Chemical reaction is a process, which results in change of matter. In other words, substance(s) participating in a chemical reaction turns into another substance. In first approximation we can say that molecules of reacting chemicals exchange, unite or expel atoms or group of atoms. Formal expression of chemical reaction is called chemical equation.

CaCO3 + HCl --> CaCl2 + H2O + CO2

This record means that calcium carbonate interacts with hydrogen chloride and as a result calcium chloride, water and carbon dioxide are formed. Substances on the left side of chemical equation are called reactants and on the right side are products.

Because matter is indestructible, number of atoms of the same element on the left side must be equal to the number of the atoms on the right side. It is said that equation must be balanced.

CaCO3 + 2HCl --> CaCl2 + H2O + CO2.

As you see you cannot change subscripts because number of atoms in a molecule is constant. Therefore you can change only number of molecules what in turn changes total number of atoms.

There are four basic types of chemical reactions:
 
 

  1. Synthesis (Ex. CaO + CO2 --> CaCO3)
  2. Decomposition (Ex. 2KClO3 --> 2KCl +3O2)
  3. Single replacement (Ex. Zn2 + HCl --> ZnCl2 + H2)
  4. Double replacement or exchange (Ex. C6H6 + Br2 --> C6H5Br + HBr)
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