Decomosition reaction is single compound break into two or more simple substances under suitable conditions.
calcium carbonate is decomose by heating and form calcium oxide and carbon dioxide
CaCO3 -->  CaO + CO2       

displacement reaction in which a more reactive element displaces a less reactive element. 
 zinc react with hydrochloric acid and zinc displace the hydrogen and produce zinc chloride and hydrogen gas.
 Zn + 2HCl -->  ZnCl₂  + H₂ 
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