Popular Science Monthly/Volume 84/March 1914/Water
By E. P. WIGHTMAN, Ph.D.
THE JOHNS HOPKINS UNIVERSITY
THALES, one of the Seven Wise Men, said:
There is no doubt that Thales thought he knew a great deal about water, but even the average man to-day probably thinks he knows much more. Yet, what does he know about it?
It would be difficult to overestimate its great value to the human race, and its far-reaching importance in matters scientific.
First of all, what is its source? According to the astronomers and geologists, the earth is nothing more than a condensed and cooled portion of a vast nebula, which must have been similar to many now adorning the heavens. This nebula was a mass of self-luminous, gaseous matter, very highly heated. Of course, water, as such, could not exist in this, but was dissociated, or separated into its constituent parts, the two gases hydrogen and oxygen. Above a temperature of 2,000° C. or 3,632° F., these gases do not combine to form water, whereas the earth, in the molten, to say nothing of the gaseous condition, must have had a temperature hardly less than 6,000° O. However, the earth finally cooled sufficiently for the water to form as steam and then to condense to the liquid state.
For a long time, in fact until about 130 years ago, it was thought that water was an element. Aristotle named it as one of the four elements, earth, air, fire and water. This view of the composition of so called matter held sway for several centuries. Even after the theory was broken up, water still remained as an element. It was not until 1781 that it was found to be a compound substance. Priestly, and likewise phlogiston theory which held sway at that time. By the experimentation and study of later workers on this subject, this theory was overthrown and water was proved to be a combination of hydrogen and oxygen in the proportion of two parts by volume of the former to one part by volume of the latter, or by weight, 2.016 parts of the former to 16 of the latter. The proof is as follows: Known quantities of hydrogen and oxygen are exploded and the water formed is weighed or the amount of each gas used is measured. Also water is decomposed by electrolysis and the hydrogen and oxygen thus formed (the only things formed) are measured., showed that when hydrogen is burned, water is the outcome. The ideas of the former, however, were in conformity with the
The heat given off in the combination of the gases is enormous, indeed, it is the most exothermic of all chemical reactions, 67,500 calories or heat units being evolved in the combination of 16 grams of oxygen with 2.016 grams of hydrogen—a calorie is that amount of heat which will raise one gram of water 1 degree centigrade. In the absence of indifferent gases, or an excess of one of the reacting gases, the reaction is not only so violent as to raise the gases to the combination temperature, 2,000° C, but to carry them beyond to 2,844° C, at which temperature only about one third of the gases combine, the remainder doing so gradually as the temperature falls. Almost any non-reactive (catalytic), highly heated substance, such as platinum sponge, or wire, stone, porcelain, glass, etc., will bring about a combination.
An oxy-hydrogen blowpipe is an arrangement for utilizing this heat energy, by bringing the two gases together in such a way that they will produce a sharp, intensely hot flame. The apparatus is so fashioned that the gases are conducted separately through the exit where they are to be lighted, thus avoiding any possibility of explosion, which otherwise takes place, if they are mixed. By means of such a flame, a temperature of 2,000° C. can be obtained.
Having learned that water (at least in the form of its components) is older than even the earth itself, that its constituent parts existed practically at the beginning of things, and also that it is not an element, but built up of two gases combined in a definite proportion, let us now take up the substance itself and study it in its various forms. These are quite numerous, but may all be classified under three fundamental heads: gaseous, liquid, and solid water.
The fact that it can exist in these three states is not so remarkable, since it is possible to transform every known substance, elementary or combined (provided the latter do not decompose) into these three states of aggregation; but that the three should all be within the range of ordinary temperatures is rather extraordinary. There are only a few common substances of which this is true, e. g., ammonia, benzene, etc. It will be seen, moreover, that water has a good many other noteworthy properties. As compared with other substances it is nearly always exceptional, and stands at the extremes.
Gaseous Water.—Steam and atmospheric water vapor belong in this category. It is not until we go to some of the arid desert regions of our earth that we realize the importance of the latter. Where there is no moisture in the atmosphere, there can be no clouds formed, and hence there can be no rain, which means, of course, that such a place must be devoid of life; for example, the Sahara, the deserts of Asia, of western United States, etc. Yet the presence of moisture can be very disagreeable, as in hot, humid climates. The amount in the atmosphere varies considerably, depending upon the complex condition of climate and topography, therefore no general data can be given.
Steam.—The very word signifies the sublime, the wonderful! What could we do at present without it? How many thousands of mills, shops, locomotives, etc., derive their power from it? Power? Let us stop and consider—1 gram of water in the form of steam occupies 1,700 times the space that a gram of water in the liquid form does. Is it any wonder that steam is a mighty agent? If a sufficient quantity is confined and superheated, as was the case when the volcanic mountain of Krakatoa was almost completely annihilated, there is nothing that can withstand it.
