Which of the following is NOT a type of Combination reaction?

Heating of limestone

The correct answer is option 2. Heating of limestone.

Here's why:

Combination reactions involve two or more elements or compounds combining to form a single compound.

Adding water to Quick lime (calcium oxide): This is a combination reaction. Calcium oxide reacts with water (calcium hydroxide), forming a single compound.

Formation of water: This is also a combination reaction. Hydrogen and oxygen elements combine to form water \((H_2O)\).

Burning of Coal: This is a combustion reaction, a specific type of combination reaction. Coal (a complex mixture of hydrocarbons) reacts with oxygen (from the air) to form various products, including carbon dioxide and water.

Heating of limestone (calcium carbonate): However, in this process, calcium carbonate decomposes into calcium oxide and carbon dioxide. It does not involve two or more elements or compounds coming together to form a new one. Therefore, it's not a combination reaction.

In summary, while the other options involve two or more substances combining to form a single product, heating limestone simply breaks down a single compound into two separate products.