Which of the following is a sign a chemical reaction has occurred? A liquid changes to a solid when cooled. A liquid forms bubbles when heated. Two clear liquids combine and mix together. Two clear liquids combine and turn cloudy.

The best answer is, "Two clear liquids combine and turn cloudy."

When water freezes form a liquid to a solid. That's a physical change.

When water boils, it forms bubbles. That's a physical change.

When you mix ethanol and water, they form a clear solution. That's a physical change.

The last one "two clear liquid combine and turn cloudy" shows visual evidence that the two have combined and changed.