Whenever a water molecule in the liquid state gains enough kinetic energy, it escapes the bonds of the liquid molecules and becomes a gas molecule. And that means our compound goes from the liquid state to the gas state. But note that our molecular formula remains H 20. And that means evaporation is a physical reaction. Now, on the contrary, whenever a compound goes from one molecular formula to a different compound with a different molecular formula, this type of reaction is known as a chemical reaction. Now, examples include combustion of hydrocarbons in which a hydrocarbon burns in the presence of oxygen to produce carbon dioxide and water.