Water has some unusual properties that most of us do not really appreciate or understand. These properties are crucial to life and they originate from the structure of the water molecule itself. This sidebar will provide an overview of water's properties that will be useful in understanding the behavior of water in Earth's environment.
A molecule of water is composed of two atoms of hydrogen and one atom of oxygen. The one and only electron ring around the nucleus of each hydrogen atom has only one electron. The negative charge of the electron is balanced by the positive charge of one proton in the hydrogen nucleus. The electron ring of hydrogen would actually prefer to possess two electrons to create a stable configuration. Oxygen, on the other hand, has two electron rings with an inner ring having 2 electrons, which is cool because that is a stable configuration. The outer ring, on the other hand, has 6 electrons but it would like to have 2 more because, in the second electron ring, 8 electrons is the stable configuration. To balance the negative charge of 8 (2+6) electrons, the oxygen nucleus has 8 protons. Hydrogen and oxygen would like to have stable electron configurations but do not as individual atoms. They can get out of this predicament if they agree to share electrons (a sort of an energy "treaty"). So, oxygen shares one of its outer electrons with each of two hydrogen atoms, and each of the two hydrogen atoms shares it's one and only electron with oxygen. This is called a covalent bond. Each hydrogen atom thinks it has two electrons, and the oxygen atom thinks that it has 8 outer electrons. Everybody's happy, no?
However, the two hydrogen atoms are both on the same side of the oxygen atom so that the positively charged nuclei of the hydrogen atoms are left exposed, so to speak, leaving that end of the water molecule with a weak positive charge. Meanwhile, on the other side of the molecule, the excess electrons of the oxygen atom, give that end of the molecule a weak negative change. For this reason, a water molecule is called a "dipolar" molecule. Water is an example of a polar solvent (one of the best), capable of dissolving most other compounds because of the water molecule's unequal distribution of charge. In solution, the weak positively charged side of one water molecule will be attracted to the weak negatively charged side of another water molecule and the two molecules will be held together by what is called a weak hydrogen bond. At the temperature range of seawater, the weak hydrogen bonds are constantly being broken and re-formed. This gives water some structure but allows the molecules to slide over each other easily, making it a liquid.
Studies have shown that clustering of water molecules occurs in solutions because of so-called hydrogen bonds (weak interaction), which are about 10% of the covalent water bond strength. This is not inconsiderable and energy is required to break the bonds, or is yielded by the formation of hydrogen bonds. Such bonds are not permanent and there is constant breaking and reforming of bonds, which are estimated to last a few trillionths of a second. Nonetheless, a high proportion of water molecules are bonded at any instant in a solution. But this structure leads to the other important properties of water.
We will consider, for the purposes of this course, only six of these important properties:
As mentioned above, these properties have importance to physical and biological processes on Earth. Effectively, large amounts of water buffer Earth surface environmental changes, meaning that changes in Earth-surface temperature, for example, are relatively minor. Thus, the high heat capacity of water promotes continuity of life on Earth because water cools/ warms slowly relative to land, aiding in heat retention and transport, minimizing extremes in temperature, and helping to maintain uniform body temperatures in organisms. However, there are other effects of water properties as well. Its low viscosity allows rapid flow to equalize pressure differences. Its high surface tension allows wind energy transmission to sea surface promoting downward mixing of oxygen in large water bodies such as the ocean. In addition, this high surface tension helps individual cells in organisms hold their shape and controls drop behavior (have you seen "An Ant's Life"?). Also, the high latent heat of evaporation is very important in heat/water transfer within the atmosphere and is a significant component of transfer of heat from low latitudes, where solar energy influx is more intense to high latitudes that experience solar energy deficits.
Take a few minutes to learn why water is the most fascinating and important substance in the universe.
Water does not give up or take up heat very easily. Therefore, it is said to have a high heat capacity. In Colorado, it is common to have a difference of 20˚ C between day and night temperatures. At the same time, the temperature of a lake would hardly change at all. This property originates because energy is absorbed by water as molecules are broken apart or is released by molecules of water associating as clusters.
Take a few minutes to watch the video below to help you understand heat capacity.
A calorie is the amount of heat it takes to raise the temperature of 1 gram (0.001 liters) of pure water 1 degree C at sea level. It takes 100 calories to heat 1 g. water from 0˚, the freezing point of water, to 100˚ C, the boiling point. However, 540 calories of energy are required to convert that 1 g of water at 100˚ C to 1 g of water vapor at 100˚ C. This is called the latent heat of vaporization. On the other hand, you would have to remove 80 calories from 1 g of pure water at the freezing point, 0˚ C, to convert it to 1 g of ice at 0˚ C. This is called the latent heat of fusion.
