VOL.6, NO. 4
WHAT Is TRUTH?
715
WHAT IS TRUTH?* S. FRANCIS HOWARD, NORWICHUNIVERSITY, NORTHPI&LD, VERMONT
Is i t not about time to ask this 1900-year old question of our chemistry textbooks? We surely cannot tell the whole truth for neither time nor the principles of pedagogy will permit. But can we tell the truth and nothing hut the truth? At least four cases have forced themselves on my attention for a number of years and I desire to discuss each one briefly for the purpose of bringing out certain fundamental principles. Several textbook writers state that the standard atmospheric pressure is the average pressure exerted by the atmosphere and is measured by 760 mm. of mercury a t sea level and a t 45 degrees latitude. Either the word average is used in a special sense or the statement is not the truth. Some years ago I called the attention of one of our noted teachers and textbook writers to this error. He replied: "760 mm. is the average pressure of the atmosphere a t the sea level a t a latitude of 45'. Vermont is farther up than that, so, naturally, the average is not the same up there." As he would not admit his error I wrote to one of our most eminent physical chemists who wrote: "The definition of the atmosphere as equal to 760 nun. is an arbitrary convention. The value comes somewhere near the average atmospheric pressure a t the sea level in the latitude of 45' but is, in strictness, not defined with reference to this average." The average atmospheric pressure would,he very difficult if not absolutely impossible to obtain. Shall it be the average for every second or hundredth second and for ten, or a hundred, or ten thous&d years? None of these would be the true average. A true average could only result from measurements at infinitely small intervals and for infinite time. The value 760.000. . . a s our standard has not been so determined. "760 mm., a convention," is the truth. "760 mm., an average," is an untruth. Which shall we teach? 2. Some statements need qualification in order to he strictly accurate. Dulong and Petit's law is not expressing the truth unless the clause, "in the solid state," is included. What about Graham's law of gaseous diiusions as often stated? Problem: If 16 grams of oxygen diiuse through an orifice in a certain time, how many grams of hydrogen will diffuse under identical conditions? Answer: 4 grams. Law: Gases diffuse by weight a t rates directly proportional to the square roots of their densities. Problem: If, as above, 10 liters of oxygen diffuse, how many liters of hydrogen will diffuse under identical conditions? Answer: 40 liters. Law: Gases diffuse by volumeat rates inversely proportional to the square roots of their densities. The law as usually stated does not tell whether we are dealing with *Read before the Division of Chemical Education at the 76th Meeting of the A. C. S., September 12, 1928, at Swarnpscott, Mass.
JOUIWAL on CHEMICAL EDUCATION
716
APRIL, 1929
weight or volume and so while not, perhaps, telling an untruth, is not telling the whole truth. Would the addition of the two words, "by volume" in any way interfere with the teachableness of this law? Are we justified in leaving out these two words because we usually are interested in volumes and seldom in weights? When two words will make a statement accurate and their omission makes the meaning doubtful, are we justified in leaving out these qualifiers? Boyle's law might be too "cluttered up" for teaching if we tried to include in it all of van der Waal's deductions. However,. either the textbook or the teacher, it seems to me, should bridy note its shortcomings. A teacher must always tell the truth and when it can be told simply he should try to tell the whole truth. Teaching is not more efficient when inexact. We must not follow the line of least resistance. That which is most easily taught is not necessarily the best taught. 3. Thirty years ago the equations in the textbooks for the formation of sodium bicarbonate were written: NHl
+ COz + NaCl + H,O
=
NaHC03 4- N&Cl
It was stated that, as the sodium bicarbonate was less soluble than the other possible salts, it was thrown down. Today many texts state that concentrated salt solutions are saturated with ammonia and carbon dioxide and that there results ammonium bicarbonate. This, they state, reacts later with the sodium chloride and forms sodium bicarbonate. Why is the ammonium salt fist formed and how, and what makes the bicarbonate radical shift over to sodium Fter? Certainly this description of the process is not any more simple than the older one but what are the facts? Sodium bicarbonate is less soluble than ammonium bicarbonate and its heat of formation is 19,000 calories greater. Ammonia gas will precipitate sodium chloride from its saturated solution. Properly to teach the Solvay process, then, we must account for the formation of ammonium bicarbonate, provided it is formed, for its breaking up so that the sodium salt may be precipitated, and for such effects as result from different solubilities and different heats of formation. We should seek the truth with reference to these things and teach accordingly. If we do not know the truth, let's say so. 4. MendelCeff, in his "Principles," tells how to obtain perfectly pure salt as follows: "a saturated solution of table salt is prepared, and hydrochloric acid gas is passed through it. This precipitates the sodium chloride which is not soluble in a strong solution of hydrochloric acid." This seems fairly simple and quite teachable. Why do we have to bring in the ions and say that the gas adds chlorine ions, which, with those present, increase the chlorine ion factor in the equation: (Na+) X (Cl-) = K. Consequently, in order to keep K constant, the factor [Na+] must decrease and it does so by sodium ions uniting with extra chlorine ions, forming sodium
chloride molecules which are precipitated. True, this is in terms of the electrolytic dissociation theory but theories are for the purpose of generalization and the following facts cannot be brought under the common ion i i l e : saturated solutions of sodium chloride, potassium iodide, and potassium bromide are each precipitated by hydrogen chloride, hydrogen bromide, and hydrogen iodide and also by ammonia gas. I think that Mendel4eff would generalize and say that these salts are not as soluble in the strong solutions of the gases as they are in pure water, and would he not thus sufficiently explain the situation? In the cases where there is a common ion (HCl NaCl, HBr KBr, and H I KI) doubtless the common ion principle should be considered but when there is no common ion and we get precipitation, i t is evident that our teaching must include some other factors. The three hydrogen halides are very soluble in water and cause precipitation of the salts. Ammonia gas is also very soluble in water and it, too, precipitates salts from saturated solutions of sodium chloride, potassium bromide, and potassium iodide. A number of interesting questions come up a t this point but my purpose is not to explain how these precipitations take place nor even to tell what these precipitates are. I have not stated that hydriodic acid precipitates sodium chloride nor am I sure that this acid precipitates only sodium chloride. I did state that there were precipitations. I am,however, questioning the advisability of teaching a series of phenomena in terms of a theory which cannot hold for all cases. In other words, until we know what is truth, should we state the facts as they appep and offer explanations which are not in accord with the facts? Is the common ion efect a teachable explanation for very strong solutions of salts which are precipitated by the addition of very soluble gases? A teacher dislikes to call the attention of his classes to the shortcomings of the textbook. Misstatements and especially wrong and impossible explanations when brought to the attention of the pupils make them lose confidence in the text and perhaps in the teacher, too, or lead them to think that it is more or less guesswork and that "one man's guess is as good as another's." This should be avoided, if possible, and in order to eliminate some of these, perhaps minor, defects in our textbooks, this paper has been presented. After Pilate asked the question, "What is truth?" he said, "I find no fault." I, too, would prefer to be classed with the truth-seekers rather than with the fault finders.
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