2.4–2.7: Water has features that enable it to support all life.

Three drops of water bead on a blade of grass.

Every so often, a tadpole or small fish gets an unexpected—and life-ending—surprise from above as a giant spider moves quickly across the surface of the water, reaches down with its two front legs, and plucks the animal from the water. The spider, known as the fishing spider (Dolomedes triton), injects its prey with venom, carries it back to shore—moving across the water’s surface as if across solid ground—and eats it. How can these spiders walk on water?

The fishing spider, like numerous other insects such as the water strider, makes use of the fact that water molecules have tremendous cohesion. That is, they stick together with unusual strength. This molecular cohesiveness is due to hydrogen bonds between the water molecules.

Each water molecule is V-shaped (FIGURE 2-13; see also Figure 2-11). The hydrogen atoms are at the ends of the two arms and the oxygen is at the bottom end of the V, between the two hydrogen atoms. Oxygen’s strongly positively charged nucleus pulls the circling electrons toward itself and holds on to them for more than its fair share of the time. Consequently, the oxygen at the bottom of the V has a slight negative charge and the other end of the water molecule, containing the hydrogen atoms, has a slight positive charge.

Figure 2.13: Walking on water! A multitude of hydrogen bonds makes this possible for some animals.

Because of their unequally shared electrons, water molecules are polar (see Figure 2-11). Consequently, large numbers of water molecules orient themselves so that the negative side of one molecule is near the positive side of another. Hydrogen bonds form between the relatively positively charged hydrogen atoms and the relatively negatively charged oxygen atoms of adjacent water molecules.

Hydrogen bonds, as we’ve noted, are much weaker than covalent and ionic bonds, and they don’t last very long. Nonetheless, to the fishing spider, the cumulative effect of all the hydrogen bonds in water is to link together all the water molecules in the stream just enough to give the water a surface tension with some net-like properties.