Popular Science Monthly/Volume 83/July 1913/Suspended Changes in Nature
|SUSPENDED CHANGES IN NATURE|
ASSOCIATE PROFESSOR OF CHEMISTRY, UNIVERSITY OF WISCONSIN
IN the Physical world we are familiar with the fact that changes of all kinds are continually taking place, Prominent among these are changes of state, such as the evaporation of water, the melting of ice and the condensation of steam. These familiar transformations seem to have a common property; as usually observed they take place at very definite temperatures. To be sure, in the laboratory it is possible to heat water to 105-106° without boiling it, and to cool vapors below the temperature at which they ought to condense, but such experiments have usually been regarded as quite exceptional. Within the last few years, however, it has been found that such a reluctance to enter a new state is by no means unusual, that many cases similar to the above actually exist; moreover, that they are not restricted to ultra-refined laboratory experiments, but are a matter of common experience.
If ice is heated it melts, and the temperature at which this process takes place is sharp and definite, as is evidenced by the fact that mixtures of ice and water are used to calibrate and test the accuracy of the most delicate thermometers. No one has ever succeeded in heating ice above the temperature of its melting point, zero degrees, which is a property that ice shares with other solids such as lead, gold, saltpeter and ordinary table salt. But the reverse is not true, for many liquids can be cooled below their freezing points without solidifying. Water, for example, can be cooled below zero without changing to ice. When water freezes in nature, as in ponds and lakes, the transition of water to ice takes place at this temperature and is fairly sharp, but if a clean flask is filled with water the surface of which is protected from dust by means of a layer of oil, it is easily possible to cool the water ten degrees below its freezing point. True, it becomes solid on shaking the flask or upon the introduction of a fragment of ice, but it can be kept for hours in the liquid state, offering a passive resistance to the forces of nature which are operating to-change it to ice, the stable form of water at this temperature. Water that has been cooled below its freezing point in this way is said to be in the metastable state; it is also called undercooled water, that is, water that has been cooled under the temperature at which it ought to solidify. The phenomenon of undercooling is not restricted to water, but is shown by aqueous solutions of salts and also to a marked extent by melted phosphorus, carbolic acid, and many organic substances like thymol and betol that are seldom encountered outside a chemical laboratory.
Although many undercooled liquids are similar to water in that they may be changed to the stable state by agitation, this property is by no means general. An infallible test for undercooling is the addition of a fragment of the stable substance. Thus a bit of ice causes water to
solidify, similarly undercooled thymol becomes a crystalline mass upon the addition of a crystal of thymol, while undercooled sodium acetate solutions at once separate needle-like crystals of sodium acetate when a crystal of that substances is added. The addition of a solid fragment for the purpose of causing subsequent crystallization of the liquid is called inoculation or vaccination. The name is particularly fortunate, for the growth and spread of the crystals resembles a bacterial growth. Undercooled liquids are as sensitive to the presence of a crystal of the solid material as milk is to the presence of certain bacteria, and Just as much care must be exercised in their preservation. In working with undercooled sodium acetate under no circumstances may an open flask of this substance be brought into a room in which sodium acetate has been ground in a mortar, for the dust in the air carries enough finely divided sodium acetate to inoculate the solution. Frequently the
investigator must take his solution into the open air, but here again he must be careful, for his clothes and hair may carry enough powered material to inoculate the solutions.
Fig. 2. Crystals Growing in a Tube of Undercooled Sodium Acetate Solution. The amount of material required to inoculate a solution is very, very small, far beyond the sensitiveness of the most delicate balance. Nowhere in science is the importance of the fact that "the very small is as real as the very great" better illustrated than in the case of undercooled liquids; a human hair lightly brushed against a crystal of thymol will collect enough of this material to inoculate a flask of the undercooled liquid thymol. A tube of undercooled sodium acetate may be divided by a piece of parchment paper into two parts (Fig. 3). The inoculation of the solution in A causes the separation of crystals which ultimately appear in B via the parchment. The pores in the parchment, which are of microscopic size, become filled with the undercooled solution, and as the crystals forming in the pores can not be larger than the pores it is evident that these crystals are of microscopic dimensions only.
How far can a liquid be cooled below the temperature at which crystals ought to separate? This depends entirely upon the substance used. With some liquids if the undercooling is greater than a degree or two, crystals at once separate spontaneously from the solution. In the case of water the undercooling has been carried to as low as twenty degrees below zero before crystals of ice separated.
