(c) Anthony K. Grafton 2003

Wonderful Wetting Water

Water is the basic ingredient for all life. But what is water, and why does it have such amazing properties?

Every water molecule contains one oxygen atom and two hydrogen atoms. Each hydrogen atom is chemically attached to the oxygen atom. It is the fact that the three atoms bond together to make a bent molecule (rather than bonding together in straight line) that gives water most of its unusual properties. Because of this angular shape, each water molecule is polar, which means that it has positive end and a negative end. In the case of water, the negative end is near the oxygen atom, and the positive end is near the hydrogen atoms.

Molecules that are polar act toward each other like little bar magnets: the positive end of one molecule is attracted to the negative end of another molecule, and vice versa. In water, this strong attraction between molecules results in very high melting (0 ^oC) and boiling (100 ^oC) points when compared to other molecules of similar size. If it weren’t for such a high boiling point, water would be a gas at Earthly temperatures and we likely wouldn’t be here to think about it.

Another property of water that results from the strong attractive forces between molecules is a high surface tension. Surface tension is a measure of the amount of force needed to stretch or deform the surface of a liquid: the higher the surface tension, the more force is needed to stretch the surface of the liquid. The high surface tension of water allows some spiders and insects such as the water strider to walk along the surface of a puddle or pond without getting wet. These creatures simply don’t have enough mass to break through the surface of the water.

Here’s an interesting experiment to try at home. Get permission from a parent, and ask to borrow three small sewing needles of different sizes. If you place each needle on its side very gently on the surface water in a small glass, it will “float”. (Be careful, even a small amount of pressure from your finger will cause the needle to break through the surface and sink.) Once all three needles of different sizes are suspended on the surface, warm the water slowly, perhaps by placing it in a sunny window. Which needle will break through the surface and sink first? Why would warming the water cause the surface tension to change enough to allow the needles to break through? Send your responses in to Science Corner.