Final answer:
Burning firewood is a chemical change that produces new substances and releases heat energy, indicating the molecular structure of the wood is being transformed.
Step-by-step explanation:
When Miss Lopez burns firewood to keep warm, a chemical change is occurring. This process involves the firewood reacting with oxygen in the air to produce new substances such as carbon dioxide and water vapor, as well as releasing heat energy. The conversion of the wood to different chemical substances and the release of energy are clear indicators that a chemical change is taking place, as opposed to a physical change which would only alter the substance's form without changing its underlying molecular structure.