Sodium acetate on being strongly undercooled shows an interesting property, for as the temperature becomes lower and lower the liquid becomes less mobile until at fifty degrees below zero just before spontaneous crystallization the liquid assumes a viscous glassy appearance. Its similarity to glass is more than superficial. When molten glass Fig. 3. is cooled it gradually becomes more and more viscous until finally it has all the appearances of a solid. But at no definite temperature does it suddenly harden, as would be the case in cooling mercury or molten lead. The glass differs from the undercooled sodium acetate in one respect only: its viscosity has reached the solid stage. Glass is an undercooled substance and like other undercooled substances is in the metastable state, consequently it has the power of returning to
the stable condition. Old glass, especially glass tubes through which water has been allowed to pass, frequently shows this property when heated for a few minutes. The glass crystallizes, taking on the appearance
of ground glass. Its surface becomes rough and the glass is no longer transparent. Pieces of glass apparatus which have partially crystallized can he found in any chemical laboratory. It is called devitrified glass.
Certain glassy appearing minerals like obsidian are really undercooled substances, natural glasses, which have cooled without crystallizing. In the course of time they begin to crystallize. It is not unusual to find in nature numerous samples of these minerals existing in all stages: the metastable, partially Fig. 6. Crystals of Needle-like or Monoclinic Sulphur. When first formed they are bright yellow. After standing for a few hours they change to a dull yellow color and become very brittle. A microscopic examination of a fragment shows that it is now made up of minute rhombic crystals (Fig. 7). crystallized, and the stable or completely crystallized mineral.
The rate of the growth of crystals is a specific property depending on the substance used. Much information has been obtained on this subject by studying the rate at which crystals are deposited from undercooled liquids. By filling a narrow glass tube with the desired liquid, as shown in figure 2. inoculating at one end and measuring the time necessary for the crystal surface to travel the length of the tube, the rate of growth of a crystal can be measured. Undercooled phosphorus crystallizes 200 feet a minute, water at two degrees below zero at the rate of 8 inches a minute, Crystals of Rhombic Sulphur. while one thirty-second of an inch a minute is the velocity of crystallization of betol. In nature crystals often grow much more slowly than this. The examples cited above deal with undercooled liquids, but the phenomenon of undercooling is by no means restricted to this class of substances.
When molten sulphur is allowed to cool slowly, long lustrous needle shaped (monoclinic) crystals separate from the liquid. On standing for a few days the appearance of the crystals changes; they lose their luster, and examination with the microscope shows that their crystalline form is no longer needle-like, but consists of so-called rhombic figures. The temperature at which the transition from needles to the rhombic form takes place is 96°.
The change by which water is transferred to ice and its reverse may be represented as follows:
At 0° ice ⇄ water.
The arrows pointing in opposite directions indicate that the process is reversible. The significance may be expressed as follows: at 0° ice can be changed to water or water to ice. Similarly in the case of the sulphur.
At 96° sulphur (monoclinic) ⇄ sulphur (rhombic)
Now just as the water at 0° does not always change to ice, but may be undercooled, so the transformation of monoclinic to rhombic sulphur does not take place immediately, but the needles can exist in a metastable state analogous to the undercooled water. And as the addition of a fragment of ice causes undercooled water to solidify, so the addition of a crystal of rhombic sulphur accelerates the change of monoclinic sulphur to its stable form. The rapidity of the transformation of the metastable solid, however, is by no means as rapid as the change of metastable liquids. This is not surprising when we consider how much more inert solids are than liquids, especially when considered from a chemical standpoint.
A most interesting example of the retarded transformation of metals has been furnished by Professor Cohen of Utrecht. Tin is a white crystalline metal which does not corrode readily and under ordinary conditions appeal s very permanent. After a particularly cold winter in one of the small towns of northern Germany, it was noticed that in one of the churches the tin pipes of the organ were full of holes and that the tin around the edges of those holes was brittle and would crumble to powder very easily.
A similar occurrence had been reported in St. Petersburg where, after a severe winter, blocks of tin which had been stored in the custom house were found to have crumbled to powder and a number of cases of tin buttons used for military uniforms had undergone a similar change. It was noticed that in the case of the organ pipes, and also on the tin roofs of certain public buildings, the tin had taken on in spots a wartlike appearance; moreover that the warty growth seemed to have the power of spreading. Wherever the tin had changed in this way it had lost its original properties and would easily crumble to gray powder. Because of the appearance of the tin and the spread of the warts over its surface this phenomenon was called the "tin pest" or the "tin disease." That the powder found was still tin and not a product of the corrosion of the metal was easily demonstrated, but the transition of a bright malleable metal to a dull gray powder was for many years a great mystery. But just as there are two kinds of sulphur which can be transformed into each other, so Professor Cohen showed that tin exists in two forms, white tin with a specific gravity of 7.28 and gray tin with a specific gravity of 5.79 Fig. 9. The Spread of the Tin Disease on a Piece of Sheet Tin. These two forms can be transformed into each other, the transition temperature being 18°:
At 18° tin (white) ⇄ tin (gray).
