What is the major indicator that a chemical reaction has taken place?  

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A chemical reaction involves a chemical change. This means that the composition of products would be different from those of the reactants. In comparison, a physical change does not involve any change in the chemical composition of a material (although a phase change may be there).

There are a number...

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A chemical reaction involves a chemical change. This means that the composition of products would be different from those of the reactants. In comparison, a physical change does not involve any change in the chemical composition of a material (although a phase change may be there).

There are a number of indicators that a chemical reaction has occurred. These include the following:

  • Exchange of heat with the atmosphere: A chemical reaction may be endothermic or exothermic. If the reaction requires energy to take place, then it is termed as an endothermic reaction. If the reaction produces energy, it is known as an exothermic reaction. Thus, energy exchange typically takes place during a chemical reaction.
  • Generation of gases: A number of chemical reactions involve the generation of gases. These may be observed as bubbles during the reaction.
  • Precipitate formation: a number of chemical reactions may also result in the formation of precipitates. 

Some of the other indicators of chemical reactions include a change in color, odor generation, etc.

Hope this helps. 

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