Ice

We have plenty of ice here in Minnesota. It is the most common form of much of the water here for most of the year. Ice is bad when it’s on roads but good when it’s on lakes. We drive on it, skate on it, and drill holes through it to fish beneath it. People win or lose bets on when the ice “goes out” in the spring. In the long winters up here, we take our entertainment where we can find it.

We had the most ice when the Laurentide Ice Sheet oozed across the landscape like a gigantic amoeba, extending a lobe here, retreating there. This is not the behavior of ice most of us are familiar with today. When it is a few inches thick, ice shatters when struck hard. As it thickens further, ice becomes strong and solid enough to support a moose weighing 300 kilograms walking across a frozen beaver pond. At the same time, beaver and fish are protected from predators by the strength and winter-long permanence of the ice.

How can something which shatters when it is only a few inches thick become as strong as concrete and then flow like toothpaste as it progressively thickens? Why does ice float on the surface of water when the solid form of almost every other substance is denser than its liquid form? And why are the ice crystals so beautifully hexagonal?

Natural history questions like these arise from observations on the behavior of objects and species at human scales. But to understand them, we often need to investigate the properties of nature more deeply at finer and finer scales. This method of science is known as reductionism and, while it has taken some hits in recent decades, reductionism remains a powerful method in ecology and science in general when properly used.

The secrets of the natural history of ice – indeed, of every property of water – lie deep in the geometry and electrical properties of the water molecule. Every schoolchild knows that water is H20: two hydrogen atoms bonded with one oxygen atom. The geometry of these bonds is the source of the properties of liquid water and ice. In a water molecule, the two hydrogens “donate” their electrons to the oxygen in the sense that the two donated negative electrons spend more time around the oxygen atom than around the hydrogen atoms. This gives the oxygen atom a negative charge and the hydrogen atoms a positive charge.

Unlike kernels in a row of corn, the hydrogen atoms do not line up with the oxygen atom in a neat straight row. Instead, the hydrogen atoms in a water molecule connect to the oxygen offset from each other at an angle of about 105 degrees. The now positively charged hydrogen atoms are on one side of a water molecule, but the negatively charged oxygen is on the other side. Each water molecule is like a little dipole magnet. The positive hydrogen end of each molecule is attracted to the negative oxygen end of other surrounding water molecules. These additional bonds between the molecules are much weaker than the bonds within the molecule. As heat makes the molecules in liquid water jiggle about, the bonds between water molecules form and break rapidly while the bonds between oxygen and hydrogen within the molecule remain stable. The continuous forming and breaking of the weak bonds between molecules in liquid water is what makes water slippery when it is a thin layer on your kitchen floor or on the surface of an ice-covered pond.

As the earth moves along its orbit and the Northern Hemisphere turns away from the sun, sometime around the autumn equinox the water cools, the heat–induced jiggling of water molecules slows down, and it is easier for each molecule to form bonds with its neighbors. The electrical attraction between the positive hydrogen atoms in one molecule and the negative oxygen atoms in its neighbors draws them closer together. Molecules are closest to their neighbors at about 4 ˚C and this is when water is most dense. As lakes cool down to this temperature in the autumn, the colder and denser surface water sinks while the slightly warmer but less dense deeper water rises. The lake now “turns over”, mixing completely from top to bottom. This happens again in most lakes in the spring as they warm up to 4 ˚C. This turnover brings oxygen dissolved in water from the surface to the bottom of the lake and nutrients from the sediments at the bottom to the top, replenishing the food web throughout the lake with essential oxygen and nutrients.

As the earth continues in its orbit from the autumn equinox to the winter solstice, the Northern Hemisphere turns more and more away from the sun and gets colder. The jiggling between water molecules and neighboring water molecules diminishes further, allowing them to form an orderly structure with each other. In a water molecule, the hydrogen atoms are almost exactly 1 Ångstrom away from the oxygen atom (an Ångstrom is one–billionth of a meter). About 2.74 Ångstroms further away is the negatively charged oxygen atom of the neighboring molecules. At temperatures slightly above 0 ˚C, the neighbors’ oxygen atoms will be slightly off to the right and to the left of the 105 degree interior angle by about 7 degrees. So the total angle between a water molecule and its two neighbors opens up to 105 + 7 + 7 = 119 degrees, almost precisely the120 degree interior angle of a perfect hexagon. That we know these distances and angles with such precision is one of the real aesthetic pleasures of research and one of the great triumphs of chemistry.

