Place the other end of the straw into the vial of water and bromthymol blue labelled A. Have another partner remove the tie on the balloon and slowly untwist the balloon. Keeping the neck of the balloon pinched to control the flow of gas, gently squeeze the balloon so the gas slowly bubbles through the solution. Repeat the same procedure with the other balloons and their respective vials. In some cases, the bromthymol blue solution will change color, from blue to yellow, indicating the presence of carbonic acid formed from CO2. Analyze each of the samples by titrating them with drops of dilute ammonia.
Ammonia neutralizes the carbonic acid.
The bromthymol blue will return to a blue color when all the acid has reacted. Add drops of ammonia to each of the samples that turned yellow, carefully counting the number of drops needed until they are about the same color as your control. Record the results. Ask the students which samples had the most and the least carbon dioxide. Carbon dioxide is a natural part of our atmosphere, but too much CO2 could make the Earth warmer through an increased greenhouse effect.