According to the theory of kinetic energy, the molecules of all substances are in rapid motion, and at the surface of liquids, water in particular, there is a tendency for some of the rapidly moving particles to be thrown off into the atmosphere and to form vapor. Likewise, some of the vapor molecules pass back into the liquid again. When the tendency of each to pass into the other is exactly counterbalanced, we have what is called a state of equilibrium between the two phases. This tendency of the molecules to pass off into the atmosphere, even at lower temperatures, gives rise to a certain amount of pressure, called "vapor tension." The atmosphere, or any artificial pressure which may be applied, tends to overcome this. At every temperature only a certain amount of water vapor can exist under a given external pressure, viz., the vapor tension of water at that temperature. At that pressure you have a "saturated vapor." Stronger pressure causes liquefaction; reduced pressure, an increase of vapor.
Steam is that condition or phase of water which is stable at temperatures above 100° C, at ordinary atmospheric pressure (760 mm. mercury). At this temperature and pressure the vapor tension of the liquid water is so great that none of it can remain in the liquid state. Increased pressure tends to drive back the steam into the liquid state again, the temperature of boiling being increased directly in proportion to the temperature. Up to a certain temperature, the "critical temperature," 360° C, water can be made to remain in the liquid state by applying sufficient pressure. Above that it can exist only in the form of a gas, no matter how great the pressure. It is possible, by using a small enough quantity of water and a sufficiently strong apparatus, to determine the critical temperature and pressure by experiment.
The amount of heat absorbed in the transformation of a unit quantity, 1 gram, of water at 100° C, into steam, that is, its heat of vaporization, is 537 calories (this is exactly the same in amount as its heat of condensation). It is easy to see, then, why it takes so long to boil away a large quantity of water. The amount of heat absorbed which is necessary to raise the water to its boiling point and keep it there is simply enormous. It may be said here that after the water once reaches the temperature 100° C, it remains there until the whole of the liquid boils away, even though the amount of heat applied is somewhat in excess of that required to keep it at the desired temperature.
Liquid Water.—Very much more interesting and important than any other form of water is liquid or "wet water." In this form it is the most fascinating of all chemical substances, besides being the most useful. In the first place it forms 75 per cent, of the human body and without it nothing could live. It covers about two thirds of the earth's surface to an average depth of about 12,500 feet. It is the best solvent known; as will be shown later, it is an essential to almost all chemical action. Here again life as well as nearly all branches of science would be at a standstill if it did not exist. It occurs as rain, fog, dew, river and ocean water, spring water, etc.
When the vapor of the atmosphere condenses around small particles of dust in the air, clouds are formed, or, if down near the surface, a fog. Whenever these small particles run together they produce drops which fall as rain. Dew is nothing more than water which has condensed out of the atmosphere on to cold objects. Only so much moisture can be held in the air at a given temperature if this is lowered, as would happen after the sun goes down, the dew separates out. If pure, water is an odorless, tasteless and in small quantities, colorless, transparent liquid. In bulk it becomes blue in color and very nearly opaque. It never occurs pure in nature, the nearest approach to it being rain-water after it has rained for some time (at first the rain gathers up a large amount of impurity from the atmosphere); and melting snow. Water can be readily purified by distillation. For ordinary purposes one distillation is enough, but for certain scientific work a special method of distillation must be resorted to. In this degree of purity it is almost a non-conductor of electricity.
Water is only slightly compressible. For every atmosphere (15 pounds per square inch) of additional pressure, it is made smaller by 0.0005 of its volume. The effect of pressure upon its freezing point is also exceedingly small—only 0.00757° C. lowering for each atmosphere. Nevertheless, it can be prevented from freezing by a pressure of 138 tons to the square inch at 1.11° C. Any further lowering of temperature requires a proportional increase of pressure. In passing from the liquid to the solid state there is an increase in volume equivalent to one eleventh that of the liquid.
The boiling point is affected to a much greater extent. Under a normal pressure of 760 mm., water boils at 100° C, or rather, this value is arbitrarily assigned to it under these conditions, and all other values of temperature are referred to this and to the freezing point, 0° C, as standards. If the pressure is changed, the raising or lowering of the boiling point is directly in proportion.
Egg albumen coagulates only very slowly at temperatures below 100° C, and since the atmospheric pressure on the top of high mountains is quite a bit lower than at their foot, we see from the above why an egg takes so very much longer to cook at such elevations, if it cooks at all.
Water is a powerful refractor of light. This can be best shown by holding a stick in it in a slanting position, so that part of it protrudes above the surface. The stick appears to be bent. An interesting curiosity which makes use of this principle is the fish-eye camera, which makes things in front of it appear just as they would to a fish under water, that is, instead of a limited view of the scenery, or whatever it may be, everything within a radius of 180° is shown in the picture. The camera is a box filled with water; in the back is placed the plate, and the light enters through a small hole in the front.
