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Why does stainless steel float when coated with wax?

  1. Jul 22, 2010 #1
    Properties of Matter (Fluids)
    Why is it possible to float a wax-coated stainless steel needle on the surface of water?
     
  2. jcsd
  3. Jul 22, 2010 #2
    I think that it's surface tension that's keeping up the needle, regardless if it is waxed or not.

    As long as the needle isn't submerged in water, it should be able to float.
     
  4. Jul 23, 2010 #3

    K^2

    User Avatar
    Science Advisor

    Clean steel will be pulled into the water.

    It's about energy, really. Think of it this way, why does needle sink? Because that reduces needle's gravitational potential energy. But you also have to move some water particles out of the way. That may or may not require energy.

    If you have two water molecules next to each other, they are 'connected' by a hydrogen bond. To move the two molecules apart, you need to invest some energy to break that bond. This is what gives rise to surface tension. When the needle moves down, it moves molecules apart. If no new bonds are formed, that energy has to be supplied by the needle.

    A waxed needle forms no hydrogen bonds of its own with surrounding water molecules. That means, it requires energy for needle to become submerged, and it actually turns out to be greater than amount of available potential energy.

    A clean needle, however, will form its own bonds with water molecules. Perhaps not as strong as before, but enough to offset the above, and make it advantageous for the needle to sink.
     
  5. Jul 23, 2010 #4
    Thank you.
     
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