Interestingly, the latent heat and freezing and boiling points are controlled by the way water molecules interact with one another. Because molecules acquire more energy as they warm, the association of water molecules as clusters begins to break up as heat is added. In other words, the energy is absorbed by the fluid and molecules begin to dissociate from one another. Considerable energy is required to break up the water molecule clusters, thus there is relatively little temperature change of the fluid for a given amount of heating (this is the heat capacity measure), and, even at the boiling point, it takes far more energy to liberate water molecules as a vapor (parting them from one another). On the other hand, when energy is removed from water during cooling the molecules of water begin to coalesce into clusters and this process adds energy to the mix, thus offsetting the cooling somewhat.
When water is a liquid, the water molecules are packed relatively close together but can slide past each other and move around freely (as stated earlier, that makes it a liquid). Pure water has a density of 1.000 g/cm3 at 4˚ C. As the temperature increases or decreases from 4˚ C, the density of water decreases. In fact, if you measure the temperature of the deep water in large, temperate-latitude (e.g., the latitude of PA and NY) lakes that freeze over in the winter (such as the Great Lakes), you will find that the temperature is 4˚ C; that is because fresh water is at its maximum density at that temperature, and as surface waters cool off in the Fall and early Winter, the lakes overturn and fill up with 4˚ C water.
However, as dissolved solids are added to pure water to increase the salinity, the density increases. The density of average seawater with a salinity of 35 o/oo (35 g/kg) and at 4˚ C is 1.028 g/cm3 as compared to 1.000g/cm3 for pure water. As you add salts to seawater, you also change some other properties. Incidentally, increasing salinity increases the boiling point and decreases the freezing point. Normal seawater freezes at -2˚ C, 2˚ C colder than pure water. Increasing salinity also lowers the temperature of maximum density. This effect also helps explain why you are supposed to add salt to ice when making ice cream or to add salt to water when cooking spaghetti (although, in this case, the effect on boiling point is minor and the added salt is mainly for flavor).
When water freezes, however, bonds are formed that lock the molecules in place in a regular (hexagonal) pattern. For nearly every known chemical compound, the molecules are held closer together (bonded) in the solid state (e.g., in mineral form or ice) than in the liquid state. Water, however, is unique in that it bonds in such a way that the molecules are held farther apart in the solid form (ice) than in the liquid. Water expands when it freezes making it less dense than the water from which it freezes. In fact, its volume is a little over 9% greater (or density ca. 9% lower) than in the liquid state. For this reason, ice floats on the water (like an ice cube in a glass of water). This latter property is very important for organisms in the oceans and/or freshwater lakes. For example, fish in a pond survive the winter because ice forms on top of a pond (it floats) and effectively insulates (does not conduct heat from the pond to the atmosphere as efficiently) the rest of the pond below, preventing it from freezing from top to bottom (or bottom to top).
If water did not expand when freezing, then it would be denser than liquid water when it froze; therefore it would sink and fill lakes or the ocean from bottom to top. Once the oceans filled with ice, life there would not be possible. We are all aware that expansion of liquid water to ice exerts a tremendous force. Have you or a family member (you wouldn't admit to this would you?) ever left a full container of water with a tight-fitting lid (or even a can of soda?) in the freezer? In other words, 10 cups of water put into the freezer is going to turn into 11 cups of ice when it freezes (oops). The force of crystallization of ice is capable of bursting water pipes and causes expansions of cracks in rocks, thus accelerating the erosion of mountains!
A rough sketch of water molecules in ice crystal form is below.
Next to mercury, water has the highest surface tension of all commonly occurring liquids. Surface tension is a manifestation of the presence of the hydrogen bond. Those molecules of water that are at the surface are strongly attracted to the molecules of water below them by their hydrogen bonds. If the diameter of the container is decreased to a very fine bore, the combination of cohesion, which holds the water molecules together, and the adhesive attraction between the water molecules and the glass container will pull the column of water to great heights. This phenomenon is known as capillarity. This is a key property that allows trees to stand high, for example, because surface tension stiffens stems and trunks. Plants "wilt" because they are unable to acquire sufficient water to maintain the required surface tension. And, of course, water droplets (rain) and fog condensing as droplets on surfaces are a function of water's surface tension. Without this property, water would be a slimy coating and cells would not have shape. Surface tension decreases with temperature and salinity.
Please take a few minutes to watch this amusing video to learn more about the surface tension of water.
This is, of course, another key property of water because more substances dissolve in water than any other common liquid. This is because the polar water molecule enhances "Dissolving Power." Dissolution involves breaking "salts" into component "ions." For example, NaCl (common salt) breaks down into the ions Na+ and Cl- because of the attraction for ions (atoms or groups of atoms with a charge) to water molecules is high.
Cations, such as Na (Sodium) have a net positive charge, whereas anions (such as Cl, Chloride) have a net negative charge. There are many individual elements and compounds that form ions. Thus, water can hold considerable concentrations of various chemical species depending on their particular properties. Note how the water molecules surround the individual ions, keeping them isolated from other ions in solution. This occurs until the capacity of water to isolate the ions is exceeded, at which point the solution is "saturated" with those ions and cannot dissolve more (salt will begin to precipitate—form a solid).