7.6: What Are Salts?

For those of us who grew up near the ocean and spent many a day splashing around in it, the saltiness of its waters seems natural and barely merits our notice. But for those people who were raised far inland, who were perhaps already in their teens or adult years prior to even seeing the ocean for the first time, seawater, when tasted, serves up a rather rude welcome. The sting of that first swallow on the virgin tongue begets an immediate spit and a loud ugh. “It’s salty,” they exclaim, as if somehow their elementary school teacher was only kidding when she said the sea was made of salt. “Yes, my dear,” you can hear their teacher explaining on the student’s retelling of the horrific tale, “seawater contains salt. It won’t harm you.”

Salt is the general term applied to all the dissolved substances that we find in seawater. But the dissolved substance in the greatest proportions in seawater has a familiar name and formula: sodium chloride (NaCl)—salt. As it turns out, seawater contains nearly all known natural elements in the periodic table—even gold. It’s said that if you could extract all the gold dissolved in the ocean—about 20 million tons—you would be a very rich person. Of course, what isn’t said is that the cost of extracting the gold using currently known chemical methods would exceed the amount you would make in selling it, or that if you put such a large quantity of gold on the market, the price per ounce would plummet. So don’t believe everything you read on the internet about gold in the ocean.

As solids, sodium and chlorine atoms stack up like a jungle gym, a playground structure made of crisscrossing metal pipes. They form a crystal lattice, the three-dimensional, symmetrical, and repetitive arrangement of atoms that forms a crystal. If you examine a sample of halite, a mineral formed from sodium chloride, you’ll notice regularly repeating cubes—the crystal lattice structure of sodium chloride. Diamonds also exhibit a crystal lattice structure. In a crystal of salt, the sodium and chloride atoms have exchanged electrons; the sodium atoms have given up an electron to the chlorine atom, and the chlorine atom has accepted that electron. This kind of bond, where atoms exchange electrons, is called an ionic bond. The ionic form of sodium is referred to as the sodium ion while the ionic form of chlorine is called the chloride ion. It is one of the two major kinds of chemical bonds, the other being the aforementioned covalent bond formed when atoms share electrons.

Dissolved Salts

As we learned above, water is a polar molecule. Not only do the positive and negative charges on the water molecule make it ideal for bonding with itself, they also allow water to bond with a great number of other kinds of atoms and molecules. In fact, any polar molecule will dissolve in water because opposite charges attract and form bonds. Even complex molecules, including some normally “insoluble” hydrocarbons, are at least partially soluble owing to a charge on their molecule. Strictly nonpolar molecules—those with a balance of charges and no dipole—are, of course, insoluble. Thus, a mixture of water and a nonpolar molecule, such as olive oil, will not combine. You know the saying, “Oil and water don’t mix.” Now you can explain why.

So powerful is water’s ability to bond with other atoms and molecules that it has been named the universal solvent. A solvent is a substance that dissolves another substance. The substance being dissolved is known as a solute. When water acts as a solvent, the substance that it dissolves is the solute.

Now, I want to make sure that you understand what the word “dissolving” means. When you dissolve one thing in another, the thing being dissolved becomes part of the solution of the other. But what does that mean exactly?

If you sprinkle a little bit of table salt in a bottle of water, the sodium and chlorine atoms in the salt become a few atoms among many molecules of water. In essence, they find a place to live within the spaces between the water molecules. They become part of a solution of water molecules and sodium and chlorine atoms. If you cap the bottle and keep it in a safe place for a hundred years and come back after that time and gaze upon that undisturbed, capped bottle of slightly salty water, where will you find the salt? You won’t. The salt will remain invisible because it is dissolved. It’s part of the matrix of the water. You will not—as many of my students erroneously answer when first asked—“find it at the bottom of the bottle.” The salts stay dissolved. As long as conditions inside the bottle remain unchanged (i.e., as long as we don’t freeze or evaporate the water), the salt will remain permanently dissolved, held in solution by water.

Now, in defense of those students who thought that the salt would be found at the bottom of the bottle, they did have one thing correct: Gravity acts on solutions. While the salts do not come out of solution—they do not turn into solids again—they can be very slightly concentrated in their dissolved form at the bottom of a column.

