Write combination and decomposition reaction with one example each.

Hint:These two reactions are very common in chemistry and we can easily find either of them in any chemical reaction. As it can be proposed from their names itself, combination reaction is formation of bonds and decomposition is breaking of bonds.

Complete step by step answer:
Firstly, let us discuss about combination reactions-
Combination Reaction- It is also known as a synthesis reaction where two or more elements or compounds, known as the reactants, react with each other and combine to form a single compound, called the product. We can represent these reactions as-
     \[X+Y\to XY\]
The combination of two or more reactants to give a single product is known as combination reaction. Example of a combination reaction is-
     \[MgO+C{{O}_{2}}\to MgC{{O}_{3}}\]
Combination reactions can be between two different compounds like in the above reaction where magnesium oxide combines with carbon dioxide to give magnesium carbonate.
It can also be carried out between two elements or a compound and an element as shown in the reactions below-
     \[\begin{align}
  & C+{{O}_{2}}\to C{{O}_{2}} \\
 & 2CO+{{O}_{2}}\to 2C{{O}_{2}} \\
\end{align}\]
Here, carbon dioxide is formed in both the cases but in the first reaction (element + element), Carbon is completely burnt in oxygen to get carbon dioxide whereas in the second reaction (compound + element), oxygen combines with carbon monoxide and carbon dioxide is produced.
Combination reactions are generally exothermic. When a bond is formed between the reactants, heat is evolved during the process making it exothermic.
Now coming to decomposition reaction,
Decomposition reaction- It is the type of chemical reaction in which a single compound breaks down into two or more elements. These reactions can be represented as-
     \[AB\to A+B\]
These reactions are generally endothermic, as breaking of bond requires a high amount of energy.
A simple example of decomposition reaction is hydrolysis of water where a water molecule is broken down into hydrogen and oxygen gas.
 We can write the reaction as-
     \[2{{H}_{2}}O(l)\to 2H(g)+{{O}_{2}}(g)\]
Some compounds (like metal carbonates) readily decompose when exposed to high heat while others do not-
     \[MC{{O}_{3}}\xrightarrow{\Delta }MO+C{{O}_{2}}\]
The metal is denoted as ‘M’ in the above reaction where metal carbonates when heated decomposes into its oxide and releases carbon dioxide.

Note: It should be noted here that for combination reactions there is no specific number of reactants needed.
It is also important to remember here that to break a chemical bond heat is required and while formation of a bond, heat is released.