As the temperature of the water approaches ˚C, the jiggling between molecules decreases further and it becomes easier to maintain the 119–120 degree angle and the 2.74 Ångstrom/arm length distance between neighboring water molecules. Collections of six neighboring water molecules start to form semi-rigid hexagonal rings and ice forms. This is the reason for the hexagonal crystalline form of ice. But there is a hole of empty space in the center of each ring. Such holes do not exist in liquid water because the jiggling of heat prevents them from being permanent. However, at 0 ˚C, water longer has the heat energy to close these holes and “gives up”, taking the path of least resistance and letting the holes form. The rings link up in sheets and the sheets begin to stack atop one another. The water molecules are starting to form a six-sided ice crystal. These holes within each hexagon actually connect into open channels throughout the crystal, pushing the molecules farther apart than they were at 4 ˚C. The ice is therefore less dense and can float atop the slightly warmer but denser water below. A thin film of ice begins to float atop a pond as winter comes upon us.

Eventually, so many of the molecules are locked solidly in hexagonal rings connected into stacked sheets that the ice is strong enough for moose and wolves to walk on and for people to skate on, build ice houses on, even drive trucks on. Someone breaks some of the rigid bonds between neighboring water molecules when he drills a hole through the ice with an auger to fish for walleye and lake trout. But no reason to fear. There are plenty of other strong hexagonal rings in the ice to keep this guy from going into the drink (usually).

Although the bonds between water molecules are strong enough in their hexagonal crystalline structure to hold trucks when the ice is a few feet thick, they are not strong enough to resist deforming by the weight of ice when it is greater than 50 meters thick. The ice is now a glacier which begins to ooze as the planes of hexagonal rings begin to slip past one another. The oozing is a property of the entire mass of glacial ice, not of single crystals or even molecules. Walk up to the snout of the glacier and whack it with a hammer and the brittleness of ice at small scales allows you to break off a chunk which you can melt for drinking water. Hence, ice on lakes or at the snout of a glacier is brittle but hundreds of meters of glacial ice oozes under its own weight.

Technically, at temperatures on earth, the molecules locked in hexagonal rings have formed a crystalline structure known as Ice I. As the temperature decreases or as pressure on the ice increases, the distance and angle between the molecules becomes distorted, and up to eight different forms of ice (known imaginatively as Ice II through VIII) can form. Because the distance and angles between the molecules are distorted, these forms of ice are denser than water and will therefore sink, unlike Ice I which floats. Except in laboratories, only Ice I exists on Earth. It is possible that at least some of the other forms of ice could be found on the outermost planets with their very high atmospheric pressures and frigid temperatures that would make International Falls, Minnesota, the Icebox of the Nation, seem like a tropical forest. (Kurt Vonnegut imagined a new form of ice, Ice IX, which was a military weapon in his novel, Cat’s Cradle. Fortunately, the militarily useful properties ascribed to Ice IX by Mr. Vonnegut are thermodynamically impossible and, equally fortunately, reality is not compelled to obey Mr. Vonnegut’s wonderful imagination).

As the earth continues in its orbit towards the spring equinox, the Northern Hemisphere turns back towards the sun. Then one day in late April or early May, the angle of the sun is much higher above the horizon than back in January and a warm breeze comes up from the south. Each molecule now has the energy to dance more with its neighbors. One day, those bonds between molecules break, the hexagonal rings melt away, and someone in a bar wins a bet on when the ice would “go out” and buys a round of drinks. Boats, canoes, and kayaks are slipped into the lake, and the hydrogen bonds between water molecules part easily before their bows.

For Further Reading

Eisenberg, D. and W. Kauzmann. 1969. The Structure and Properties of Water. Oxford University Press.

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