As Mia mixed two substances in a test tube, she noticed that the test tube became warm. She also noticed gas coming from the test tube. Based on Mia's observations, what was most likely occuring inside the test tube?

The correct answer would be - a new substance is forming.

Explanation:

A reaction can lead to two types of changes in the substances reacting, it could be chemical change or physical change or both. Physical changes are deal with the change in the state of the matter and other physical changes whereas chemical change leads to the formation of a new substance by the altering composition of the substances.

Chemical changes involve a change in temperature, formin and releasing gas, or changing the color of reactants. In the given case, Mia mix two substances which leads to an increase in temperature and release gas by this reaction which tells there is a chemical reaction forming a new substance.