A key observation that most likely indicates that only a chemical change has taken place is the formation of new substances. This can be evidenced by:
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Color Change: A color change that is not simply a result of mixing colors but indicates the formation of a new substance (e.g., rust forming on iron).
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Temperature Change: A change in temperature (exothermic or endothermic) that occurs without external heat being added or removed can indicate a chemical reaction.
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Gas Production: The production of gas bubbles that occur when substances react (e.g., vinegar reacting with baking soda to produce carbon dioxide).
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Formation of a Precipitate: The formation of a solid from a liquid mixture indicates a chemical change.
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Change in Odor: The development of a new odor that was not present before.
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Light Emission: Release of light energy during a reaction can also suggest a chemical change.
In contrast, physical changes do not result in the formation of new substances, such as changes in state (e.g., melting ice). If an observation shows that new substances are formed, it is strong evidence of a chemical change.