Precipitation (chemistry)

“ D. On applying heat to a portion of the water just uncorked, and boiling it quickly, till it was reduced to one half or even one third of its original bulk, no precipitation whatever took place; but on continuing the evaporation, a white feathery crystalline substance appeared on the surface of the fluid, and on pushing the process still further, a saline matter of a pale yellowish green colour appeared, which continued to increase till the whole was reduced to a dry yellowish mass. ”

“ The water in all rivers and springs contains salt to a certain degree, and where it runs into a basin like that of a lake with no outlet, through the process of evaporation pure water [Pg 41] is being constantly carried off, leaving the salt behind. It is easy to see that if this process is kept up long enough the water will become in time a saturated solution, when crystallization sets in and precipitation follows, accounting for the deposits of rock salt. ”

“ Rain-water is seen to sink beneath the surface with every shower, and since this source seems altogether adequate for ground-water, and since no other source is known whence any considerable amount of ground-water might come, it is concluded that atmospheric precipitation is its chief source. ”

“ One might be almost led to imagine that the vapour molecules endured a strain similar to that of water cooled below its freezing point, or heated beyond its boiling point; and that, on the strain being relieved by the sudden yielding of the opposing force, the particles rushed together, and thus filled in an instant the clear atmosphere with aqueous precipitation. ”

“ The virtue of the solution depends on the fact that neither the solution nor the precipitation is complete; the proportion of alcohol must be regulated to prevent more than a small part of the chemical salts from precipitation. ”

“ It is undoubtedly the case that mingled with these little bubbles of moisture there are fine particles of solid water hanging on and carried along with them. Undoubtedly this is true; at least just before the final act of condensation takes place; and when the little hollow spherules collapse they are gathered together in drops of water larger or smaller according to the rapidity of condensation. [Pg 70] There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. ”

“ A fourth factor involved in the deposition of mineral matter from solution is pressure. Pressure increases the solvent power of water with respect to minerals directly; it produces the same effect indirectly by its effect on the solution of gases. As water charged with gas comes to the surface, the pressure is relieved and some of the gas escapes. Such mineral matter as was held in solution by the help of the gas which escapes is then precipitated. (5) Precipitation is also sometimes effected by the mingling of waters containing different mineral substances in solution. ”

“ After twenty or thirty minutes (the hotter the water, the quicker the precipitation) the precipitate has settled in large flocks at the bottom, and the clear water is drawn off into the pure water reservoir. The precipitating and settling of the impurities can also take place in cold water; it will require, however, a pretty long time. ”

“ Precipitation and evaporation are the converse of each other; and the same air can not precipitate and evaporate, take up and let down water, at one and the same temperature. As the temperature of the air is raised, its capacity for receiving and retaining water in the state of vapor is increased; as the temperature of the air is lessened, its capacity for retaining that moisture
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is diminished. ”

“ A great number of bodies which arise from industrial processes, domestic combustion of coal, natural changes in vegetable and animal matter, terrestrial disturbances as tornadoes and volcanic eruptions, vital exhalations, etc., are discharged into the atmosphere, and, whether by solution or mechanical contact, descend to the surface of the earth in the rain, leaving upon its evaporation in many instances the most incontestable evidences of their presence. The acid precipitation around alkali and sulphuric acid works is well known ”

“ By the process of evaporation, it is transformed into vapor and is carried over the land by currents of air. As it comes into contact with colder currents, condensation ensues and then precipitation, and our raindrop descends to earth once more. ”

“ There is much more evidence of solution by sea-water than of precipitation from it. The ocean appears to be under-saturated with lime carbonate on the whole, though it is still possible that deposition may take place in favorable situations, as, for example, where the very calcareous waters of rivers are spread out in thin sheets on the surface of the heavier salt water, and thus exposed to exceptional evaporation, or where there is very exceptional agitation and aëration. ”

“ If the rain-drops fall through a stratum of air below freezing temperature and strike an object whose surface is also below freezing temperature a varnish of ice will be formed, and it is likely to increase in thickness so long as precipitation continues. When the temperature is very slightly above the freezing point, and the surface air is dry, it is possible that rapid evaporation may chill the varnish of water below the freezing point and change it to ice. Rain-drops in the air may be chilled to a temperature several degrees below the freezing point ”

“ Again, large bodies of water will be precipitated by smaller amounts of alum than one would infer from experiments on a small scale, as the mechanical action of the precipitant here, in enveloping and carrying down suspended matter, is greater in a large body of water than in a small one. It will be better, however, to err on the side of too large an amount, for even then the amount of alum added will be insufficient to impart any detrimental properties to the water. The water, after precipitation has taken place, is perfectly clear and sparkling, and has acquired neither taste nor smell. ”

“ On the one hand the precipitation is increased, and on the other, the absorbent power of the air being less, the rate of evaporation is diminished. In so dry a climate the precipitation is increased in greater ratio than the humidity, and the rate of evaporation is diminished in less ratio; while of the increased precipitation an increased percentage gathers in streams and finds its way to the lake. ”

“ Let us now briefly consider some of the chemical processes of weathering. The solvent effect of perfectly pure water upon rocks is very slight and slow. But such water is not found in nature because certain atmospheric gases, especially oxygen and carbonic acid gas, are always present in it, and they notably increase the solvent power of the water. ”

“ The average precipitation equals the evaporation. The heat that is absorbed and evolved in the process of lifting up and letting down such a body of water has a powerful influence upon climates as well as upon winds; it is the chief source of that motive power which gives to the winds their force, to the storm its violence. Six hundred and twenty pounds of aqueous vapour occupy in the open air the space which it takes one thousand pounds of dry air at the same temperature to fill. ”

