All of the given options can indicate that a chemical change has occurred when two substances come in contact with one another, but some are more reliable indicators than others.
Among the options provided, the most reliable ways to observe that a chemical change has occurred include:
- Test for the release of heat: This indicates an exothermic reaction, which is characteristic of a chemical change.
- Observe a color change: This can also signify that a new substance has formed, indicating a chemical change.
- Monitor for the release of a gas: The formation of gas bubbles is typically a strong indicator of a chemical change.
Conducting a test to determine the chemical makeup of the molecules is a more definitive approach but usually requires more time and equipment than simply observing physical changes or measuring heat.
However, based on common classroom observations, observing a color change, monitoring for the release of a gas, and testing for the release of heat are all reliable indicators of a chemical change.
If you are looking for just one response that is a reliable indicator, I would highlight observe a color change as it is often one of the most immediately visible signs of a chemical change.