Option 'd. a substance evaporates' does not represent a chemical change but rather a physical change because the substance's molecular structure remains unchanged during the process.
The question asks to identify which option is not an example of a chemical change. By definition, a chemical change occurs when the molecular structure of a substance is altered, resulting in the formation of new substances with different chemical properties.
Examples of chemical changes include burning a candle, where wax reacts with oxygen to produce carbon dioxide and water, or when natural gas is burned, changing methane and oxygen into new substances. Evidence of a chemical change includes temperature changes, light being given off, unexpected color changes, and bubbles being formed without boiling because a new gaseous substance is created.
Looking at the options provided:
- Heat is given off - This can be an indication of a chemical change due to reaction exothermicity.
- Light is released - This often accompanies chemical changes that are energetically favorable and release energy as light, like in combustion.
- Bubbles are formed - This can signify the production of a gas as a result of a chemical reaction.
- A substance evaporates - This is a physical change, not a chemical one because it involves a phase change from liquid to gas without altering the substance's molecular structure.
Therefore, the answer is 'd. A substance evaporates' because evaporation is a physical change and does not result in the formation of a new substance with different chemical properties.