Theresa creates an experiment where she mixes two red-colored substances together in a container and observes the solution slowly changing from red to blue for the next eight minutes. The solution becomes completely blue after eight minutes.

The gradual change from red to blue is evidence that a chemical reaction is occurring as the substances interact. It begins when Theresa mixes them and continues while the color changes, finishing when the solution is completely blue after eight minutes. Therefore, the correct choice is “A chemical change started immediately and finished at eight minutes.”

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