7.5: The Water Molecule

Writing water as H-O-H helps us visualize why water looks like Mickey Mouse. When the hydrogen and oxygen atoms come together as a water molecule—which they do because they are attracted to each other—they share their electrons in a covalent bond. Because oxygen is a larger atom with a bigger nucleus, it’s a bit more “powerful” than hydrogen, and, frankly, it hogs the electrons. Because electrons are negatively charged, being held closer to oxygen gives oxygen a slightly negative charge. We say that the oxygen atom in a water molecule is electronegative. Because the hydrogen atoms are stripped of some of their electron blanket, they become slightly positive, or electropositive. This separation of charges—the positive charge on the hydrogen atoms and the negative charge on the oxygen atoms—makes water a polar molecule, one that exhibits electropositive and electronegative ends. This property of the bonding of two hydrogen atoms and one oxygen atom gives water its characteristic V shape. When the atoms are represented as circles, the water molecule looks like Mickey Mouse.

Hydrogen Bonds, Cohesion, and Surface Tension

Water’s extraordinary ability to dissolve comes from its polarity. In the world of atoms, opposites attract. Positive charges attract negative charges, and vice versa. So it is with water. Because the oxygen atom of the water molecule is electronegative and the hydrogen atoms are electropositive, water molecules attract each other. In fact, the oxygen atom of one water molecule forms a bond with the hydrogen atom of another water molecule, a bond known as a hydrogen bond. This is not a strong bond—it’s a weaker covalent bond—but it’s strong enough to give water molecules some fascinating properties.

If you place a drop of water on a tabletop, what does it do? Rather than flattening out and running off the table like other liquids do, water beads up. Rain on a car hood will form beads of water.Dew on grass appears as beads of water. Beads of water pop out of your forehead when you sweat. The beading up of water comes about because of hydrogen bonds. It’s a property of the water molecule known as cohesion, the “sticking together” of molecules. Water is a sticky molecule; it likes to stick to other water molecules.

At the surface of a body of water, the stickiness of water gives rise to another property: surface tension—the enhancement of cohesion at the surface of a liquid. It’s this property that allows insects to skate across the top of a pond without falling in. A needle placed lengthwise on the top of a glass of water will remain afloat because of surface tension. The water molecules at the surface form stronger hydrogen bonds than the water molecules beneath them because there are no water molecules above them competing for their bonds. Think of it like being able to hold onto something with two hands instead of one—two hands are stronger. That’s how it is with surface water molecules. The result is a kind of “skin” on the surface of water in a glass, pond, lake, or ocean.

Water Is Dense

Another somewhat obvious property of water is that it weighs a lot. If you’ve ever lugged a five-gallon container of water, you know what I mean. A single gallon of water weighs eight pounds! The weight of water doesn’t come from the atoms that make it up; hydrogen is the lightest atom in the periodic table, and oxygen only has an atomic mass of 16. Compare that to the heaviest atom, uranium, at 238 atomic mass units. Where liquid water gets its weight is from its density. Density refers to the amount of matter packed into a given volume, or mass per unit volume. Liquid water is 800 times more dense than air. So there are a lot of water molecules packed into a liter of water compared to a liter of air, and that makes a bottle of water relatively heavy.

Most substances, especially gases, exhibit a change in density when their temperature changes or when they are subject to changes in pressure. Typically, an increase in temperature will reduce a substance’s density—its molecules spread apart—while a decrease in temperature increases its density—the molecules move closer together. Greater pressure increases density, while lower pressure reduces it.

Pure water, however, doesn’t quite obey the temperature rule. Pure water reaches its maximum density at about 39.2°F, or 4°C. Below 39.2°F, water becomes less dense. The reason for this involves some complicated aspects of the behavior of water molecules. But you can think of it this way: like people, water molecules prefer a certain amount of personal space. Get too close, and they balk. “Get back,” they say. That limit of tolerance for closeness to other water molecules happens at about 39.2°F. But let’s be clear: this holds only for pure water. Seawater doesn’t exhibit this property.

Water, including seawater, also doesn’t obey the pressure rule. For the most part, water is incompressible, unable to be compressed. However, water can be compressed slightly, and this gives rise to some interesting changes in seawater’s temperature as pressure increases with depth. We’ll visit this topic in our chapter on deep circulation.

Water’s abundance on Earth makes it useful as a standard for comparing the density of other liquids, a quantity known as specific gravity. Scientists define specific gravity as the density of a substance divided by the density of water. Thus, substances with a density greater than water will have a specific gravity greater than one, and substances with a density less than water will have a specific gravity less than one. See how that works? The density of water, and especially factors that change the density of seawater—namely, temperature and the saltiness of the seawater, its salinity—prove critical to understanding layering of the ocean and ocean circulation, as we shall see in later chapters.