Most substances, when dissolved in water, lower its freezing point. That is one reason why salt is used in the freezing mixture when making ice cream, the temperature of the ice salt mixture surrounding the can in a "freezer" often reaching a temperature of—21° C. In this connection it may be said that the stirring which is carried on serves two purposes; it brings the entire contents of the can into contact with the cold walls of it, which radiate the heat very rapidly to the outside; it likewise causes a more rapid crystallization of the contents, and in consequence makes the crystals much smaller.
Mention should also be made here of the undercooling which takes place when a solution is cooled. Instead of ice forming at the freezing temperature of the solution, by keeping it quiet and out of contact with the air, the solution will remain in the liquid state several degrees below that point. A small crystal of the solvent, or a sharp-edged body, or even a jar, will cause it to freeze suddenly.
Similar to this is a supersaturated solution, or one in which more of the substance is dissolved than it can ordinarily hold, a crystal of the dissolved substance, or the other treatments spoken of, causing crystallization.
Besides lowering the freezing point, dissolving a substance in a liquid also raises the boiling point. Much could be written concerning both phenomena but space does not permit. It is enough to say that the relationships established by a study of them are some of the most important of all science. Of course every substance has its own effect and the amount of each dissolved, has to be taken into account as well.
When an acid, base or salt is dissolved in water, it is dissociated, that is, the molecules of the substance are split up into two parts, each part being charged with equivalent quantities of opposite kinds of electricity. These charged particles are called ions, and a compound which yields ions is called an electrolyte; all others, such as sugar, for instance, are called non-electrolytes. Solutions of the former will easily conduct an electric current, while solutions of the latter will do so no more than the pure water itself. Of all common liquids which dissociate substances, water has the highest power. It is dissociated itself only to the very slightest extent.
A fact which can be explained only by the theory of electrolytic dissociation is, that whenever an acid in solution is acted upon by an equivalent quantity of a base in solution, both solutions being dilute, and no matter what the acid or base, the same amount of heat is liberated in the reaction. The only thing here which can and does take place is for the hydrogen ion, which is the essential part of the acid, to combine with the so-called hydroxy 1 ion, the essential part of the base, to form a definite quantity of water, the same in every case, and hence giving off the same quantity of heat. The other parts of the acid and base remain unchanged, as ions, in the solution. In concentrated solutions, other factors come into play which necessarily cause the amount of heat to be variable.
We see from the above that water instead of being a side issue in chemical reactions, as we have been prone to place it, is really the most important and most fundamental thing in them. Moreover, it is made up of what constitutes both acid and base and yet has not the slightest trace of the properties of either. It is perfectly neutral.
When a soluble solid, no matter how great its specific gravity, is placed in the bottom of a vessel and is covered with water, it will in time diffuse through the entire liquid until the whole is perfectly homogeneous, even though the force of gravity is pulling continually against it, tending to keep it at the bottom. Diffusion is said to be due to osmotic pressure, but as this has never been explained satisfactorily, we are about as far from answering the question as to its cause as if we had left it alone. All we know of osmotic pressure is, that if we separate two solutions of different concentrations by a membrane, water will pass through the membrane from the more dilute to the more concentrated solution, which, if the latter side is enclosed, will set up a pressure on that side. This is called osmotic pressure, and there are certain laws governing it. These have been thoroughly studied and have been shown to correspond exactly to the laws of gases, but the cause for the pressure is as yet unknown. Diffusion is not a property of water only, but of all liquids. However, it has been studied in the case of water more thoroughly than in any other.
Another property of all liquids which has a special interest where water is concerned is surface tension. It is this property which causes a liquid to rise in a capillary tube and also aids in the formation of drops (pressure of the atmosphere likewise tending to reduce the liquid to the smallest, most stable geometric shape possible). It is due to capillarity that the minute blood vessels of living animals are supplied with blood, that a blotter sucks up ink, that moisture tends to come to the surface of the earth and that a good many other essential things of a similar nature take place. In fact we could not do without this important force.
Solid Water.—Here we have snow, hail, frost and ordinary ice.
Snowflakes are assemblages of minute crystals of ice formed from the aqueous vapor in the atmosphere. They vary in size from one fourteenth of an inch to one inch in diameter. The smaller ones are formed when the temperature is very low, but the larger ones not until it is near 0° C. They always assume a hexagonal shape and from each corner of the hexagon protrudes a ray at an angle of 60° to the ray on either side of it. This fundamental form is the same, no matter how much the crystals otherwise vary in shape.
