One covalently bonded molecule that plays a major role in living systems is water-H₂O. In fact, water is the most abundant molecule in your body, making up about two-thirds of your body weight. Although it seems to be a simple molecule, water has many surprising properties. For example, of all the common molecules on Earth, only water exists as a liquid at the Earth's surface. When life on Earth was beginning, this liquid provided a medium in which other molecules could move around and interact. Life evolved as a result of these interactions. And life, as it evolved, maintained these ties to water (Figure 4).

Figure 4 Water is the cradle of life. This mass of frog eggs is attached to a rock in the watery environment of a stream bottom.

   Three-fourths of the Earth's surface is covered by water. Where water is plentiful, such as in the tropical rain forests, the land abounds with life. Where water is scarce, such as in the desert, the land seems almost lifeless except after a rainstorm. No plant or animal can grow and reproduce without some amount of water.

   The chemistry of life, then, is water chemistry. Water has a simple molecular structure: one oxygen atom bonded by single covalent bonds to two hydrogen atoms. The resulting molecule satisfies the octet rule, and its positive and negative charges are balanced. Because the oxygen atom in each water molecule contains eight protons and each hydrogen atom contains only one proton, the electron pair shared in each covalent bond is more strongly attracted to the oxygen nucleus than to either of the hydrogen nuclei. Although the electrons surround both the oxygen and hydrogen nuclei, the negatively charged electrons are far more likely to be found near the oxygen nucleus at a given moment than near one of the hydrogen nuclei. Because of this situation, the oxygen end of the water molecule has a partial negative charge. The hydrogen end has a partial positive charge. (The shell model and space-filling model both show this.) Molecules such as water that have opposite partial charges at different ends of the molecule are called polar molecules. Water is one of the most polar molecules known.

The polarity of water contributes to its ability to attract other molecules and form special types of chemical bonds with them. These weak electrical attractions between molecules are called hydrogen bonds, and have approximately 5% to 10% the strength of covalent bonds. In fact, water forms hydrogen bonds with other water molecules because the hydrogen atoms of some water molecules are attracted to the oxygen atoms of other water molecules. (These bonds are shown as dotted lines between water molecules in the illustration of the molecular structure of water and ice in the Just Wondering box.) The ability of water molecules to form weak bonds among themselves and with other molecules is the reason for much of the organization and chemistry of living things. Although hydrogen bonds are weak, these bonds are constantly made and broken. (Each lasts only 1/100,000,000,000 of a second!) The cumulative sans-serif">effect of very large numbers of hydrogen bonds is responsible for the many important physical proper ties of water. Surface tension-the ability of water molecules to "stick together" at the water's surface-is one such property. It is the reason that water molecules can bear the weight of organisms such as the water strider shown to the right.

Water Is a Powerful Solvent

Water molecules gather closely around any particle that exhibits an electrical charge, such as ions and polar molecules. For example, sodium chloride (table salt) is made up of the positively charged sodium (Na⁺) and negatively charged chloride (Cl^-) ions. These ions are attracted to one another and cluster in a regular pattern, forming crystals. When you put salt in water, some ions break away from the crystals because the positive ends of some water molecules are attracted to the Cl^- ions, while the negative ends of other water molecules are attracted to the Na⁺ ions. These attractions are shown in the right-hand portion of Figure 5.

Figure 5 How salt dissolves in water.

   These attractions are stronger than the attraction between the ions that keeps the crystal together. Therefore, the ions are pulled from their positions in the crystal as is shown on the left. Water molecules then surround each ion, forming a hydration shell, which keeps the ions apart. The salt is said to be dissolved.
      Similarly, hydration shells form around all polar molecules and ions. Compounds that dissolve in water this way are said to be soluble (SOL-you-ble) in water. Chemical interactions readily take place in water because so many kinds of compounds are water soluble and therefore move among water molecules as separate molecules or ions.

Water Organizes Nonpolar Molecules

   Remember the old saying that "oil and water don't mix?" This statement is true because oil is a nonpolar molecule and cannot form hydrogen bonds with water. Instead, the water molecules form hydrogen bonds with each other, causing the water to exclude the nonpolar molecules. It is almost as if nonpolar molecules move away from contact with the water. For this reason, nonpolar molecules are referred to as hydrophobic (HI-dro-FO-bik). The word hydrophobic comes from Greek words meaning "water" (hydros) and "fearing" (phobos). This tendency for nonpolar molecules to band together in a water solution is called hydrophobic bonding (see left).

Crude petroleum from an oil spill floats on the surface of the ocean becuase it is hydrophobic and less dense (lighter) than water.

Hydrophobic forces determine the three-dimensional shapes of many biological molecules, which are usually surrounded by water within organisms.