Lose the Blues with CO2

Carbon dioxide (CO2) is essential for life on Earth, but too much of it in the atmosphere is bad for the climate and for the oceans. Scientists have ways of testing the amount of CO2 in the atmosphere. In the activity below, you can do your own test for CO2!

Here's what to do:

  1. Tear up two leaves of red cabbage and place the pieces in a zip-closing plastic bag. Add about 1 cup of room temperature water. Get as much air out of the bag as you can and close the bag securely
  2. While holding the bag, use your other hand to squish the leaves in the water until the water turns a medium to dark blue. This is your indicator solution. It will change color when certain substances are added to it.
Squishing red cabbage leaves in a plastic bag with water.
  1. Open a corner of the bag and pour the indicator solution into a clean plastic cup.
  2. Place 2 small clear plastic cups on a white piece of paper. Pour 1 tablespoon of the blue indicator solution into each cup.
  3. Ask your adult partner to put a straw all the way down into one of the cups of indicator and blow air into the indicator as you watch from the side. What did you notice?

    Save the unused indicator solution for the next part of the activity.
Blue indicator in two cups with a straw in one of them.

What to expect

As air is blown into the indicator, the color of the indicator should change from blue to purple. 

Blue indicator in one cup and purple in the cup with straw.

What's happening in there?

The carbon dioxide gas from exhaled breath makes the water slightly acidic which makes the indicator change color.

What else could you try?

If carbon dioxide is making the indicator change color, maybe more carbon dioxide would make it change color even more. Let’s try it!

What you'll need: 

  • Indicator solution
  • White piece of paper
  • Clear plastic cups (1 small, 1 Medium, 1 Tall)
  • Baking soda
  • Vinegar
  • Measuring spoons

Be safe

Be sure to review the safety instructions on page 1 before proceeding.

Here's what to do:

  1. Pour 1 tablespoon of indicator solution into a clean empty small cup.
  2. In the medium cup, add 1 tablespoon of vinegar.
Adding tablespoon of vinegar to empty cup.
  1. Add 1 teaspoon of baking soda to the vinegar and quickly place the small cup with the indicator into the cup with the vinegar and baking soda.
  2. Place a cup on top as shown so that the carbon dioxide gas is trapped inside the cups.
Cup with blue indicator placed in cup with vinegar and baking soda reaction with an upside down cup on top.
  1. Gently swirl the entire set of cups and watch the color of the indicator. What did you notice? 

What to expect

As you swirl the cups, the color of the indicator should change from blue to purple. The color should be more reddish purple than the color of the indicator when your adult partner blew into the indicator.

Purple indicator in cup in vinegar and baking soda reaction.

What's happening in there?

When the carbon dioxide mixes with the water in the red cabbage indicator, it creates a very weak acid called carbonic acid. This acid reacts with the red cabbage indicator and changes its color. The vinegar and baking soda reaction produced a lot more carbon dioxide than the exhaled breath, so more carbonic acid was produced which caused a greater color change.

Blue and purple indicator in small cups.