The idea that water has a ‘skin’ is confusing.
I just saw Adam Hart Davis explain surface tension by showing that the molecules link together to provide a surface.
But I was asking, why only on the surface? surely the molecular linking is the same ‘inside’ as it is on the surface.
So what is different about the border between water and air that allows something very light to sit on top of the water (but heavy enough to sink if it goes below the surface)?
I’m no expert, but I can bring two facts to the table:
No, it is NOT a myth, it is scientific fact.
It has to do with the “adhesive” property of water.
I’m sure there’s some science people who can elaborate further.
The molecules all have an attraction for each other - a slight force pulling them together - all through the water. The result of this is an apparent “tension” at the surface, trying to minimize the surface area. But the actual force responsible for it is all through the water, not just at the surface.
CurtC got it pretty much right.
Think of it this way; in the middle of a cup of water these forces would act in all directions and pretty much cancel out. The difference occurs in the boundary area between water and air. You have an imbalance in the forces since one side of the boundary (the air side) is not subjected to the same forces.
Hope this helps, I’ll try to come up with a better explanation.
Or wait, maybe it was “cohesion” not adhesion. Whatever…
Imagine a bunch of people all holding hands in a line. Each person is pulling hard on the arms of the people on either side. All the people in the middle each have a person on either side pulling equally, so they don’t move. But the two people on either end get pulled tightly in, because they are only being pulled from one side. Expand that model to three dimensions and you have surface tension.
Interesting slightly related thread
Secret of walking on water uncovered. Fascinating article on how aquatic insects move
To visually confirm surface tension, fill a cup with water until you get a dome of water on the top - there’s slightly more water there than the cup holds. Now add a drop of soap, which works (partly) by disrupting surface tension. The dome should collapse, spilling a bit of water on your counter.
Alcohol doesn’t really have much in the way of surface tension - if you’ve got something really really high proof kicking around, you’ll notice that it doesn’t have any doming effect should you pour a shot of it.
Surface tension is real. However, there’s a myth about “breaking” surface tension by adding soap, etc., as if the skin gets torn. Soap just LOWERS surface tension. It’s impossible to “break” surface tension, since there’s no “skin” there to be broken.
And another by the way, surface tension is what keeps just plain water from doing a good job cleaning clothes, etc. The purpose of soap is to “break” the surface tension (sorry, bbeaty, that’s the common term) which makes the water “wetter.” It then gets all around the globs of dirt to dissolve them.
I’ve heard that explanation, mostly in older books. Soap and detergent molecules generally work by offering a good bonding environment for both water (a polar, usually negatively charged, ‘hydrophilic end’) and oils (a non-polar hydrocarbon-like ‘lipophilic end’). When they dissolve oily deposits on clothes, soap molecules form a sphere around droplets of grease, with the lipophilic ends (usually drawn as ‘tails’) on the inside and the hydrophilic ends (drawn as ‘heads’) on the outside. This permits the grease, surrounded by soap, to become soluble in water.
Detergents would also affect cohesion at the air-water interface by disrupting the usual water-water bonding (i.e. hydrogen bonding).