The substances became cooler and expanded. The substances physically changed the plastic. The substances became warm and stretched the bag. The substances chemically reacted and produced a gas.
I gave my students this multiple choice question: A science teacher mixes two different substances in a plastic bag and seals the bag. The bag expands and breaks open. What is the most likely reason the bag expanded after the two substances were mixed?

Only one answer is correct among these 4 choices:
- The substances became cooler and expanded.
- The substances physically changed the plastic.
- The substances became warm and stretched the bag.
- The substances chemically reacted and produced a gas.

Could you tell me which one is correct?

The substances chemically reacted and produced a gas.