Which of the following observations most clearly indicates that a chemical reaction has occurred?

The correct answer to the question "Which of the following observations most clearly indicates that a chemical reaction has occurred?" is Option C: Two clear liquids are mixed, and a solid precipitate forms..

The other options — A) A substance changes from a liquid to a gas when heated., B) Sugar dissolves in water, forming a clear solution., D) An iron bar expands slightly when its temperature increases. — are incorrect for this GED Exam Prep question. As explained above: The formation of a precipitate (a solid) from two liquids indicates that new substances have been created, which is a key sign of a chemical reaction. Options A, B, and D describe physical changes.