Final answer:
Melting is a physical change because it only involves the change in the state of a substance, not its chemical structure. Burning, or setting a substance on fire, is a chemical change because it results in new substances with different properties.
Step-by-step explanation:
When a substance melts, it undergoes a physical change. This is because the substance changes its state from solid to liquid due to an increase in temperature, but without altering its chemical structure. Its molecules are simply moving more freely, but remain the same molecules they were in the solid state.
On the other hand, when a substance is set on fire, it undergoes a chemical change. It's because burning involves a reaction with oxygen and results in newly formed substances like ash and smoke. These new substances have different chemical properties than the original substance, indicating that a chemical change has taken place.
Learn more about Physical and Chemical Changes