But if gray tin is stable below 18° how can we explain the fact that tin pails and tin pans remain bright year after year? The average temperature of the northern part of the United States is far below 18°, consequently why do not our tin utensils crumble into the gray modification of the metal? Fortunately for the housewife, the white tin exhibits to a marked degree the property of metastability. It remains unchanged at temperatures far below 18° and even contact with changes it but slowly to the stable form. But once the gray tin let the transformation of the tin begin, and the spread of the disease is certain. The surface of the tin becomes disfigured with blotches winch gradually spread until the whole substance succumbs to the disease. Results of the tin pest are frequently found in museums. A tin vase in the British Museum which was found in Appleshaw, Hampshire County, England, and which dates back to 350 B.C. shows very strikingly the effects of the tin disease. The metal is not corroded, but it is dull in color and is so brittle that it can be broken with the fingers. Some of the fragments on being melted gave white tin with its original toughness and luster.
Every one who has studied the advance of science during the last few centuries realizes that our modern inventions and processes of manufacture have been in many cases foreshadowed in the ancient world. The use of gunpowder by the Chinese and their extraordinary success with glazes, as well as the perfection obtained by certain of the old civilizations in the use of cements, pigments, dyestuffs and in metallurgical processes, is familiar to every one. What the modern scientist discovers by painstaking investigation was learned in those days either by accident or as the result of centuries of experience. Consequently the fact that the tin disease was known in those days ought not to be surprising. Professor Cohen has pointed to an observation of Aristotle.
The knowledge of the tin disease is no more modern than the knowledge of most other diseases.
In this connection it is interesting to speculate on the antiquity of the use of tin. This metal is one of the easiest to obtain from its ores, and may have been used far earlier in the history of mankind than is generally supposed. Evidence in the form of utensils, etc., would of course have been destroyed by the tin disease.
We are indebted to the investigations of Professor Cohen for a more striking example of a metastable metal, that of the "explosive" antimony. By passing an electric current through a solution of antimony chloride this metal may be deposited on platinum in the form of a thick metallic coating. This electrolytic antimony is in the metastable condition exhibiting the same state of passive resistance towards change as the metastable sulphur, sodium acetate and water. If scratched with a file it changes to the stable form of antimony with explosive violence, heat is given off and dense clouds of whitish vapor are evolved. The metal has changed to the ordinary antimony, used so much in manufacturing as a basis for bearing metals. The method of bringing this change about and the velocity of transformation reminds one forcibly of the transition of undercooled water to the stable form.
That many other metals have the property of existing in the metastable state is highly probable. In this connection the hardening of steel is of especial interest, particularly so since the manufacture of steel has played so important a role in the advance of civilization. The method of tempering steel has been the subject of numerous trade secrets. In a book of recipes published in the sixteenth century the reader is told that steel may be hardened by quenching it in rain water in which snails have been boiled: also that
Fortunately for this type of young man the modern steel manufacturer uses other methods for hardening steel.
The discovery of hardening steel by the quenching process is of course as much of a mystery as the method of raising bread by fermentation, we only know that it is an ancient process and moreover of great interest from the standpoint of delayed transformations.
If the alloy that we call steel is taken at a high temperature and allowed to cool very slowly it becomes soft and tools made from it will not have a cutting edge. Sudden chilling, however, produces in the metal a decided hardness. The results obtained by different rates of cooling have been explained by the investigations of the last few years. When cooled slowly the steel undergoes a transformation changing to a form very much softer than that which existed at a higher temperature. If chilled suddenly the steel remains in the same form that was stable at high temperatures, consequently the property of hardness is retained. Here again the analogy to the other cases of delayed transformation is evident, for the quenched steel is exhibiting the same state of passive resistance as the white tin that remains unchanged at a temperature below 18°. Now since the tin is not permanent under those conditions the question occurs to us, does steel slowly return to the stable form and thus in time grow softer? That we do not know; we can only say that if such a change does take place hundreds of years are necessary to bring it about. Japanese swords hardened in this way and made as far back as the fifteenth century when carefully preserved are apparently as hard as ever. If, however, this kind of steel is heated to the temperature of boiling water it gradually softens, reverting to the stable form. And if heated to 150° the softening takes place in a very few minutes.
From these examples of retarded transformation an idea of the extent and the importance of this phenomenon in the physical sciences may be obtained. New cases are constantly being discovered, in fact, the reluctance of substances to assume a new state seems to be pretty general. And as it so often happens in science that discoveries which seem at first to be of theoretical importance only, ultimately are shown to be intensely practical, so the study of this phenomenon has cleared up the mystery of the tin pest and promises to be of great importance in the study of metallurgy and many other branches of applied chemistry.
- Solutions may be said to be undercooled when they have been cooled to a temperature at which crystals of the dissolved substance ought to separate from
the solution, without such a separation taking place. Such solutions are usually said to be supersaturated.