Heat caused an increase in gas production. Heat caused a new substance to form. Heat caused a decrease in energy. Heat caused a change in state.
I gave my students this multiple choice question: When frozen carbon dioxide (CO_{2}) is heated, it changes directly into a gas. Which statement best indicates that a physical change has taken place?

Only one answer is correct among these 4 choices:
- Heat caused an increase in gas production.
- Heat caused a new substance to form.
- Heat caused a decrease in energy.
- Heat caused a change in state.

Could you tell me which one is correct?

Heat caused a change in state.