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The Destructive Power of Ice Formation

Frost weathering (also called freeze-thaw weathering, frost shattering, frost wedging, or cryofracturing reduces boulders to rubble as a result of the tremendous forces created by the 9% increase in volume of water as it freezes. To understand these forces, we need to look into hydrogen bonds between water molecules.

Weathering involves disintegration of rocks through contact with the atmosphere. Weathering does not involve abrasion resulting from movement, so it is distinguished from erosion due to moving water or wind.

Figure 1: Frost damaged rock
Figure 2: Talus slope[1]

Although some geologists argue that the forces are not sufficient to create the enormous "talus slopes" or "screes" of rock rubble that results, the video found on this YouTube Video or below:

In the video, water is poured into a cast iron container, which is tightly sealed. The container is then placed in a acetone/dry ice sludge, which is at a temperature of -77°C. After a short period of time, the ice freezes, expands, and causes the cast iron container to explode, blasting off the cover of the acetone/dry ice bath, and spraying the bath itself everywhere. Even though the cast iron container had ⅛ inch thick sides, the pressure of the expanding ice was still able to blow it apart.

The force required to shatter the cast iron container results from the formation of a hydrogen bonded structure in ice that has large open spaces or voids, as shown in Figure 3. Although there are several crystal forms of ice, the one that forms under typical atmospheric pressures has the structure shown in Figure3.

Figure 3. Two computer images of the structure of ice. The water molecules have been arranged, so that each oxygen atom is surrounded by four hydrogen atoms in tetrahedral geometry. Two of these atoms are covalently bound to oxygen, while the other two are hydrogen bonding with the oxygen.
Figure 4. Hydrogen bonded structure of ice

There is clear evidence of hydrogen bonding in the structure of the solid. Figure 3 shows two computer-drawn diagrams of the crystal lattice of ice. In the model we can clearly see that each O atom is surrounded by four H atoms arranged tetrahedrally. Two of these are at a distance of 99 pm and are clearly covalently bonded to the O atom. The other two are at a distance of 177 pm. They are covalently bonded to other O atoms but are hydrogen bonded to the one in question. The situation is thus:

Image:Covalent and H Bonding in Water.jpg

As in the case of HF, the distance between molecules is abnormally short. The sum of the van der Waals radii of H and O is 260 pm, considerably larger than the observed 177 pm.

The tetrahedral orientation of H atoms around O atoms which results from hydrogen-bond formation has a profound effect on the properties of ice and of liquid water. Ice is the only known non-metallic substance to expand when it freezes. The density of ice is 0.9167 g/cm³ at 0°C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Comparing the volumes of equal masses of ice and water,


\frac{V_{ice}}{V_{water}}= \frac{\frac{m}{D_{ice}}}{\frac{m}{D_{water}}} =\frac{\frac{1}{0.9167 \text{g/ml}}}{\frac{1}{0.9998 \text{g/ml}}} =\frac{0.9998}{0.9167} = \frac {1.091}{1.000}

So freezing water at 0°C leads to a 9.1% increase in volume, from 1.000 to 1.091 mL, for example, and the expansion of freezing water in rock crevices widens the crevices until the rock is split. The same process happens to roadways, and is the reason for new cracks and potholes seen on roads after a cold winter.

Liquid water is densest, essentially 1.00 g/cm³, at 4°C and becomes less dense as the water molecules begin to form the hexagonal crystal as the freezing point is reached. The Density of ice increases slightly with decreasing temperature and has a value of 0.9340 g/cm³ at −180 °C (93 K)[2].

When ice melts, some of the hydrogen bonds are broken and the rigid crystal lattice collapses somewhat. The hexagonal channels become partially filled, and the volume of a given amount of H2O decreases. This is the reason that ice is less dense than water and will float on it. As the temperature is raised above 0°C, more hydrogen bonds are broken, more empty space becomes occupied, and the volume continues to decrease. By the time 4°C has been reached, increased molecular velocities allow each H2O molecule to push its neighbors farther away. This counteracts the effect of breaking hydrogen bonds, and the volume of a given amount of H2O begins to increase with temperature.

Since water has maximum density at 4°C, water at that temperature sinks to the bottom of a deep lake, providing a relatively uniform environment all year around. If ice sank to the bottom, as most freezing liquids would, the surface of a lake would not be insulated from cold winter air. The remaining water would crystallize much more rapidly than it actually does. In a world where ice was denser than water, fish and other aquatic organisms would have to be able to withstand freezing for long periods.

Hydrogen bonding also contributes to the abnormally large quantities of heat that are required to melt, boil, or raise the temperature of a given quantity of water. Heat energy is required to break hydrogen bonds as well as to make water molecules move faster, and so a given quantity of heat raises the temperature of a gram of water less than for almost any other liquid. Even at 100°C there are still a great many unbroken hydrogen bonds, and almost 4 times as much heat is required to vaporize a mole of water than would be expected if there were no hydrogen bonding. This extra-large energy requirement is the reason that water has a higher boiling point than any of the other hydrides.

The fact that it takes a lot of heat to melt, boil, or increase the temperature of water, makes this liquid ideal for transferring heat from one place to another. Water’s ability to store heat energy is also a major factor affecting world climate. Persons who live near large lakes or oceans experience smaller fluctuations in temperature between winter and summer than those who inhabit places like Siberia, thousands of kilometers from a sizable body of water. Freeze weathering is much more important in climates that are not moderated by large bodies of water. Ocean currents, such as the Gulf Stream, convey heat from the tropics to areas which otherwise would be quite cold.

References

  1. http://en.wikipedia.org/wiki/Scree
  2. Lide, D. R., ed. (2005), CRC Handbook of Chemistry and Physics (86th ed.), Boca Raton (FL): CRC Press, ISBN 0-8493-0486-5
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