All of the options you've provided can indicate that a chemical change has occurred. However, the most reliable observable signs of a chemical change typically include:
- Test for the release of heat (exothermic reaction)
- Observe a color change
- Monitor for the release of a gas
Among these, if you're looking for the most definitive indicators that often accompany a chemical change, testing for the release of heat or observing a significant and constant color change would be strong indicators.
If I had to choose one response based on common educational criteria for indicating a chemical change, I would suggest:
Observe a color change.
This is often highlighted as a visual indicator in many educational resources. However, remember that ideally, multiple observations and tests should be taken into account for a thorough understanding.