A student set up an experiment to show that carbon dioxide is consumed by Elodea plants during photosynthesis. She filled two test tubes with distilled water and placed a piece of Elodea in one of the tubes. She added a drop of phenol red which is a pH indicator to both tubes. She blew in each tube to add carbon dioxide, which caused the distilled water to become more acidic. She exposed both tubes to the light and recorded any color changes in the liquid in the tubes. What is the negative control in this experiment

The NEGATIVE CONTROl is the test tube that is filled with ONLY distilled water.

Explanation:

A negative control of an experiment is carried out alongside the main primary experiment except that the treatment is changed to something that is predicted to have no result. In the negative control, the student does not expect any response. This would help the student compare the result of a new experiment against an already known existing results.

Analysing the experiment carried out by the student, She filled two test tubes with distilled water and placed a piece of Elodea in one of the tubes, leaving the other without a plant. The other test tube that contains only distilled water is the negative control because she wasn't expecting any result from it. It's also served as a control sample to the primary experiment. This is observed as she blew in each tube to add carbon dioxide, which caused the distilled water to become more acidic. This would help the student know that the that experimental results are valid and to ensure that time is saved.