|MadSci Network: Physics|
Hi, 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 spilling. 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 bubble. I hope this answers your question. Here are some references you might find useful: Hydrogen bonding Surface tension Soap Bubbles
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