The cohesion of water molecules to each other is caused by the hydrogen bonding between the molecules. At the water's surface this interplay is asymmetrical and a rather unusual phenomenon occurs. It appears as though the liquid water has an invisible skin on it. (Perhaps you have seen nature videos of "army ants." When the first ones get to a gap, they link legs to form a bridge so that the following ants have an easier time in crossing. The exterior ants on this bridge are very much like H-bonded water molecules - transiently holding, yet strong.) This invisible "skin" on water can be visualized by floating sewing needles upon the surface. Of course, the metallic needle is several times denser than is water, such that if the needle is dropped onto the surface of water, it rapidly sinks to the bottom. But, using the most gentle of techniques, a needle of surprising size can be gently lowered onto the surface and it floats! Actually, this is not true floating - no more than you float in a waterbed. In the bed you lie atop a plastic membrane that acts as your "boat." The "floating" needle lies atop a film of tightly hydrogen bonded water molecules.

As said, H-bonds are transient bonds. They are constantly being formed and destroyed at phenomenal rates - zillions of formations and breakings per second. But such is "life" on the molecular level - FAST! Because the rate at which these bonds are formed and destroyed is a manifestation of Brownian Motion (see India Ink Microscopy), and since Brownian Motion IS a manifestion of temperature, we would expect to see that by increasing temperature we should see changes in the strength of the surface tension film atop water. We shall now demonstrate this magic right before your own eyes - using your own hands! - in two experiments. We don't want you to think that there are any masterful illusions going on. This is pure science! You shall "float" four differently sized needles on some ice water. Then you shall slowly raise the temperature of the water to see what happens as surface tension decreases. The thickest needle will soon no longer be able to be supported by the surface film and it will sink to the bottom. As the temperature rises more, the next thickest needle will sink, and so on. (Please note: it is not the length of the needle that counts, just the gauge or thickness of the needle.)

1. Heating the Needles' Bath Water
   1. Set-up jobs for three students
      1. One person obtains four needles from the instructor and threads them (the needle threader helps!) with different colored pieces of thread. (See data sheet for the proper sizes for this part of the experiment.) Red on the thickest needle, yellow on the next thickest one, green on the next to thinnest, and blue on the thinnest. (Mnemonic: in the sprectrum of the rainbow: red light has longest wave length, blue the shortest.)
      2. One person sets up a hotplate, a foil pan for atop the hotplate, and obtains a thermometer.
      3. One person procures about a liter of ice-water (but no ice in the water)
   2. Using a five inch piece of plastic coated wire (unfolded plastic coated paper clip or a twist tie), produce something that looks like this:
   3. Lean the thermometer in the water in such a way that the temperature can readily be seen without having to move the thermometer.
   4. Rub the "red" needle over a piece of wax (e.g.: candle) and rub the needle's sides with your fingers to smooth the wax over the surface. Next, scrape the point of the needle on the wax to place a very small bead of wax on the sharp tip. The bit of wax should be about the size of a grain of sand.
   5. Cradle the "red" needle in the holder and SLOWLY lower the needle onto the water's surface. It is a good idea to allow the far end of the piece of red thread to drape over edge of the contain - sort of like tying your boat to land. (This is important because needles are often slightly magnetic and attract each other - you must keep them separated!)
   6. Now do the same with the next smaller needle, etc., etc., until all four needles are "afloat."
   7. Turn on the heat.
   8. Have your data recording sheet ready.
   9. As each needles sinks, record the temperature next to its color. Continue taking temperatures up to boiling.
   10. Turn off the hotplate, and pour about 1/2 volume of cold water into the hot water. This should cool it sufficiently for you to safely carry the container to the sink to dump it. Don't disgard the thermometer (of course!), nor the needles.
   11. Graphing and comparison of data come later.

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2. Changing the Surface Tension
   1. Determine which experimental group your are assigned:
      1. Salt water (1% NaCl = 10 gms of NaCl mixed into your liter of water) (Read about how to make solutions.)
      2. Alcohol water (mix 10 ml EtOH into your liter of water)
      3. Detergent water (Add ONE DROP of liquid dishwashing detergent to the liter of water in your container. Slowly stir it in so as not to make foam. Again: NO FOAM!)

   2. For this experiment you will need the same set-up as earlier except you need neither the hot plate nor the thermometer. You have ice water in your container, and your needle collection will be more extensive.
   3. Starting with your "red" needle, wax it, and see if you can "float" it.
   4. Working down in sizes from the full set that the instructor has, determine the largest needle you can "float" (don't forget the dark green, purple and violet ones!)
   5. Record that answer on your data sheet
   6. Send a messenger to the other two groups to obtain their answers for your data sheet.
   7. Be ready to explain what each treatment did to the surface tension.