Snow is only white to the eye because of the great refractive power of the crystals, which, when examined under the microscope, are seen to be transparent. It forms whenever a cold enough wave passes over a moist atmosphere, the water condensing out as crystals. Hail, on the other hand, is formed when the rain passes through a region of the atmosphere sufficiently cold to freeze it.
Just as the dew condenses out of the atmosphere on a summer night, on a winter night, when the temperature is below 0° C. frost forms. The action of frost as a geological agent need hardly be mentioned, it is so well known. Suffice it to say that it has played and continues to play a very important role in the changing of the earth's topography.
Water, when it cools, contracts until it reaches a temperature of 4° C, and then it begins to expand, slowly at first, until it very nearly reaches 0° C. and is about to freeze, then it increases very markedly and suddenly in volume. The specific gravity of ice is only 0.920, whereas at 4° C, pure water has a specific gravity of 1.000, that is, at 4° C, one cubic centimeter of pure water, in a vacuum, weighs one gram. Water is therefore used as a unit for specific gravity measurements. If water contracted all the way down to its freezing point, as most liquids do, in one cold winter every river, lake, etc., would be frozen up and would stay so, because of the ice being so much heavier than water and sinking to the bottom.
In freezing, water gives off a very large amount of heat, 79.06 calories for every gram of ice formed. The amount of heat liberated in freezing a gram of water, stating it in other words, is sufficient to raise the temperature of 79.06 times its weight of water from 0° C, to 1° C. Now we see why there is always a "warming up" just before a snowstorm.
When gases are allowed to expand suddenly, they cool themselves, taking heat from all surrounding objects. Also if a substance, like ammonia, which at ordinary temperatures is a gas, can be condensed by cooling and pressure to a liquid, and the pressure is removed, it will immediately begin to evaporate rapidly, and in so doing absorb a large amount of heat from everything around. Such a principle is used in the preparation of artificial ice.
Ice is often seen to have much dirt in it. If the water were stirred while freezing so that the crystals which separate are small, they would also be very nearly pure.
So much for solid, liquid and gaseous water. There are still one or two interesting things in connection with water, however, which do not bear directly on any one of these three heads.
Certain compounds have the power to crystallize with a greater or less amount of water—"water of crystallization," as it is called. Most of them can lose this water (or part of it) by heating them, and without detriment to the substances themselves. Examples of such are copper sulphate, sodium sulphate, alum, calcium chloride, etc. Some of these, like calcium chloride, if allowed to stand in the air, will attract moisture and become wet. They are said to be deliquescent. Others like sodium sulphate tend to lose their water of crystallization on standing open to the air. They are called efflorescent. There are still other compounds, called anhydrides, which take up water readily from the atmosphere, not as water of crystallization, but by so doing form a different compound, an acid. Phosphoric anhydride (phosphorus pentoxide) is an example of this kind, and it is the finest substance known for desiccating purposes. Dehydrated copper sulphate and calcium chloride likewise are extensively used.
Sugar, oxalic acid and a number of other substances lose water when being heated, but here the loss is quite a different one from that above. The compounds themselves are completely changed, showing that the water was in direct combination with them and that it was the fundamental part of them.
Many people know that water forms a large part of the human body and of the nourishment of the same, but few know what an enormous percentage of the whole this is. A human body weighing 150 pounds contains about 113 pounds of water (75 per cent., as was stated above), and requires daily for its sustenance, either as a liquid or combined with food, about 5.5 pounds of water. This equals more than half a gallon.
One can see from the following table from what source a large part of this water is derived:
|Butter||11 to 16|
|Cheese||25 to 50|
|Apples and Grapes||80|
It would take volumes to tell of all the effects of water as a dynamic agent in geology—of the action of frost, of percolating waters, of rain, of waves, of rivers, glaciers, lakes, oceans, subterranean waters, etc., of all these and more on the exterior and interior of the earth. As justice can not be done to any one of these topics in a few words, they can only be mentioned here.
The prime importance of water to chemical reactions has already been spoken of above, but in conclusion, one or two examples will help to further show how really essential it is.
Concentrated sulphuric acid and metallic sodium will react with the most explosive violence if brought together in the presence of only a trace of water, but if proper precautions are taken to exclude every particle of moisture, drying them first and then bringing them together as quickly as possible, there will be no reaction whatever. The fuming of hydrochloric acid and ammonia in the presence of each other is proverbial in the chemical laboratory. They combine to form ammonium chloride, which appears in the form of a white cloud. Here again there is no combination, if the two are perfectly dry. Soda and tartaric acid (both solids) can be intimately mixed together, in solid form, without undergoing any reaction. But as soon as water is added, a tremendous effervescence takes place.
Many other cases might be cited, but these, as well as what has gone before, will, I hope, give some idea, at least, of the all importance of this wonderful yet common substance.