Question

Why are decomposition reactions called the opposite of combination reactions? Write an equation for these reactions.

Answer 1

In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.
For example: $CaC{O}_3\left(s\right)\to CaO\left(s\right)+C{O}_2\left(g\right)$
While, in a combination reaction, two or more substances simply combine to form a new substance.
For example: $CaO\left(s\right)+C{O}_2\left(g\right)\to CaC{O}_3\left(s\right)$
Hence, they are opposite to each other.