LECTURE III WATER: ITS CHEMISTRY AND PROPERTIES; IMPURITIES AND THEIR ACTION; TESTS OF PURITY

I have already had occasion to refer, in my last Lecture, to water as a chemical substance, as a compound containing and consisting of hydrogen and oxygen. What are these water constituents, hydrogen and oxygen? Each of them is a gas, but each a gas having totally different properties. On decomposing water and collecting the one of these two gases, the hydrogen gas, in one vessel, and the other, the oxygen gas, in another vessel, twice as large a volume of hydrogen gas is given off by the decomposing water as of oxygen. You may now notice a certain meaning in the formula assigned to water, H₂O: two volumes of hydrogen combined with one of oxygen; and it may be added that when such combination takes place, not three volumes of resulting water vapour (steam), but two volumes are produced. This combination of the two gases, when mixed together, is determined by heating to a high temperature, or by passing an electric spark; it then takes place with the consequent sudden condensation of three volumes of mixture to two of compound, so as to cause an explosion. I may also mention that as regards the weights of these bodies, oxygen and hydrogen, the first is sixteen times as heavy as the second; and since we adopt hydrogen as the unit, we may consider H to stand for hydrogen, and also to signify 1—the unit; whilst O means oxygen, and also 16. Hence the compound atom or molecule of water, H₂O, weighs 18. I must now show you that these two gases are possessed of totally different properties. Some gases will extinguish a flame; some will cause the flame to burn brilliantly, but will not burn themselves; and some will take fire and burn themselves, though extinguishing the flame which has ignited them. We say the first are non-combustible, and will not support combustion; the second are supporters of combustion, the third are combustible gases. Of course these are, as the lawyers say, only ex parte statements of the truth; still they are usually accepted. Oxygen gas will ignite a red-hot match, but hydrogen will extinguish an inflamed one, though it will itself burn. You generally think of water as the great antithesis of, the universal antidote for, fire. The truth is here again only of an ex parte character, as I will show you. If I can, by means of a substance having a more intense affinity for oxygen than hydrogen has, rob water of its oxygen, I necessarily set the hydrogen that was combined with that oxygen free. If the heat caused by the chemical struggle, so to say, is great, that hydrogen will be inflamed and burn. Thus we are destroying that antithesis, we are causing the water to yield us fire. I will do this by putting potassium on water, and even in the cold this potassium will seize upon the oxygen of the water, and the hydrogen will take fire.

Specific Gravity.—We must now hasten to other considerations of importance. Water is generally taken as the unit in specific gravities assigned to liquids and solids. This simply means that when we desire to express how heavy a thing is, we are compelled to say it is so many times heavier or lighter than something. That something is generally water, which is regarded, consequently, as unit or figure 1. A body of specific gravity 1·5, or 1½, means that that body is 1½ or 1·5 times as heavy as water. As hat manufacturers, you will have mostly to do with the specific gravities of liquids, aqueous solutions, and you will hear more of Twaddell degrees. The Twaddell hydrometer, or instrument for measuring the specific gravities of liquids, is so constructed that when it stands in water, the water is just level with its zero or 0° mark. Well, since in your reading of methods and new processes, you will often meet with specific gravity numbers and desire to convert these into Twaddell degrees, I will give you a simple means of doing this. Add cyphers so as to make into a number of four figures, then strike out the unit and decimal point farthest to the left, and divide the residue by 5, and you get the corresponding Twaddell degrees. If you have Twaddell degrees, simply multiply by 5, and add 1000 to the result, and you get the specific gravity as usually taken, with water as the unit, or in this case as 1000. An instrument much used on the Continent is the BeaumÉ hydrometer. The degrees (n) indicated by this instrument can be converted into specific gravity (d) by the

