Chemical change
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A chemical change is a process in which reactants are changed into one or more different products. A chemical change occurs whenever new compounds are formed or existing compounds decomposed. During this reaction, chemical bonds between atoms are broken and new chemical bonds formed. This results in a rearrangement of the chemical bonds, thus differing from physical change which is a change of a substance's state (solid, liquid, gas).
There are several different types of chemical change. These include synthesis, decomposition, single displacement, double displacement, neutralization, precipitation and redox.
Indicators of a chemical change include a colour change, the formation of a precipitate, the formation of gas, production (exothermic) or loss (endothermic) of heat, production of light, the appearance of a new substance, a use-up of a starting substance or a change in temperature or energy. When new substances are formed, a chemical change has occurred, and a chemical reaction has taken place. Note that, in a few cases, exothermic reactions may be hot enough cause certain chemicals to also undergo a change in state; for example in the case of aqueous solutions, bubbles may not necessarily be newly produced gas but instead water vapour.
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