Water (previous version): _{Properties and Behavior}

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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.

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 (Figure 1). 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 (Figure 2).

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.

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.  

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.

Water as a solvent

The partial charge that develops across the water molecule helps make it an excellent solvent. Water dissolves many substances by surrounding charged particles and "pulling" them into solution. For example, common table salt, sodium chloride, is an ionic substance that contains alternating sodium and chlorine ions (see Figure 3).

When table salt is added to water, the partial charges on the water molecule are attracted to the Na⁺ and Cl^- ions.  The water molecules work their way into the crystal structure and between the individual ions, surrounding them and slowly dissolving the salt.  The water molecules will actually line up differently depending on which ions are being pulled into solution.  The negative oxygen ends of water molecules will surround the positive sodium ions; the positive hydrogen ends will surround the negative chlorine ions, as shown in Figure 4.

In a similar fashion, any substance that carries a net electrical charge, including both ionic compounds and polar covalent molecules (those that have a dipole), can dissolve in water. This idea also explains why some substances do not dissolve in water. Oil, for example, is a nonpolar molecule. Because there is no net electrical charge across an oil molecule, it is not attracted to water molecules and therefore does not dissolve in water.