Ebullition or Boiling of Water, Steam.—The atmosphere around us is composed of a mixture of nitrogen and oxygen gases; not a compound of these gases, as water is of hydrogen and oxygen, but a mixture more like sand and water or smoke and air. This mass of gases has weight, and presses upon objects at the surface of the earth to the extent of 15 lb. on the square inch. Now some liquids, such as water, were it not for this atmospheric pressure, would not remain liquids at all, but would become gases. The pressure thus tends to squeeze gases together and convert them into liquids. Any force that causes gases to contract will do the same thing, of course—for example, cold; and ceteris paribus removal of pressure and expansion by heat will act so as to gasify liquids. When in the expansion of liquids a certain stage or degree is reached, different for different liquids, gas begins to escape so quickly from the liquid that bubbles of vapour are continually formed and escape. This is called ebullition or boiling. A certain removal of pressure, or expansion by heat, is necessary to produce this, i.e. to reach the boiling-point of the liquid. As regards the heat necessary for the boiling of water at the surface of the earth, i.e. under the atmospheric pressure of 15 lb. on the square inch, this is shown on the thermometer of Fahrenheit as 212°, and on the simpler centigrade one, as 100°, water freezing at 0° C. But if what I have said is true, when we remove some of the atmospheric pressure, the water should boil with a less heat than will cause the mercury in the thermometer to rise to 100° C., and if we take off all the pressure, the water ought to boil and freeze at the same time. This actually happens in the CarrÉ ice-making machine. The question now arises, "Why does the water freeze in the CarrÉ machine?" All substances require certain amounts of heat to enable them to take and to maintain the liquid state if they are ordinarily solid, and the gaseous state if ordinarily liquid or solid, and the greater the change of state the greater the heat needed. Moreover, this heat does not make them warm, it is simply absorbed or swallowed up, and becomes latent, and is merely necessary to maintain the new condition assumed. In the case of the CarrÉ machine, liquid water is, by removal of the atmospheric pressure, coerced, as it were, to take the gaseous form. But to do so it needs to absorb the requisite amount of heat to aid it in taking that form, and this heat it must take up from all surrounding warm objects. It absorbs quickly all it can get out of itself as liquid water, out of the glass vessel containing it, and from the surrounding air. But the process of gasification with ebullition goes on so quickly that the temperature of the water thus robbed of heat quickly falls to 0° C., and the remaining water freezes. Thus, then, by pumping out the air from a vessel, i.e. working in a vacuum, we can boil a liquid in such exhausted vessel far below its ordinary boiling temperature in the open air. This fact is of the utmost industrial importance. But touching this question of latent heat, you may ask me for my proof that there is latent heat, and a large amount of it, in a substance that feels perfectly cold. I have told you that a gasified liquid, or a liquefied solid, or most of all a gasified solid, contains such heat, and if reconverted into liquid and solid forms respectively, that heat is evolved, or becomes sensible heat, and then it can be decidedly felt and indicated by the thermometer. Take the case of a liquid suddenly solidifying. The heat latent in that liquid, and necessary to keep it a liquid, is no longer necessary and comes out, and the substance appears to become hot. Quicklime is a cold, white, solid substance, but there is a compound of water and lime—slaked lime—which is also a solid powdery substance, called by the chemist, hydrate of lime. The water used to slake the quicklime is a liquid, and it may be ice-cold water, but to form hydrate of lime it must assume a solid form, and hence can and does dispense with its heat of liquefaction in the change of state. You all know how hot lime becomes on slaking with water. Of course we have heat of chemical combination here as well as evolution of latent heat. As another example, we may take a solution of acetate of soda, so strong that it is just on the point of crystallising. If it crystallises it solidifies, and the liquid consequently gives up its latent heat of liquefaction. We will make it crystallise, first connecting the tube containing it to another one containing a coloured liquid and closed by a cork carrying a narrow tube dipping into the coloured liquid. On crystallising, the solution gives off heat, as is shown by the expansion of the air in the corked tube, and the consequent forcing of the coloured liquid up the narrow tube. Consequently in your works you never dissolve a salt or crystal in water or other liquid without rendering heat latent, or consuming heat; you never allow steam to condense in the steam pipes about the premises without losing vastly more heat than possibly many are aware of. Let us inquire as to the latent heat of water and of steam.

Latent Heats of Water and Steam.—If we mix 1 kilogram (about 2 lb.) of ice (of course at zero or 0° C.) with 1 kilogram of water at 79° C., and stir well till the ice is melted, i.e. has changed its state from solid to liquid, we find, on putting a thermometer in, the temperature is only 0° C. This simply means that 79° of heat (centigrade degrees) have become latent, and represent the heat of liquefaction of 1 kilogram of ice. Had we mixed 1 kilogram of water at 0° C. with 1 kilogram of water at 79° C. there would have been no change of state, and the temperature of the mixture might be represented as a distribution of the 79° C. through the whole mass of the 2 kilograms, and so would be 39½° C. We say, therefore, the latent heat of water is the heat which is absorbed or rendered latent when a unit of weight, say 1 kilogram of water as ice, melts and liquefies to a unit of water at zero, or it is 79 heat units. These 79 units of heat would raise 79 units of weight of liquid water through 1° C., or one unit of liquid water through 79°.

