage juice to form carbonic acid, which causes the pH of the solution to drop, making the pigment in the cabbage juice turn pink.

Why is this interesting? About a quarter of the carbon dioxide released by activities like burning fossil fuels and burning down rainforests is absorbed by our world's oceans. This results in the ocean water becoming more acidic, like the cabbage juice in the experiment, and can have an effect on sea life, like coral. To learn more about ocean acidification and the chemistry of ocean acidification, check out NOAA's amazing website.

If you have some cabbage juice left over, you can soak white coffee filters in it, dry them and cut them into strips to make litmus paper. It's also fun to pour 1/4 cup of cabbage juice into each of two cups, add a Tbs. baking soda to one cup, 2 Tbs. of vinegar to the other cup and then pour one cup into the other to see lots of carbon dioxide bubbles form as the vinegar (acetic acid) reacts with the baking soda solution (sodium bicarbonate.)