19. Which is nota possible clue that a chemical change has occurred The substance melts. The color changes. A precipitate forms. Bubbles are produced.?

The statement "The substance melts" is not a possible clue that a chemical change has occurred. Melting is a physical change, as it involves a change in the state of matter from solid to liquid without altering the substance's chemical composition. In contrast, color changes, precipitate formation, and bubble production typically indicate chemical changes.