When the vapour pressure of any liquid becomes equal to atmospheric pressure, liquid start converting into gaseous from, it is called “BOILING” while the temperature at which it occurs is called boiling point of that liquid.
At normal condition the vapour pressure is very much lower than the atmospheric pressure. When the liquid is heated the energy of molecules increases so the vapour pressure of liquid also increases continuously. After the certain time the vapour pressure of liquid becomes equal to atmospheric pressure (760 Torr) therefore, liquid is converted in to gaseous form and boiling occurs.
“Those Liquid which boils at room temperature and heating is not necessary are called VOLATILE LIQUID”.