What are three clues that indicate a chemical change has occurred?

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ncchemist | eNotes Employee

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One big indication that a chemical change has occurred is a color change.  Many times a chemical reaction will start as relatively colorless or only lightly colored (often times yellow).  During the course of a chemical reaction, reactions will often turn a darker color (dark yellow, brown, even black), thus indicating a chemical change.  Another indicator of a chemical change can be smell.  Some chemical reactions produce a gas either as a main product or as a byproduct.  It this gas has a particular odor like ammonia or a sulfur compound, this is also an indicator of a chemical change (chemical bonds are broken and reformed to produce the gas).  Finally, change in pH can often indicate a chemical change.  pH is the measure of the acidity of a chemical or a mixture, so if an acid is produced or consumed as part of a reaction, this will change the pH of the chemical reaction mixture.