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In a common experiment, you can make a paper clip float on water due to the strong surface tension. Adding just a tiny bit of soap immediately destroys the surface tension and lets the paper clip sink.

At the same time, pure water tends not to make stable bubbles (with air for example), soapy water on the other hand is very good at that.

Isn't the stability of the soap bubble dependent on the surface tension of that soap-water mixture? Why is it that in one experiment, the surface tensions drops whereas in the other one it increases? Where is my mistake in tackling that problem?

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  • $\begingroup$ It's interesting to note that surface tension is not the only force that helps keep the clip afloat. The clip also displaces water and so Archimedes principle applies. There is a buoyant force. Of course the steel is more dense than the water, and so the force due to gravity and mass outweigh buoyancy. Nevertheless to properly model the static equilibrium and position of the clip relative to the water surface,one needs to take into account all three forces. $\endgroup$ – docscience Feb 27 '15 at 4:15
  • $\begingroup$ "Adding just a tiny bit of soap immediately destroys the surface tension and lets the paper clip sink." This immediately points out that it is not surface tension that makes bubbles stable and the hydrophobic nature of soap molecules has to be invoked to change the properties of the surface. $\endgroup$ – anna v Mar 13 '18 at 6:37
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Surface tension occurs because water molecules attract on another. That means that water prefers to form compact shapes with little surface area. Creating a large, extended area, as you do in a soap bubble, is actually opposed by surface tension. This is why you can't get stable bubbles with pure water: the bubble wants to collapse into a compact shape.

Soap changes the situation. Soap molecules have one end that is attracted to water molecules (the hydrophilic end); the other is not (the hydrophobic end). In water, the soap molecules will try to arrange themselves so their hydrophilic ends are in the bulk of the water but the hydrophobic ends are at the surface of the water (or clustered together to minimize contact with the water - these clusters are called micelles). This means it is no longer so costly for the water to form large surfaces, because the hydrophobic soap ends are the parts that are exposed at the surface instead of the water itself. In a very soapy solution, it's easy to form bubbles: you will have two layers of soap molecules, one on the inside and one on the outside. In between, a layer of self-attracting water molecules will hold the bubble together.

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  • $\begingroup$ To clarify, the reason why water doesn't form stable bubbles is exactly because it has such a high surface tension whereas soapy water allows (due to smaller surface tension) to form larger surfaces. $\endgroup$ – ahemmetter Feb 27 '15 at 5:30
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    $\begingroup$ Not exactly. Any amount of surface tension in pure liquids will always oppose bubbles, because it wants to minimize the surface. However, you need the liquid molecules to attract one another to provide the strength for the walls of the bubble. So stable bubbles require liquids with different properties at the surface than in the bulk. Soap does this by coating the surface with a hydrophobic layer. $\endgroup$ – user27118 Feb 28 '15 at 15:51
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Why is it that in one experiment, the surface tensions drops whereas in the other one it increases?

The surface tension decreases in both cases

Where is my mistake in tackling that problem?

To form a long-lasting bubble you need lower surface tension and something that reduces the rate of evaporation. The soap provides both these. The hydrophobic ends of the soap molecules project from the surface and provide some protection to the water molecules.

See

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