Which of the following facts best demonstrates that baking a chocolate chip cookie involves a chemical reaction?

• A. The cookie dough is shaped into round balls before baking.
• B. Chocolate chips melt and spread throughout the cookie.
• C. The dough changes color and produces a new aroma during baking.
• D. The cookie dough becomes firmer as it cools after baking.

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction involves the formation of new substances with different properties from the original materials.
2. Analyze each fact to determine if it indicates a physical change (change in state or shape) or a chemical change (new substances formed).
3. Recognize that shaping dough into balls is a physical change because it only changes the shape, not the chemical composition.
4. Note that melting chocolate chips is a physical change because it involves a change of state from solid to liquid without forming new substances.
5. Identify that a change in color and the production of a new aroma during baking indicate a chemical reaction, as these are signs of new substances being formed.