MadSci Network: Physics

Re: why do bubbles disappear when they reach the water surface?

Date: Tue Aug 8 02:12:20 2006
Posted By: Joe Fitzsimons, Grad student, Quantum and Nanotechnology Theory Group, Department of Materials, Oxford University
Area of science: Physics
ID: 1153098384.Ph


Good question! The water molecule, H2O, comprises of a central oxygen atom with two hydrogen 
atoms bonded to it. Hydrogen atoms consist of a positively charged proton and a negatively 
charged electron. The bond between the oxygen and hydrogen atoms draws the hydrogen's 
electron away from  the proton towards the oxygen. This results in a slightly negative charge 
around the oxygen, due to the extra electrons, and a slightly positive charge around hydrogen. 
Since opposite charges attract, the hydrogen atoms within a water molecule are then attracted 
towards the oxygen atoms in other water molecules. This type of inter-molecular force is called 
hydrogen bonding.

When a water molecule is deep within a volume of water, the forces acting on it are balanced, 
since it is surrounded by other water molecules. For a water molecule on the surface, however, 
the situation is different. There is no water above it to pull it upwards. The result of this is that 
the surface behaves like an elastic sheet. Some insects can stand on the water surface due to 
surface tension, and it is also the reason you can fill a glass a little above the top without it 

When a bubble forms on the surface of water, water tension pulls in all directions, keeping it 
tight. If some point in the bubble becomes slightly thinner than the rest, it will stretch, becoming 
even weaker until the bubble bursts. As a result of this bubbles in water are unstable and will 
burst almost immediately. This is why bubbles in water burst when they reach the surface.

In order to make a bubble last, you need to add something (like soap) to the water to decrease 
its surface tension. As a soap bubble thins a weak point the soap is driven away from that area 
and the surface tension increases, strenghening that patch. In this way, the soap stabilizes the 
bubble, strengthening weak points, and weakening strong points. The result of this is that the 
weak points no longer become weaker and burst the bubble, giving you a much longer lived 

I hope this answers your question. Here are some references you might find useful:

Hydrogen bonding
Surface tension 
Soap Bubbles

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