Fill 'er Up!


Cars are really useful inventions. But they're also really big polluters! For example, the gases in their exhaust cause something called acid rain. You'll see how in this Fab Lab.



Bromothymol blue indicator (BTB). (See "More Info" below.)
Clear plastic cups
Distilled water or clean tap water

Can't get BTB? Click here!


Work in teams of 2 to 6.

1) Fill two cups halfway (not more!) with clean water.
2) Squirt some BTB (at least 5-10 drops) into one of the cups and stir it. Write down what color the water turns. Set that cup aside.
3) Now, pass around the OTHER cup (the one with no BTB in it). Have each kid take 30-60 seconds to blow air bubbles into the water through their straws. This forces carbon dioxide gas into the water.
4) After you're done, record what the water looks like. Does it look any different from before you started blowing air into it?
5) Now add a squirt of BTB and stir. Record the color you see. Is it different from the color in the other cup?


Your breath contains carbon dioxide gas, which is also found in car exhaust. When carbon dioxide dissolves in water, carbonic acid forms. Carbonic acid contributes to acid rain, which hurts the environment. (The BTB indicator turns greenish yellow in the presence of acid.)

Other gases from cars, including nitrogen oxides, play an even bigger role in causing acid rain. The chemistry is similar: the gases react with rainwater to create acids.

Note: If your BTB indicator turned greenish yellow in BOTH cups, try the experiment again using bottled water, or water from another tap.

More Info

Bromothymol blue (BTB) is a kind of indicator -- it tells you how much acid is present in a liquid. Your school science lab probably has it. You can also get it cheaply from a pet store that sells fish, or from a mail order service on the Internet.

If you can't get BTB, you can make an indicator from cabbage juice. Here's how:

Combine equal parts thinly sliced red cabbage and water. Heat to almost boiling and then cool. Strain out the cabbage, and substitute the purple juice for BTB in the experiment. (Note: Instead of turning greenish yellow, the juice will change to a slightly pinker shade of purple.)