How come a needle can float on top of water? asks Loretta, via email.
Have you ever seen a long-legged insect skitter across a pond? Water striders can stroll on water, thanks mainly to surface tension. Surface tension, a property of liquids that makes the surface act like an elastic skin or membrane, also causes water to bead up on wax paper. That same tension keeps water hanging in a big drop on the end of a faucet as if it’s held in a tiny, see-through balloon.
Surface tension also allows a metal sewing needle to rest on the surface of a bowl of water, even though it’s too dense to “float.”
What causes surface tension? Imagine a bowl full of water. Water molecules under the surface are strongly attracted to their fellow water molecules. Each molecule of water, or H2O, has two hydrogen atoms and one oxygen atom. Because of how the molecules are constructed, the hydrogen end has a positive electrical charge, while the oxygen end has a negative charge. Since opposites attract, water molecules tend to pull together, the hydrogen atoms in one molecule attracted to the oxygen in another.
This ever-shifting electrical attraction known as hydrogen bonding holds the molecules together in a loose liquid mass. But at the surface, water molecules aren’t equally attracted to gas molecules in the air above them. Their main attraction is “downward and inward,” to other water molecules in the bowl, and to their fellow H2O molecules on the surface. The effect of this attraction is to create a tight surface, like a thin, rubbery skin on the water.
(Different liquids have “skins” that are stronger or weaker. Alcohol, for example, has a weaker surface tension than water. But liquid mercury’s surface tension is six times stronger, which is why a broken mercury thermometer spills out those gray (and very toxic) little balls.)
So if we gently place a lightweight sewing needle lengthwise on the surface of a bowl of water (using tweezers helps), the needle should float. (It also helps if the needle is a little oily, or water-repellant.) Look closely, and you’ll see a little indentation where the needle rests, as if the water’s surface were a sheet of rubber. As long as the needle isn’t too heavy, the forces exerted along its length by surface tension will trump the downward-pulling force of gravity.
But put a few drops of liquid detergent in the water, and you’ve changed the balance of force. Detergents (and soaps) are surfactants, or “surface-active agents.” Surfactants turn down the surface tension of water. They actually make water wetter, by disrupting and weakening the attraction between water molecules. The result: Gravity wins; your needle sinks like a stone.
But you can also sink your needle in plain water simply by pushing it under the springy surface. Or just turn it on end, piercing the surface “skin.” Finally, try gradually raising the temperature of the water. As water molecules gain energy and pull apart, surface tension will decrease, and the needle will fall.
