The Shape of Water

September 25, 2011

WaterdropIMG_2148acutresepod (3)

Photographer: Phil Lachman
Summary Author: Phil Lachman

Why is a drop shaped like a drip? What keeps the water drop in the above photo joined to the leaf by a water bridge?

Water is made of two hydrogen atoms and one oxygen atom - its chemical name is dihydrogen monoxide. The chemical bonds that join the three atoms together are called covalent bonds -- the atoms share their outer electrons. However, having eight positive protons in its nucleus to hydrogen's one, the oxygen atom pulls the electrons closer to itself. This makes the oxygen atom slightly negative and the two hydrogen atoms slightly positive. With water, the covalent bonds are in fact polar covalent bonds. In addition, water molecules are bent into a V shape. Thus, it's a molecule that's a dipole - it has a negative side (the point of the V where the oxygen atom is positioned) and a positive side (where the hydrogen atoms are positioned -- at the tips of the V). This dipole charge on every water molecule makes them attractive to each other -- positive to negative and negative to positive. This electrostatic attraction creates a force called cohesion in the water, so it tends to pull together. The water drop tends to be spherical because of this, as shown by pictures of water drops in space.

The drip shape occurs because there's another force called adhesion. Water molecules also tend to be attracted to other substances or surfaces. Remember that glues are called adhesives. The water in the drop wants to pull together into a sphere, but also hang on to the leaf. This play between adhesion and cohesion creates a water bridge for a split second between the leaf and the drop, distorting the drop into a drip shape. Photo taken in Sydney, Australia on June 26, 2011.