A reliable way to observe that a chemical change has occurred when two substances come in contact with one another is to monitor for the release of a gas. This is a common indication of a chemical reaction, as gas can be produced through various chemical processes, signifying that new substances have formed.
However, observing a color change and testing for the release of heat can also indicate a chemical change, but they may not be as definitive as monitoring for gas release. Conducting a test to determine the chemical makeup of the molecules can confirm a reaction after it has occurred but does not serve as an immediate observation.
Therefore, the best initial reliable observation among the options given would be to monitor for the release of a gas.
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