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A chemical reaction is defined as a process by which one or more substances (chemical elements or their compounds) called as reactants are converted into one or more different substances called as products by a rearrangement of atoms.

Chemical equations

A Chemical equation is used to represent a chemical reaction. The equation uses chemical formulae with the reactants on the left side of the equation and the products on the right side. The reactants and products are separated by an arrow. This arrow stands for “yield” and indicates the direction and the type of reaction. A chemical equation must be balanced. In a balanced equation the number of atoms for each substance should be the same on both sides of the equation.
Here is how the chemical equation is represented
A + B → C + D
Where A and B are the reactants that undergo a chemical reaction to generate the two products C and D.

Types of chemical reactions

There are different types of chemical reactions. The main categories are listed here:


Synthesis reaction / Combination reaction: In this reaction two or more reactants combine to form a complex product. The product in a synthesis or direct combination reaction is always a compound. It can be represented by the following equation:
A + B → AB
Example: Iron and sulphide combine to form iron sulphide.
8Fe + S8 → 8FeS

Decomposition reaction: This is the reverse of synthesis reaction. In this reaction a complex reactant breaks down into simpler products. Decomposition reactions often involve an energy source such as heat or light that causes the breakdown of the compound. It can be represented by the following equation:
AB → A + B
Example:  Electrolysis of water to make oxygen and hydrogen.
2H2O → 2H2 +O2

Single replacement/ single displacement reaction: In this reaction one uncombined reactant replaces another reactant in a compound or exchanges places with it. It can be represented by the following equation:
A + BC → AC +B
Example: Magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas.
Mg + 2H2O → Mg(OH)2 + H2

Double replacement reaction: In this  reaction the anions (negative ions) and the cations (positive ions) exchange places to form two new compounds. Double replacement reactions are also known as metathesis. It can be represented by the following equation:
AB + CD → AD + BC
Example: Lead nitrate with potassium iodide form lead iodide and potassium nitrate.
Pb(NO3)2 + 2KI → PbI2 + 2KNO3

Balancing chemical equations examples
As mentioned earlier a chemical equation must be balanced. For balancing the chemical equations you can use the following guidelines.

Add the number of each type of atom. The total number of atoms will be the same on both the sides of the equation.
Account for all types of atoms. Elements present on one side of the equation must be present on the other side of the equation. You should not factor out the coefficients.

Example 1:     6CO2 + 6H2O  → C6H12O6 + 6O2

Example 2:     2AgI + Na2S → Ag2S + 2NaI

To write balanced equation the first step is to write the unbalanced equation. The next step is to balance the equation using the above rules. When balancing an equation you cannot change the subscript. You can add coefficients. You have to look for the how many atoms you have on each side of the equation and add coefficients to the molecules to balance out the number of atoms.

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