XX: Miscellanea No. 2 KAROL 1. MYSELS University of Southern California, Los Angeles THE MEANING OF "ELEMENT"
Professor A. Sementsov of Lafayette College, Easton, Pa., notes that students are often confused by the failure of textbooks2 to distinguish clearly between elements and elementary substances. This difficulty becomes particularly clear during the discussion of the composition of compounds which are said to contain certain elements but which clearly would not be pure if they contained any of that element as a substance, i.e., uncombined. Thus pure copper sulfide contains 20.1% of sulfur but copper sulfide can he readily purified to contain less than 0.1% of sulfur! When the elementary substance has a special name, there is no danger of confusion: 4.197% of carbon in washing soda may be a sign of purity hut the same percentage of graphite would certainly reduce its cleansing value! Unfortunately few elementary substances have such special names which were made necessary by their allotropy (diamond, graphite; red and black phosphorus; ozone) but even here the most common allotrope often has the name of the element (carbon, oxygen). The trained chemist, of course, always knows from the context whether the element or the elementary substance is involved hut the beginning student can readily be perp1exed.l A possible path towards clarification may be to use in the early stages terms such as elemental sulfur or elemental oxygen when describing elementary substances and simply sulfur and oxygen when discussing the elements. Another approach might be to warn the student explicitly of the ambiguous character of the word element and of the names of most elements. THE SOLUBILITY OF PHENOL IN CARBONATE SOLUTIONS
carbonate solutions and ascribe this to its weakness as an acid. In fact, however, phenol dissolves in sodium carbonate solutions and this is due to its strength as an acid which is greater than that of the bicarbonate ion. Phenol is quite soluble in water, the saturated solution containing about 8% of phenol which corresponds to about 0.9 M. As is well known, it becomes completely miscible with water at higher temperatures. A test-tube experiment shows readily that, at room temperature, addition of sodium carbonate increases its solubility and that it dissolves freely in concentrated solutions of this salt. However, a t higher salt concentration a phenoxide-rich, water-miscible layer separates. This in turn gives a phenol-rich layer upon acidification. The equilibrium involved CsHsOH
+ GOa- = CsHsO- + COIH-
gives an equilibrium constant which can be expressed in terms of the corresponding dissociation constants which are4 about 1.3 X 10-lo for phenol and 4.4 X 10-" for COaH-.
Thus phenol being appreciably more acidic than bicarbonate, tends to form the phenoxide a t the expense of the carbonate ion. 'If the equilibrium concentration of carbonate were equal to the solubility of phenol, the above equation indicates that the solubility of phenol would be more than doubled since the stoichiometry gives (CsH60-) = (C03H-) and under these conditions
Professor F. J. Guerin of Merrimack College, North Andover, Massachusetts, points out that many textbooks2 state that phenol does not dissolve in sodium Suggestions of material suitable far this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many detailils aaspossihle, and particularly with references to modern textbooks, to Karol J. Mysels. Department of Chemistry, University of Southern California, Las Angeles 7, California. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent standard books to be presented. In connection with perplexing words the reader may try this statement: "If the ions were to form s. union, the ionized particles would be unioniaed and the unionized ones would be ununionized!'
5Ea
Needless to say that in sodium bicarbonate solutions there is no appreciable increase in the solubility of phenol since it is a much weaker acid than carbonic acid. THE CHANGE OF VAPOR PRESSURE WITH TEMPERATURE
Michael Avinor of the Scientific Department of the Ministry of Defense of Israel calls attention to the fact that textbooks2 seldom point out the exact con'%andbook of Chemistry and Physics," Chemical Rubber Publishing Co., Cleveland, Ohio, 1955.
JOURNAL OF CHEMICAL EDUCATION
ditions for the validity of thermodynamic equations describing the change of vapor pressure with temperature. In particular, the familiar equation dP/dT
=
AHITV.,
in which the volume of the liquid does not appear is often given as an approximation without noting that this is true only for the liquid under its own vapor pressure (such as water in vacuum). On the other hand, this equation is actually the exact one if the liquid is kept under a constant pressure (for example, approximately, water in the presence of air). This can be shown as follows. The differential changes of chemical potential must he equal in the two phases under equilibrium conditions and can be wrib ten V'dP1 - SUT
=
u"dPw- SedT
(1)
where the superscripts refer to the phase, and the quantities V and S refer t o the same amount of the material (e.g., one mole). This can be rearranged t o (8' - SL)dT = VYdP"- V'dP1
(2)
The difference of entropies on the left side of the equation corresponds t o the heat of transition from liquid to vapor AH" so that we can write AHZ0dT/T = VOdP*- V'dP'
(3)
Now we. can apply this general relation t o two cases. The first case is when the pressure on the two phases is the same, dPn = d P f . This is the usual case of a liquid exposed simply to its pure vapor. The above equation then simplifies t o
which gives
VOLUME 35, NO. 10, NOVEMBER, 1958
only if we neglect the volume of the liquid. The second case is when the pressure of the liquid is maintained constant. This less familiar case can be realized exactly if the vapor-liquid interface is curved so that surface tension supplies the difference of pressure between the two phases. I n small capillaries this difference can be appreciable. The other and less perfect way of realizing this case is by the presence of an inert and insoluble gas in the vapor. Then P v is the partial pressure of the vapor in the vapor phase and P' is the sum of the pressures of the vapor and the inert gas. This is approximately realized when water is exposed t o air a t constant total pressure. Now d P = 0 so that our equation (3) becomes AHLsdT/T = VDdP'
and gives without any approximation. It may he noted, however, that the value of AH'., the heat of transition, has to be taken under the conditions of the process considered and may diier slightly in the two cases. Only the first case corresponds exactly to the usual definition of heat of vaporization. DISCUSSED ELSEWHERE
The Action of Phosphorus Patachloride on Ether. J . Zussman and E. de Barry Barnett of The Grammar School, Hastings, England, examined5 the evidence for the formation of alkyl chloride and phosphoryl chloride by this reactcon as is sometimes stated in texts. They conclude that no such reaction occurs but that an error probably slipped into a translation about a hundred years ago! "USSMAN,
271 (1957).
J., AND E. DE BARRYBARNETT, School Sei. Rev., 37,
