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A water molecule is polar: its oxygen pulls electrons away from its two hydrogens, and its bent shape leaves one partly negative end and one partly positive end. Neighboring water molecules therefore attract each other through hydrogen bonds. These bonds explain the properties that make water suited to life: cohesion and adhesion, a high specific heat, a high heat of vaporization, the lower density of ice than liquid water, and the ability to dissolve many polar and ionic substances. The way water excludes nonpolar molecules, the hydrophobic effect, also drives the formation of membranes and the folding of proteins.
Pair each property with a quick demonstration and a biological consequence: count drops of water that fit on a coin (cohesion, which helps water move up xylem), compare how fast sand and water warm in sunlight (specific heat, which keeps coastal climates mild), and watch ice float (density, which keeps lakes from freezing solid). Then shake oil and water together and relate the separation to membrane formation.
Cells are mostly water, and water's unusual behavior traces back to the shape and charge of one small molecule.
In H2O, oxygen is more electronegative than hydrogen, so the shared electrons spend more time near the oxygen. The molecule is bent, with an angle of about 104.5 degrees between the two O–H bonds. The oxygen end carries a partial negative charge and the hydrogen ends carry partial positive charges. When water molecules meet, the hydrogen of one is attracted to the oxygen of another. This attraction is a hydrogen bond. Each one is weak and short-lived, but in liquid water huge numbers of them form and break constantly.
Several properties follow. Cohesion is the attraction of water molecules to each other, and it produces surface tension strong enough to support a water strider. Adhesion is the attraction of water to other polar surfaces, such as the walls of xylem cells. Together they let tall trees pull continuous columns of water up from their roots.
Water also resists changes in temperature. Its specific heat, about 4.18 joules per gram per degree Celsius, is high because much of the heat added goes into breaking hydrogen bonds before molecules can move faster. Oceans and lakes therefore warm and cool slowly, and so do bodies made mostly of water. Water's heat of vaporization is high too, which is why evaporation of sweat or of water from leaves cools them so well.
Unlike most substances, water is less dense as a solid. In ice, hydrogen bonds lock each molecule to four neighbors in an open lattice. Liquid water is densest at about 4 °C. So ice forms at the surface of a lake and floats, insulating the water below, where fish and other organisms survive the winter.
Water is an excellent solvent for polar and ionic substances. Its partial charges surround ions and polar molecules in hydration shells and pull them into solution. Substances that dissolve readily are hydrophilic. Nonpolar substances such as oils are hydrophobic. Water molecules bond to each other in preference to them, so the nonpolar molecules end up clustered together. This hydrophobic effect makes phospholipids assemble into membranes and makes proteins fold with nonpolar side chains on the inside.
Finally, a tiny fraction of water molecules split into hydrogen and hydroxide ions. In pure water at 25 °C the two are equal and the pH is 7. Cells use buffers to hold pH steady, because hydrogen ions affect protein shape.