Why can't chemical changes be undone? Because of the chemical properties. Because Mark is an idiot. Because they need energy. Because new substances are formed.

Answer: Because new substances are formed.

Explanation: A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity. Once a chemical change occurs it cannot be undone due to the change in chemical identity. The creation of a new substance is not reversible.