In the early 1800s, Gay-Lussac wondered something similar: Can the gravity-driven settling of salts explain the higher salinities in the deep ocean? He performed an experiment with six-foot-tall glass tubes in which he placed a salt solution for several months. At the end of that time, he measured the salinity at the top and the bottom of the tubes. He found no difference (e.g., Wallace 1974).

Nonetheless, calculations made by chemical oceanographer Ricardo Pytkowicz (1929–1991) indicate that had Gay-Lussac extended the experiment for years, and had he been able to measure salinity to an accuracy of thousandths, he would have found a salinity of about 34.994 parts per thousand (ppt) at the top and 35.006 ppt at the bottom, assuming a starting salinity of 35.000 ppt (Pytkowicz 1962). The salts remain dissolved, of course, but they do concentrate ever so slightly under the force of gravity as long as the water remains undisturbed. A minor point, perhaps, but one that illustrates how scientists do their best to think of every possibility.

Anions and Cations

When sodium chloride comes into contact with water, the ionic bonds between the sodium and chlorine atoms disappear. The atoms in the salts separate—dissociate—from one another. That’s because the polar water molecules have a stronger attraction for the sodium and chlorine atoms than the two elements have for each other. With the ionic bond gone, the sodium atom takes on a positive charge, and the chloride atom takes on a negative charge. They become what are known as ions—atoms or molecules whose number of positively charged protons differs from their number of negatively charged electrons.

Ions by definition have a net electrical charge—the sum of their positive charges and negative charges does not equal zero. Ions with a negative charge, such as chloride, are called anions, while ions with a positive charge, such as sodium, are called cations. In the written language of chemistry, the ionic form of an atom or molecule is represented by a superscript of + or − following the chemical formula. For example, the sodium ion is represented as Na⁺ and the chloride ion is represented as Cl^–. For cations and anions that have two or more positive or negative charges, respectively, the appropriate number is placed before the + or – sign, as in sulfate, SO₄^2– or magnesium, Mg²⁺. When salt dissolves in water, the electropositive hydrogen atoms of the water molecules surround the electronegative chloride ion, and the electronegative oxygen atoms of the water molecule surround the sodium ion. The sodium and chloride ions, in essence, become part of the fluid. As long as conditions remain constant, the sodium ions and chloride ions remain dissolved in solution.

States of Saturation

One final set of definitions completes our discussion of the dissolving power of water. The maximum concentration of a solute, including gases, that can be dissolved by a solution under a given set of conditions is called the saturation concentration. Two other states are possible: A solution may be undersaturated with respect to the solute, meaning the solution contains less of a substance than the saturation concentration; or the solution may be supersaturated, meaning the solution contains more of a substance than the saturation concentration.

Of course, the saturation concentration is not fixed; environmental factors such as temperature and pressure can change it. For example, at higher temperatures, the saturation concentration of salts in water increases. That is, water dissolves more salts at higher temperatures. Conversely, lowering the temperature reduces the saturation concentration. Thus, increases or decreases in temperature can cause a body of water to become undersaturated or supersaturated, respectively. States of undersaturation and supersaturation prove important to understanding dissolved gases in the ocean, too. However, gases behave differently in solution than do solids, as we shall see.

Electrostriction

As further testament to the magic of seawater, a curious thing happens when you dissolve NaCl in it: the volume of the solution decreases by about 3 percent (e.g., Pilson 2013). The reduction in volume of a solution when salts are added is called electrostriction, from electrical constriction. This reduction in volume results from the rearrangement of water molecules around solutes: positive hydrogens orient toward anions and the negative oxygen skews toward cations. In short, water and salts rearrange themselves into a tighter, more intimate space. Other salts cause electrostriction too. This property may help explain why seawater lacks the density maximum at 39.2°F of pure water (e.g., Pilson 2013).

The Periodic Table of the Ocean Elements

We can now examine the cast of characters—the elements—found in seawater. In 1997 Japanese oceanographer Yoshiyuki Nozaki (1947–2003) published an interesting electronic variation on the periodic table, one that included illustrations of the distribution of the elements in the water column. Nozaki’s work inspired researchers at the Monterey Bay Aquarium Research Institute to produce an online version, which they called the Periodic Table of the Elements in the Ocean (PTEO). The PTEO provides a fun and useful visual tool for studying the ocean elements.