In physical science lab, Kim and Dawn added small pieces of magnesium to hydrochloric acid. They noticed that bubbles formed, the test tube got hot, and the magnesium disappeared. What is a sign that a chemical reaction has taken place in this experiment? Question options: odor gas is produced magnesium appeared temperature decreased

The answer to this question would be: B. gas is produced

In the chemical reaction of magnesium with hydrochloric acid, hydrogen gas will be formed. That gas is the one that makes the bubble appear in the solution.

Disappear also doesn't mean react. Sugar disappear in water but it was because it dissolved. Temperature change could be an indicator that a reaction has occurred. But in this case, the temperature should be increased since the tube become hot.