Which of the following is a definite indicator that a chemical change has occurred?

A.

A solid melts to form a liquid.

B.

A powder dissolves when mixed with a liquid.

C.

A gas forms when two liquids are mixed.

D.

A piece of copper is minted into a penny.

C. A gas forms when two liquids are mixed.

Explanation:

In a physical change, no change in the composition of the material occurs. There might be change in the state of the matter. Examples of physical changes: Melting, cutting, freezing. option A, B and D are all physical changes.

In a chemical change, the composition of the matter changes. In option C, when two liquids mix, a gas forms which has different composition than the individual liquids.

The answer is C!