Final answer:
A chemical change takes place when a substance undergoes a reaction that alters its molecular structure and produces new substances, such as the burning of a candle or the combustion of natural gas.
Step-by-step explanation:
In which example is a chemical change taking place? Chemical changes involve a process where new substances with different properties from the original ones are formed.
One clear example of a chemical change is the burning of a candle, where the wax reacts with oxygen in the air to produce carbon dioxide and water. This reaction involves a change in molecular structure, resulting in substances with different chemical properties than those of the reactants. Another example is the combustion of natural gas in a furnace, where methane (CH4) reacts with oxygen to form carbon dioxide and water, again a process where the molecular structure is altered and new substances are formed.
Physical changes, on the other hand, do not produce new substances -- the composition remains the same even if the form appears different, such as when water changes from solid to liquid or gas.