“ When the air—or rather, the water vapor itself—is near the point of saturation it is moist to the senses. Hygroscopic substances, such as sugar, salt and many other substances, absorb moisture; sized paper and starched fabrics swell and become limp. ”

“ Water, when heated to the boiling point, can no longer exist in the form of water, and must necessarily be converted into vapour or steam, whatever may be the state and temperature of the surrounding medium; this is called vaporization. But the atmosphere, by means of the caloric it contains, can take up a certain portion of water at any temperature, and hold it in a state of solution. This is simply evaporation. Thus the atmosphere is continually carrying off moisture from the surface of the earth, until it is saturated with it. ”

“ Very large precipitates carry down small quantities of bodies not belonging to the group, more especially when there is a tendency to form weak double compounds. The re-dissolving and re-precipitating of bulky precipitates should be avoided. ”

“ The precipitation of the water from the air is primarily due to the cooling to which it is subjected as it rises in the atmosphere. Over the sea the ascent is accomplished by the simple diffusion of the vapour or by the uprise through the aërial shaft, such as that near the equator or over the centres of the whirling storms. ”

“ The cause of the precipitate appears to be the same in all these cases. The moment a drop of the solution is converted into vapour, it deposits the quantity of lime or salt which it held in solution; and in the case of bodies which dissolve so sparingly and with so much difficulty, although the water be returned again to the solution, it is incapable of re-dissolving what it has deposited. We know that it would be a hopeless task to form a saturated solution of lime by agitating with the water no more than the few grains which it is capable of dissolving ”

“ Precipitates.—When a portion of the ocean-water is isolated in a region where evaporation from the surface of the water is greater than the rainfall on it, and the inflow from the tributary basin, the lime, magnesia, soda, potash, and other dissolved substances (solutes) are concentrated until the water becomes saturated. The solutes are then precipitated in the order in which they reach the point of saturation. ”

“ The diminution of evaporation is not the only mode in which under-draining affects the temperature. The increased effective hygroscopicity of the soil increases its absorbent action, and the condensation of atmospheric vapor thus produced is attended with the manifestation of heat. ”

“ Gases in many cases undergo a remarkable condensation when chemically combined. In slaking lime, the water becomes solid in the molecules of the hydrate of lime formed, and the intense heat produced arises from the liberation of that caloric which had been employed to keep the water liquid. When a solid passes into the liquid state, cold is produced by the abstraction from surrounding objects of the heat required to effect fluidity. An alteration of temperature occurs whenever chemical change takes place, as we have already shown, with a few trivial and uncertain exceptions. ”

“ If there be less pressure, as on a mountain, the boiling-point is lowered below 100 degrees. Salts dissolved in water raise its boiling-point, and lower its freezing-point to an extent depending on the kind and quantity of the salt. Water, however, evaporates at all temperatures, even from ice.
Pure water has no taste or smell, and, in small quantities, no color. It is rarely if ever found on the earth. What is taken up by the air in evaporation is nearly pure; but when it falls as rain or snow, impurities are absorbed from the atmosphere. ”

“ When rain falls and passes downward into the earth, it conveys the soluble matter to a depth below the surface, often to beyond the point where our ordinary crop plants, such as the small grains, can have [Pg 330] access to it, and this for the reason that their roots do not penetrate deeply. When dry weather comes and evaporation takes place from the surface, the fluid is drawn up to the upper soil layer, and there, in process of evaporation, deposits the dissolved materials which it contains. ”

“ Water rises in vapour from the ocean as from a still, and the salt and other dissolved matters remain behind. Meeting cold currents of air, the vapours condense in rain, and fall upon the earth. After coming in contact with the earth, the subsequent condition of that water entirely depends upon the character, as regards solubility or insolubility, of the substances composing the strata or layers of earth upon which it falls, and through which it sinks. ”

“ The actual supply of water does not depend on rainfall alone. It is diminished by evaporation and also by percolation. When the condition of the atmosphere is such that, either by reason of the heat, the prevalence of dry winds, and other such causes, the water that has fallen is exposed to rapid vaporization, the available supply necessarily becomes less. ”

“ Strictly speaking, the precipitations usually concern only a part of the substances dissolved. New substances are often taken up in the very act of throwing down those already held, and the way thus prepared for further changes. ”

“ This is not boiling, as may be proved by trying the temperature with the finger. What, then, is it?
It is the yielding back of the atmospheric gases which the water has dissolved or condensed within itself. These bubbles have been collected and by analysis proved to consist of oxygen, nitrogen, and carbonic acid, obtained from the air; but in the water they exist by no means in the same proportions as originally in the air, nor in constant proportions in different samples of water. ”

“ A zone of precipitation is thus formed, through which the first solution penetrates until the conditions are repeated, and a second zone of precipitate is thrown down. Zone after zone may thus arise as diffusion goes on." He suggests that the zones of flint may be similar phenomena, water diffusing through the masses of chalk taking up silica till such concentration is reached that precipitation takes place, the water then percolating further and repeating the process. ”

“ Rain always absorbs a little air, which, in addition to its nitrogen and oxygen, contains carbonic acid, and in minute proportions, sodium chloride, sulphuric acid and other ingredients, especially inorganic dust, organic particles and living germs. Probably the most generally efficient of these constituents are oxygen, carbonic acid and organic matter. Armed with these reagents, rain effects a chemical decomposition of the rocks on which it falls, and through which it sinks underground. ”