A Perspective on Solubility Rules Manus Monroe1 a n d Karl Abrams Indian Valley Colleges, lgnacio Boulevard, Novato, CA 94947 How often have you seen students ~ r o ~ e ruse l v solubility rules to identify the precipitate in a metathesis reaction? Looking through textbooks. I have found a wide variety of solubility rules; some just list generalities while others have a long list of exceptions. In one case, the list of exceptions was so extensive that some of my students asked for reasons for writing solubility rules. On the other hand, many high school texts du not make use of such rules at all. This article is a presentation of four generalizations which we have found to be very useful for high school students and those taking a one-semester college course. 'lhese generalizations are useful in introducing the dynamic topic of solubility and in helping the students make some order out of the tremendous number of factsavailable. Studentsneed to know some chemistrv for these rules to be useful. In using them, we emphasize ;hat these eeneralities are iust that -~redictors of how thines usual& are. The rulesare not hardand fast rules that all& the determination of the right answers to all chemical auestions. First, the four rules &I be presented, and then terminology will be explained. Rule I: Salts containing N14+and Group IA cations (Id+,Na+, K+, Rh+, and Cs+) are soluble to very soluble in deionized water. Common exception: KC104 is slightly soluble. Rule 11: Salts containing anions of strong or very strong acids are usually soluble to very soluble in deionized water. Common exceptions: Ag+,Pb2+,and Hga2+(Analytical Group One cations), as well as Cu+ ions, fom water insoluble salts with Group VIIA (F-, C1-, Br-, and I-), S042-, and Cr02- anions. Recall that HCl, HBr, HI, HzSO4, and HzC104are strongacids. (Anotherway to express these exceptions to the rule is that Agf. PbZ+HgzZ+,and CU+cations form water-insoluble salts with anions of strong or very strong acids except for NOa- ions which form soluble salts.) Other exceptions: Ca2+and Ba2+(Group IIA) ions, and also Cu+ ions, form water insoluble sulfate and chromate salts. Rule III: Salts containing anions of weak or very weak acids are usually insoluble to slightly soluble in deionized water Common exceptions: NH4+ and Group IA cations fom water-soluble salts. (See Rule I, which has ~reeedenceover Rule 111.) Other Exceptions: Group IIA (BeZ+,Mg2+, Ca2+, SP+, and BaZ+)sulfidea and cyanides are soluble. Acetate salts,other than those of Ag+ and Fe3+,are water soluble. Rule IV: Bases are usually insoluble in deionized water. Important common exceptions: Group IA, Group 11.4 (except magnesium and beryllium), and NH,+ fom bases that are soluble to very soluble in water. In these solubility rules, the tern "soluble to very soluble" means that a spatula of a salt completely dissolves in 10 mL
of deionized water (a medium-sized test tube a~oroximatelv half-filled) after h&ting and stirring. The ter&':insoluhle tb slightly soluble" means that of a s ~ a t u l of a salt, some remains undiss~olvedin water after heating and stirring. These solubility tennsaredesimed to be ~racticalso as toassist students guide during experiments on in applying solubiliiy rules metathesis and qualitative analvsis. (Even thouch there are additional exceptions to the fair rules which are not listed, students may experience only a few of them in the introductory laboratory. For example, freshly prepared AgF and Ba(0H)dIHzO are soluble, while Ba(OH2.H20 . . is sliehtlv - soluble.) An extension of Rules I1 and 111can be used as a quick determination of acid strengths of the oxyanions in these gmup. Simply add the charge on the anion t o the total number of oxygen atoms in its formula and use that positive value with the following chart. Acidic Sum of Characteristic Charge of and Oxygen Parent Atoms Acid Comments
ass
very strong and strong acids exist predominantly as ions in water 1 weak weak and very weak 0 very weak acids are predominantly molecular in form Sulfate ion, s042-,has, for example, a value of 2 (4 2 = 2) and is associated with a strong acid (H2S04),while acetate, C2H302, has a value of 1(2 - 1 = 1)and is from a weak acid (HCzHaOz). In this technique, the adding of charge to oxygen atoms in an oxyanion is equivalent t o obtaining the absolute difference between available hydrogen ions and oxygen atoms in an oxyacid because each negative charge represents the loss of a hydrogen ion from an o x y a ~ i dThis . ~ generalization does not apply t o oxyanions that have acidic hydrogen atoms, e.g., HSOA-. he manner in which these solubility rules are written allows high school students to learn readily and correctly apply them to appropriate laboratory experiments. In addition, these rules are applicable to a large number of homework problems concerning metathesis and acid-base reactions found in manv basic textbooks. 3 2
very strong strong
-
' A u h r to whom correspondence should be m e s s e d .
This technlqu, is a mcdlflcatlonof ~ i etos be presented In an article mat will be published early in 1985 In m s JOURNAL.
Volume 61 Number 10 October 1984
885
