Question

when potassium nitrate is heated it decomposes to form potassium nitrite and oxygen.write a balanced equation and add the state symbols.

Answer 1

The chemical reaction is a decomposition reaction. The balanced reaction is ----
  2 KNO3 --> 2KNO2 & O2 

Answer 2

The thermal decomposition reactions are defined as the chemical reactions in which the reactant molecules decompose due to the heating of it at high temperature. These are basically type of a decomposition reaction.  

Here potassium nitrate decomposes to form potassium nitrite $\left(2 \mathrm{KNO}_{2}\right)$ and oxygen gas $\left(\mathrm{O}_{2}\right)$.

Since the process occurs due to the heating of potassium nitrate, therefore, it is an example of a thermal decomposition reaction.  

The balanced chemical equation is  expressed as:

$2 \mathrm{KNO}_{3} \rightarrow 2 \mathrm{KNO}_{2}+\mathrm{O}_{2}$