ROBERT C. PLUMB
chemical principles exemplified
Squeak, Skid and Glide-The Unusual Properties of Snow and ice lllurtmting LeChcltelier's principle, and the Clopeyron equotion
Information provided by Professor N . H. Fletcher The University of New Enghnd Armidale, N . 8. W., Australia
"It must be really cold out, the snow squeaks!" This empirical rule is known to many who live, work, or play in cold climates. If it is relatively warm, but below freezing, the snow is a soft blanket muffling all sound; but walk on snow o u a bitter cold day or night when the temperature is - 10°F or lower, and the snow "squeaks" underfoot. The Canadian Safety Council recently came up with some interestine facts about safe driving on smooth ice. They showed that when i t is very cold, around zero, sand and studded snow tires lose their effectiveness in helping a driver maneuver. At higher temperatures their value is readily apparent. These are two more manifestations of the unusual properties of this common material, H20(s). Two better known examples of the unusual behavior of ice are the low friction of ice skating and the passage of a weighted wire through a block of ice, with subsequent healing of the cut, called regelation. Pressure melting appears to explain all of these phenomena, providing one takes care to use the local pressures which exist a t the points of contact of macroscopically rough or even molecularly rough surfaces. (This differs from the conclusions of Van Hook, et al. [J.CHEM.EDUC. 48, 116 (1971)] who, neglecting the surface roughness of the ground steel surface of ice skate blades, concluded that the lowering of the melting point by 1.6"C by a 150-lb man is not sufficient to explain skating a t - 30°C.) Qualitatively, in terms of LeChatelier's principle, we see that the equilibrium HIO(s) = HzO(l)
will be shifted to the right by an increase in pressure because the molar volume of HIO(s) is about 9% greater than that of HzO(l). Applying a pressure creates a stress which can be relieved by shifting of the equilibrium to the right. Quantitatively this is described by the Clapeyron equation, which tells how the temperature, T, of a phase transition varies with pressure, P. dP. dT
AH,.. =
T(V8 - V,)
Inserting numerical values of the molar volumes and the enthalpy of fusion, and assuming these do not change with T and P one obtains AT
=
-0.0075 deg atm-I AP
An increase of pressure by 133 atm will lower the freezing point by 1°C. What happens when you walk on a layer of snow crystals? The crystals are in contact a t a few points and the weight of your body concentrated on these points of contact produces enormous local pressures. Providing the snow is not too cold, melting will occur and the crystals slip past each other, lubricated by a water film. If the temperature is too low, the pressures are not sufficient to melt the ice and the flow is unlubricated. The squeak probably results from the pressure, as they are compacted. And when you make a snowball the liquid formed by pressure melting, upon refreezing, forms the bonds which hold the ball together-but you can't make a snowball from very cold snow, it won't stick together. And why doesn't sand improve driving on ice a t very low temperatures? For the sand to increase friction, it must be imbedded in the ice and to get i t imbedded there must be pressure melting. But if it is too cold, the local pressures of sand between a tire and smooth ice aren't great enough to cause melting. Thus the sand slides freely on the ice, providingno benefit. So when making snowballs or walking on snow on a cold night, or driving on ice, remember the Clapeyron equation-it's controlling the phenomena you are experiencing. Pain:
A Chemical Explanation
lllustroting hydrogen ion concentration, pH, and osmotic pressure
Information prouided by Olou Lindahl, Orthopedic Center, Karolinska Stockholm, Sweden
"It stings! Ouch! Oh, it's excruciating! I can't stand it!" We could arrange the various levels of response to pain on a scale, and think of a person subject to pain as a meter giving higher and higher readings as the intensity of the pain increases, but what kind of a meter is it-an ammeter, a voltmeter, or something bizarre? Recent research1 has shown that in sensing pain man is acting as a pH meter! Anyone who has ever spilled hydrochloric acid on a cut finger might suspect this. The contents of cells are acidic and highly buffered. Volume 49, Number 3, Morch 1972
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179
Extracellular fluids are less well buffered and normally neutral. As a result of mechanical, thermal, or toxic effects in the cells, they may burst; as a consequence the contents come into contact with nerve endings, which are always in extracellular positions, and the body perceives the sensation of pain. The fact that i t is the hydrogen ions in the cells which stimulate the nerve endings is indicated by a simple, elegant series of experiments in which solutions were placed in human tissue by a jet injection ( high velocity, small diameter stream which avoids the cell damage produced by a needle). Studies of a variety of solutions have shown that pain is produced directly by the hydrogen ions in the solution or indirectly by cell damage caused by the solutions. Pain is detectable when buffered solutions a t a pH of 6.2 are injected, and increases with the hy-
I
LINDAHL, OLOV,Ada Ollhop. Scandinav., 40, 741 (1970).
180
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journal o f Chemical Education
drogen ion concentration. Interestingly, an injection of pure water gives particularly intense pain; the cells in contact with pure water burst as water diffuses into them to approach osmotic equilibrium and the acidic contents stimulate the nerve cells. Some types of tumors are painful, some are notthose which are painful have a low pH while those that are not painful have a normal pH; painful inflammation of tissue is accompanied by a decrease in pH, but tuberculous inflammation, which is not painful, does not produce a decrease in pH; and apparently the formic acid in an insect sting acts directly upon the nerve receptors. A Footnote to the Sickle-Cell Anemia Exemplum The article preceding this column (p. 177) provides a wealth of additional resource material for the tewher who wishes to use sickle-cell anemia to illustrate the relevance of chemical principles.
