Patrin conducts an investigation to test the hypothesis that carbon dioxide is exhaled from her lungs. She makes an indicator solution by mixing bromothymol blue into water in an erlenmeyer flask. Bromothymol blue is a chemical indicator that turns yellow in solution in the presence of an acid. Patrin knows that carbon dioxide slightly dissolves in water to form carbonic acid. Patrin exhales through a straw into the indicator solution and observes its color changing from blue to yellow, indicating that her hypothesis is correct.