Let us now inquire what the latent heat of steam is. If we take 1 kilogram of water at 0° C. and blow steam from boiling water at 100° C. into it until the water just boils, and then stop and weigh the resulting water, we shall find it amounts to 1·187 kilograms, so that 0·187 kilogram of water which was in the gaseous steam form, and had besides a sensible heat of 100° C., has changed its state to that of liquid water. This liquid water, being at the boiling-point, has still the 100° C. of sensible heat, and hence the water in the gaseous steam form can have given up to the water at 0° C. into which it was blown, only the latent heat of gasification which was not sensible, but by virtue of which it was enabled to assume the gaseous form. But if 0·187 kilogram of steam at 100° C. can heat 1 kilogram of water through 100 degrees, then 1 kilogram of steam can raise 5·36 kilograms of ice-cold water through 100 degrees, or 536 kilograms through 1 degree, and thus the latent heat of steam is 536 heat units.

Effect of Increase of Pressure on the Boiling of Water.—Now we have referred to diminution of pressure and its effect on the boiling-point of water, and I may point out that by increasing the pressure, such, e.g., as boiling water under a high pressure of steam, you raise the boiling-point. There are some industrial operations in which the action of certain boiling solutions is unavailing to effect certain decompositions or other ends when the boiling is carried on under the ordinary atmospheric pressure, and boiling in closed and strong vessels under pressure must be resorted to. Take as an example the wood-pulp process for making paper from wood shavings. Boiling in open pans with caustic soda lye is insufficient to reduce the wood to pulp, and so boiling in strong vessels under pressure is adopted. The temperature of the solution rises far above 212° F. (100° C.). Let us see what may result chemically from the attainment of such high temperatures of water in our steam boilers working under high pressures. If you blow ordinary steam at 212° F. or 100° C., into fats or oils, the fats and oils remain undecomposed; but suppose you let fatty and oily matters of animal or vegetable origin, such as lubricants, get into your boiler feed-water and so into your boiler, what will happen? I have only to tell you that a process is patented for decomposing fats with superheated steam, to drive or distil over the admixed fatty acids and glycerin, in order to show you that in your boilers such greasy matters will be more or less decomposed. Fats are neutral as fats, and will not injure the iron of the boilers; but once decompose them and they are split up into an acid called a fat acid, and glycerin. That fat acid at the high temperature soon attacks your boilers and pipes, and eats away the iron. That is one of the curious results that may follow at such high temperatures. Mineral or hydrocarbon oils do not contain these fat acids, and so cannot possibly, even with high-pressure steam, corrode the boiler metal.

Effect of Dissolved Salts on the Boiling of Water.—Let us inquire what this effect is? Suppose we dissolve a quantity of a salt in water, and then blow steam at 100° C. (212° F.) into that water, the latter will boil not at 212° F., but at a higher temperature. There is a certain industrial process I know of, in course of which it is necessary first to maintain a vessel containing water, by means of a heated closed steam coil, at 212° F. (100° C.), and at a certain stage to raise the temperature to about 327° F. (164° C.). The pressure on the boiler connected with the steam coil is raised to nearly seven atmospheres, and thus the heat of the high-pressure steam rises to 327° F. (164° C.), and then a considerable quantity of nitrate of ammonium, a crystallised salt, is thrown into the water, in which it dissolves. Strange to say, although the water alone would boil at 212° F., a strong solution in water of the ammonium nitrate only boils at 327° F., so that the effect of dissolving that salt in the water is the same as if the pressure were raised to seven atmospheres. Now let us, as hat manufacturers, learn a practical lesson from this fact. We have observed that wool and fur fibres are injured by boiling in pure water, and the heat has much to do with this damage; but if the boiling take place in bichrome liquors or similar solutions, that boiling will, according to the strength of the solution in dissolved matters, take place at a temperature more or less elevated above the boiling-point of water, and so the damage done will be the more serious the more concentrated the liquors are, quite independently of the nature of the substances dissolved in those liquors.

Solution.—We have already seen that when a salt of any kind dissolves in water, heat is absorbed, and becomes latent; in other words, cold is produced. I will describe a remarkable example or experiment, well illustrating this fact. If you take some Glauber's salt, crystallised sulphate of soda, and mix it with some hydrochloric acid (or spirits of salt), then so rapidly will the solution proceed, and consequently so great will be the demand for heat, that if a vessel containing water be put in amongst the dissolving salt, the heat residing in that vessel and its water will be rapidly extracted, and the water will freeze. As regards solubility, some salts and substances are much more quickly and easily dissolved than others. We are generally accustomed to think that to dissolve a substance quickly we cannot do better than build a fire under the containing vessel, and heat the liquid. This is often the correct method of proceeding, but not always. Thus it would mean simply loss of fuel, and so waste of heat, to do this in dissolving ordinary table salt or rock salt in water, for salt is as soluble in cold water as in hot. Some salts are, incredible though it may appear, less soluble in boiling water than in cold. Water just above the freezing-point dissolves nearly twice as much lime as it does when boiling. You see, then, that a knowledge of certain important facts like these may be so used as to considerably mitigate your coal bills, under given circumstances and conditions.

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