A reliable way to observe that a chemical change has occurred when two substances come in contact with one another is to observe a color change. This is often a clear indicator that a new substance has formed as a result of a chemical reaction.
However, all of the options you provided can be indicators of a chemical change, depending on the context:
- Conduct a test to determine the chemical makeup of the molecules: This can confirm a chemical change but is not as immediate.
- Test for the release of heat: The evolution of heat (exothermic reaction) can signify a chemical change.
- Monitor for the release of a gas: Evolution of gas is another strong indicator of a chemical change.
Among these, observing a color change is often the most visually immediate and easily observable indication of a chemical change.
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