We believe that the main reason why water boils faster when the lid is on the pot is because of the energy that’s been compressed in one place instead of being released. Water has 3 stages which are solid, liquid and gas; when water is being boiled the water molecules start moving around the pot and becoming into gas. When putting the lid on the pot you are getting all that energy that's been released and compressing in one small place together forcing the water molecules to heat up faster and heating water a lot faster.
According to the website named Seasoned Advice, water has a boiling point. The bottom of the pot receives most of the energy which expands through the water while boiling. When the lid is on there is no air interacting with the water that's boiling at all therefore it makes it easy to boil fast.
By covering the pan, three things happen:
1. Radiation due to the increase of hot water is reflected back into the pan instead of being emitted to the air, which is a heat loss.
2. Convection is eliminated. Without a lid, the air increases as the hot water gets heated as well and rises to the top of the kitchen. Well, this air will get replace by the cooler, which the hot water will heat up again. Thus creating a heat loss, and by adding the lid you eliminate this loss.
3. Evaporative cooling is also eliminated. The increase of hot water will not only heat the air above but also evaporated into the air. Evaporation has an immense amount of heat. By putting a lid on the hot water you limit this evaporation significantly reducing the heat loss.
And this is why adding a lid to a pan with hot water will make the water boil faster.
WHAT EXACTLY IS BOILING?
Boiling is the transition from a liquid state into vapor or gas. It occurs at a temperature called the boiling point, which is where the vapor pressure of the liquid is the same as the external pressure around it. The bubbles are the vapor. In the case of boiling water or milk, the bubbles consist of water vapor. The bubbles expand as they rise, due to decreased pressure, eventually releasing to the surface as steam.