What are double displacement reactions? 

Give an example of a double displacement reaction, where gas is evolved.

What is a decomposition reaction? 

Decomposition reactions can occur by heat. Give two balanced equations for it. 

State the type of reaction for the following equation and balance if it is not balanced. 

${\mathrm{Cl}}_2+2\mathrm{KBr}\to 2\mathrm{KCl}+{\mathrm{Br}}_2$

State the type of reaction for the following equation and balance if it is not balanced. 

$\mathrm{NaOH}+\mathrm{HCl} \to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$

State the type of reaction for the following equation and balance if it is not balanced. 

$2\mathrm{HgO}\to 2\mathrm{Hg}+{\mathrm{O}}_2$