Physics of boiling water?

Question

how do air bubbles form in the water?

Answer 1

Those aren't air bubbles, they are steam bubbles. When water boils it turns from a liquid into a gas, and the bubbles are gaseous water rising up, since the gas is less dense than the water around it.

Answer 2

When you first heat water to boiling you will see air bubbles.

Cool water can hold some air dissolved into solution (Think how do fish breath? They get Oxygen from the air dissolved in the water).   Water has a negative solubility for gases such as air so as the water warms up the air is driven out of solution and will form tiny bubbles.   You can also see this effect by leaving a cold glass of water out over night. As the water warms to room temp you will find tiny air bubbles coating the side of the glass.

When you boil water the rise in temp up to boiling first causes any dissolved air in the water to be driven out. The air will form a layer of tiny bubbles in the pot.

As the water continues to rise in temp and eventually boil you will see steam bubbles form at the bottom of the pan. This is where the heat is being applied so naturally this is the hottest spot and where the water will first turn to steam.  You may even see a thin stream of steam bubbles forming and rising to the top of the pot

Eventually the water heats up to boiling completely and you get what is called a rolling boil.  Very large steam bubbles and  vigorous churning of the water.

Answer 3

Lavalamp is 100% correct.

Answer 4

When running out of a tap (faucet), Air mixes with the turbulent flow of the water. The water becomes 'Aerated' by absorbing air into itself.

When first heating the water, bubbles of the dissolved air are first seen. This is the heat driving out the air.

As the water nears boiling point, the air has all been driven out but, other bubbles begin to form.

These are bubbles of steam (vaporising water) forming.

As the boiling point is reached, the water begins bubble more and more as the water vaporises and begins to 'Seethe' more violently as the steam bubbles fill the water and burst at he surface in great numbers as the steam production increases rapidly.