Water Properties and Behavior

Water
Properties and Behavior

by Anthony Carpi, Ph.D.

In many ways, water is a miracle liquid. It is essential for all living things (on this planet at least), and it is often referred to as a universal solvent because many substances dissolve in it. These unique properties of water result from the ways in which individual H2O molecules interact with each other.
watermolecule2
Electronic Distribution in H2O

In the Chemical Bonding lesson we discussed the dipole that forms across the water molecule as a result of the polar covalent bonding between hydrogen and oxygen (see our Chemical Bonding module for more information). Because the bonding electrons are shared unequally by the hydrogen and oxygen atoms, a partial negative charge (ð-) forms at the oxygen end of the water molecule, and a partial positive charge (ð+) forms at the hydrogen ends. Since the hydrogen and oxygen atoms in the molecule carry opposite (though partial) charges, nearby water molecules are attracted to each other like tiny little magnets. The electrostatic attraction between the ð+ hydrogen and the ð- oxygen in adjacent molecules is called hydrogen bonding.
hydrogen - bond - Hydrogen Bonding between Water Molecules

Hydrogen Bonding between Water Molecules

Hydrogen bonding makes water molecules "stick" together. While hydrogen bonds are relatively weak compared to other types of bonds, they are strong enough to give water many unique properties. For example, hydrogen bonds sank the Titanic, and hydrogen bonds allow the Basilisk lizard to walk on water (as a result, the Basilisk has earned the nickname "Jesus" lizard).

Just how does hydrogen bonding do this? Well, let's start with the Titanic. The Titanic sank because it hit an iceberg - a chunk of ice floating on the surface of the ocean. The reason ice floats is because of hydrogen bonding. In water's liquid form, hydrogen bonding pulls water molecules together. As a result, liquid water has a relatively compact, dense structure. The animation below illustrates this idea.

Liquid Water and Hydrogen Bonding

Concept simulation - Reenacts hydrogen bonding between molecules of liquid water.



As water freezes into ice, the molecules become frozen in place and begin to arrange themselves in a rigid lattice structure, as shown in the animation linked below.

Ice and Hydrogen Bonding

Concept simulation - Reenacts hydrogen bonding between molecules of solid water.


The structure that forms in the solid ice crystal actually has large holes in it. Therefore, in a given volume of ice, there are fewer water molecules than in the same volume of liquid water. In other words, ice is less dense than liquid water and will float on the surface of the liquid. Throw in one really big chunk of ice and a cruise ship, and you begin to see the problems that can arise.

Surface Tension: As we just discussed, neighboring water molecules are attracted to one another. Molecules at the surface of liquid water have fewer neighbors and, as a result, have a greater attraction to the few water molecules that are nearby. This enhanced attraction is called surface tension. It makes the surface of the liquid slightly more difficult to break through than the interior.

When a small object that would normally sink in water is placed carefully on the surface, it can remain suspended on the surface due to surface tension. The Basilisk lizard makes use of the high surface tension of water to accomplish the incredible feat of walking on water's surface. The Basilisk can't actually walk on water; rather, it runs on water, moving its feet before they break through the surface.

from: http://www.visionlearning.com/library/module_viewer.php?c3=&mid=57&l=