The following are not decomposition reactions :
B. CaO + H2O ⇒ Ca(OH)2
C. 2Mg + O2 ⇒ 2MgO
Further explanation
The decomposition reaction is a chemical reaction shows the decomposition of a compound into its constituent elements or compounds
General formula :
AB⇒A+B
A. KCIO3 → KCI + O2
decomposition reaction
B. CaO + H2O ⇒ Ca(OH)2
synthesis/combination reaction
C. 2Mg + O2 ⇒ 2MgO
synthesis/combination reaction
D. 2NaOH ⇒ Na2O + H2O
decomposition reaction