A reliable way to observe that a chemical change has occurred when two substances come in contact with one another includes several observations. Among the options provided, the following responses can indicate a chemical change:
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Observe a color change - This can indicate a chemical change, as new substances may have different colors.
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Monitor for the release of a gas - This is another sign of a chemical change, as new gases may be produced during the reaction.
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Test for the release of heat - Heat release (or absorption) often indicates an exothermic or endothermic reaction, which is indicative of a chemical change.
The option "conduct a test to determine the chemical makeup of the molecules" is not a direct observation of a chemical change but rather a method for further analysis.
Therefore, while multiple options indicate a reliable way to observe a chemical change, if you must choose one, any of the first three options (color change, gas release, or heat release) would be correct, depending on the context of the observation.
If you need to select only one answer based on general classrooms observations of chemical change, "monitor for the release of a gas" is often a classic and clear indicator of a chemical reaction occurring.