Which of the following is not an indication of a chemical reaction? 1. Two substances burst into flames on contact. 2. A substance freezes. 3. Bubbles form when two substances are combined. 4. A solid forms when two substances are combined.

1. Two substances burst into flames on contact

Explanation:

For a chemical reaction to occur between two or more substances, the reaction process must be reversible in nature.

According to options 2,3 and 4, they are all reversible reactions that is the product formed can return back to their original form except for the first option which is an irreversible reaction. Hence, two substances burst into flames on contact is not an indication of a chemical reaction since the burnt substances cannot return back to their original form. For example burning of paper into ashes is not a chemical change because ashes which is the final product cannot turn back to its original form which is paper.