Question

which one of the following is a decomposition reaction?
ch4(g) + o2(g) → co2(g) + h2o(l)
caco3(s) → cao(s) + co2(g)
mg(s) + o2(g) → mgo(s)
pbco3(s) → pbo(s) + co2(g)

Explanation:

Step1: Recall decomposition reaction definition

A decomposition reaction is a reaction in which a single - compound breaks down into two or more simpler substances.

Step2: Analyze each option

• For the reaction $\mathrm{CH}_{4}(g)+\mathrm{O}_{2}(g)

ightarrow\mathrm{CO}_{2}(g)+\mathrm{H}_{2}O(l)$, it is a combustion reaction (a compound reacts with oxygen to produce carbon - dioxide and water), not a decomposition reaction.

• For the reaction $\mathrm{CaCO}_{3}(s)

ightarrow\mathrm{CaO}(s)+\mathrm{CO}_{2}(g)$, a single compound $\mathrm{CaCO}_{3}$ breaks down into two simpler substances $\mathrm{CaO}$ and $\mathrm{CO}_{2}$, which is a decomposition reaction.

• For the reaction $\mathrm{Mg}(s)+\mathrm{O}_{2}(g)

ightarrow\mathrm{MgO}(s)$, it is a combination (synthesis) reaction where two substances combine to form one compound.

• For the reaction $\mathrm{PbCO}_{3}(s)

ightarrow\mathrm{PbO}(s)+\mathrm{CO}_{2}(g)$, a single compound $\mathrm{PbCO}_{3}$ breaks down into two simpler substances $\mathrm{PbO}$ and $\mathrm{CO}_{2}$, which is a decomposition reaction.

Answer:

$\mathrm{CaCO}_{3}(s)
ightarrow\mathrm{CaO}(s)+\mathrm{CO}_{2}(g)$ and $\mathrm{PbCO}_{3}(s)
ightarrow\mathrm{PbO}(s)+\mathrm{CO}_{2}(g)$ are decomposition reactions.