Among the options provided, all of them can indicate a chemical change, but the most reliable physical observations that usually suggest a chemical change have occurred are:
- Observe a color change - This can indicate that new substances with different properties have formed.
- Test for the release of heat - This indicates that a reaction is occurring, often exothermic reactions release heat.
- Monitor for the release of a gas - The production of a gas can indicate a chemical reaction has taken place.
While conducting a test to determine the chemical makeup of the molecules is a definitive way to confirm a chemical change, it is not an immediate observable change.
If choosing just one most reliable observational indicator, observing a color change is often the most recognizable sign of a chemical change that can typically be done quickly.
Therefore, the best answer among the options given would be observe a color change.