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Hundreds of thousands of chemical reactions occur in aqueous solution. If a precipitate forms, a chemical reaction has occurred. But, how does one predict a precipitate? The following list is not original with me, but is one that I organized in a special way because my job is to get students, every student, to memorize it in less than 20 minutes. I can do this with no problem at all. Before they come to me, they know the ions. Then, I teach them how to “memorize” the solubility list. First, here’s the Ion Solubility list.
It looks formidable, does it not (until you learn a few tricks)? Here is the 20-minute memorization lesson
1.) The ions in green are ALWAYS soluble. I call these “trump” ions. If they occur in a compound, you can bet the compound is soluble and will NOT form a precipitate. The “trump” cations (positive-charged ions) are the alkali metals (Li+, Na+, K+, Rb+, Cs+, and Fr+), H+, and the only polyatomic cation, ammonium, NH4+. Just look at the Periodic Table, take the entire first column and add the ammonium ion and you have the soluble cations.
2.) The other “trump” ions that always are soluble are the acetate and the nitrate anions (C2H3O2–, NO3–), which are negatively charged.
3.) Then, there are the halides, Cl–, Br–, and I–. These ions “trump” everything unless they are fighting the SLiM brothers, Silver, Lead, and Mercury, well-known outlaws from the Old West.
4. Finally, there is the sulfate ion, SO4–, who is always soluble unless the SLiM (Silver, Lead, and Mercury) brothers come around or if Sul’s Fate (sulfate) is to be “cursed” by Sir Barah. (that is, the ions Ca2+, Sr 2+, Ba2+, and Ra2+. The mnemonic is “Cursed by Sir, BaRa,” as if BaRa were the name of a gentleman with the title, “Sir”).
5. Now, we get to the list of things mostly insoluble, these are the ones below the heavy red line. The first three are easy. Phosphate, carbonate, and sulfite (PO43-, CO32-, and SO3– ) are ALWAYS insoluble unless trumped.
6. The hydroxide ion, OH–, is always insoluble unless trumped, or if the hydroxide rides (note the rhyme) with Sir BaRa (Sr2+, Ba2+, Ra2+).
7. Finally, the sulfide ion is always insoluble unless it is trumped or rides the Earth, that is, the alkaline earth metals (second column in the Periodic Table.)
8. The final thing to note is that every exception is either silver lead or mercury (the SLiM brothers), or some part of the alkaline earth metals, the second column in the Periodic Table.
All this may seem difficult at first glance. But, the method is tried and true. Use my technique and in 20 minutes it is all committed to memory – almost forever.
Note: For those who take more than one year of chemistry, you learn that there are measures of solubility and that what I have called “soluble” are really “highly soluble,” and what I have called “insoluble” are really measures of extremely low solubility. Ignore that until you need it. Learn to walk first, then run.