A decomposition reaction is a type of chemical reaction where one compound breaks down into two or more simpler substances. To identify which reaction fits this description, let's examine each of the given options:
Option A:
[tex]\[ Si + O_2 \rightarrow SiO_2 \][/tex]
This reaction represents the combination of silicon ([tex]\( Si \)[/tex]) and oxygen ([tex]\( O_2 \)[/tex]) to form silicon dioxide ([tex]\( SiO_2 \)[/tex]). This is a synthesis reaction, not a decomposition reaction.
Option B:
[tex]\[ CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O \][/tex]
In this reaction, methane ([tex]\( CH_4 \)[/tex]) reacts with oxygen ([tex]\( O_2 \)[/tex]) to produce carbon dioxide ([tex]\( CO_2 \)[/tex]) and water ([tex]\( H_2O \)[/tex]). This is a combustion reaction, not a decomposition reaction.
Option C:
[tex]\[ 2 Li + CaCl_2 \rightarrow 2 LiCl + Ca \][/tex]
Here, lithium ([tex]\( Li \)[/tex]) displaces calcium ([tex]\( Ca \)[/tex]) from calcium chloride ([tex]\( CaCl_2 \)[/tex]) to form lithium chloride ([tex]\( LiCl \)[/tex]) and calcium ([tex]\( Ca \)[/tex]). This is a single displacement reaction, not a decomposition reaction.
Option D:
[tex]\[ 2 H_2O_2 \rightarrow 2 H_2O + O_2 \][/tex]
In this reaction, hydrogen peroxide ([tex]\( H_2O_2 \)[/tex]) breaks down into water ([tex]\( H_2O \)[/tex]) and oxygen ([tex]\( O_2 \)[/tex]). This is a decomposition reaction because a single compound breaks down into two simpler substances.
From the analysis above, it can be concluded that the only decomposition reaction is:
Option D:
[tex]\[ 2 H_2O_2 \rightarrow 2 H_2O + O_2 \][/tex]
Thus, the correct answer is:
[tex]\[ \boxed{D} \][/tex]
