Ocean Acidification/Acid Rain Demonstration with a Soda Stream

I mainly use this in outreach demonstrations to grades 3-5, but have also used it with college students to introduce a topic.

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I use this as an introduction to acidification, usually situation within a section on climate change.

Students will learn how carbon dioxide affects the acidity of water.
Students will learn how acidic water interacts with calcium carbonate.
Students will imagine and discuss what the causes and impacts of acidification in the environment would be.

Students will make predictions based on observations.
Students will test those predictions with new observations.

Prepare a pH indicator liquid ahead of time by boiling red cabbage in 2 or more quarts of distilled water until you have a deep blue liquid. Strain out the cabbage and cool liquid overnight. This indicator will turn red to pink in the presence of an acid and green to yellow in the presence of a base. Prepare common household solutions that students would be familiar with such as diluted apple juice (weaker acid-turns red/purple), vinegar (stronger acid-turns pink), milk of magnesia or Windex (weaker base- turns green), strongly diluted bleach (strong base- turns yellow, clear if too strong). Clear liquids are best.

When you begin, fill clear cups with the indicator. If you want more liquid, you can dilute with distilled water. Ask what students know about acids and bases and explain to students that the indicator will change colors depending on how strong of an acid or base different chemicals are. Then choose a product from above, have students make predictions if it is an acid or base then pour into a cup of indicator and let them observe the color change. Go through each solution and put them in order from highest to lowest pH, including distilled water in the middle, so students can easily observe.

Then introduce the SodaStream and ask if student know what it is used for to begin a discussion on carbonation. When ready, use the soda stream as directed to add carbon dioxide to the water. Have students make a prediction on what color the indicator will turn when carbonated water is added, then pour in for observation. Students will see that it turns a red to pink color, usually between apple juice and vinegar and can conclude that adding carbon dioxide to water makes it acidic.

The last part of the demonstration is to have students predict what happens to sidewalk chalk when placed in pure water versus carbonated water. For this, I used new cups that do not have indicator for the most effect. Students will see that pure water has no effect on the chalk, and that the SodaStream carbonated water effervesces wildly.

Then begin a discussion on the importance of this, including building materials made of carbonate such as concrete roads and structures, and marble buildings, statues, and headstones. You can then also discuss marine organisms such as snails, clams, and other mollusks with their carbonate shells - having students predict what happens to them as the pH of the ocean goes down. It really helps if you explain that chalk is made of many microscopic shells (or even show a picture of coccolithophores).

Make sure to give students a chance to ask their own questions, or maybe even make predictions about mixing acids and bases to see what happens to the color - be careful here though! Do your own experiments first.

If students ask what the indicator is, I tend to say they have to wait for the end to figure out the secret, because at near the end I will drink the apple juice/indicator mixture to gross them out (be careful here too).

As a demonstration, all assessment is generally formative through direct oral feedback.