Chemical reactions require reactants and products. During a chemical reaction, reactant concentration decreases and that of products increase, i.e. reactants are converted to products. The occurrence of chemical reaction can be determined by a number of methods. We can check for the concentration of reactants and products over time. If the concentration of reactants is decreasing and that of products is increasing (or is more than what we started with), the reaction has taken place. A number of reactions are exothermic, i.e. they generate energy (such as mixing of solid sodium with water), for such reactions, we can feel the reactor surface and if it is hot (i.e. even without production of flames), reaction has taken place. In some cases, gases are produced as part of products and in such a scenario, we can either see them or smell them and know when the reaction has occurred.
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There are four signs of a chemical reaction. 1 - an unexpected color change, 2 - a precipitate has formed (this means a solid has been produced), 3 - energy has been released or absorbed, and 4 - the formation of gas bubbles.