Which reactions are taking place when a saucepan of water boils on the stove?

Question

I remember that boiling itself it has something to do with vapor pressure, but what's happening during the stages when (1) a few bubbles form on the bottom and float to the top, then (2) the entire bottom of the pan looks like it's covered in tiny bubble frost, then (3) the rolling boil?

Answer 1

There are no "reactions" in the sense of chemical reactions, because what you are describing is a physical change, not a chemical change.

To bring a liquid to its boiling point requires some energy to break the intermolecular bonds, hydrogen bonds, in the case of water.  You start with the energy of the molecule and add enough to cause it to break apart of from its adjacent molecules.  Since some molecules start at higher energy it takes less additional energy to bring them to the vapor phase and these account for you early bubbles.

Gradually more molecules acquire enough energy to go to the vapor state to give you frost-like look.  Then lots of molecule reach the energy needed to go to the vapor state.  But even with a rolling boil, not all of the molecules have acquired enough energy to vaporize, otherwise the entire pan of water would go to steam at one time and that would be very, very exciting.

Answer 2

Charles' Law is your best friend here, he says as temperature increases, volume increases. You are going from liquid to liquid/gas mixture, which increases pressure and increases volume the container takes up. A few bubbles forming means you are starting to overcome the activation energy of water, which is why you see the small bubbles. You are also increasing entropy, since you are going from a more compact state, (liquid) to a less compact state (gas).  

Answer 3

The only chemical reaction is that of burning gas if gas is being used.

Heat of combustion of gas or from a heated element if electric, is adding heat energy to the water.

1. As the water warms up, dissolved air in the water begins to expand and show up as tiny bubbles on the bottom.

2. The continued addition of heat increases the rate at which the air leaves the water and, at the same time, the water at the bottom of the pan is receiving maximum heat. It begins to form bubbles of water vapour (like a cool steam) that forms more bubbles at the bottom.

3.When the water begins to reach boiling point, the vapour (steam) production is such that, larger bubbles are rising to the surface.

All of this rising of the air and steam bubbles is due to a process of 'Convection'...currents of rising hot water and falling cooler water in a circulatory fashion.

As the water actually reaches boiling temperature, this process is now quite violent and the water begins its 'Rolling' or 'Seething' action as the steam is escaping to the atmosphere.