I was cooking pasta. It is more efficient to boil with the lid on. However even with a steam hole in the lid and on the lowest heat, the water would bubble up and boil over.
Lifting the lid, a visible burst of water vapour was released upwards.
And the liquid settled down.
Here's what I think is happening: Normally as water boils, steam forms in bubbles inside the liquid. The pressure in the bubbles is higher than that in the surrounding air so when the bubbles reach the surface, the pop and release water vapour into the air. With the lid on, the water vapour can't go anywhere, so adding extra particles. It is also hotter so the particles move faster and are at a higher pressure. This means that bubbles of steam reaching the surface do not pop as easily because the pressure outside is now equal to the pressure inside. Hence it bubbles up. Lifting the lid releases the pressure because particles depart. The bubbles pop and water vapour is released.
