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The theory of a chemical reaction dates back about 200 years. Its origins are present in early experiments which classified substances into elements and compounds and in theories which explained these processes. Thus the concept of a chemical reaction had a primary role in defining the science of chemistry as it is known today.

A Chemical Reaction is a process in which one or more substances (known as the reactants), are converted into one or more different substances (known as the products). Substances can be either chemical elements or compounds. Another definition is that a chemical reaction rearranges the combining atoms of the reactants to create different substances as products.

Chemical reactions play an integral part of technology, of culture, and indeed in one’s life itself. These reactions have been used since thousands of years such as burning fuels, smelting iron; making glass and pottery, brewing beer, and making wine and cheese to name a few .Chemical reactions are also present in the geology of Earth, in the atmosphere and oceans, and in a vast array of complicated processes that occur in all living systems.

Classification of Chemical Reactions:

Chemical Reactions are classifies into six types. All the general chemical reactions come under these six categories.

1.) Combustion:

A combustion reaction is a type of reaction when oxygen combines with another compound in presence of air to form water and carbon dioxide .Reactions of these types are exothermic (i.e. they produce heat).

C10H8 + 12 O2 -> 10 CO2 + 4 H2O

2.) Synthesis:

A synthesis reaction is a type of reaction when two or more simple compounds combine to form a more complex one. The general form and example is given below.

A + B -> AB Generalized form

8 Fe + S8 -> 8 FeS Example

3.) Decomposition:

A decomposition reaction is the exact opposite of a synthesis reaction i.e. a complex molecule breaks down into simpler molecules. The general form and example is given below.

AB -> A + B Generalized form

2 H2O -> 2 H2 + O2

4) Single displacement:

This is when one element trades places with another element in a compound. i.e atoms of one molecules are exchanged with the atoms of other molecule.These reactions come in the general form of:

A + BC -> AC + B
Mg + 2 H2O -> Mg(OH)2 + H2

5) Double displacement:

This is when the anions and cations of two different molecules exchange places, forming entirely different compounds. These reactions are in the general form along with an example:

AB + CD -> AD + CB
Pb(NO3)2 + 2 KI -> PbI2 + 2 KNO3

6) Acid-base:

There are a special kind of double displacement reaction that takes place when an acid and base react to form salt and water. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation of water. These reactions are in the general form along with an example:

HA + BOH -> H2O + BA
HBr + NaOH -> NaBr + H2O


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