Ah, solubility rules, some of the most basic things students are taught to remember in an AP Chemistry class, as they are useful on reaction predictions.

So what are these rules, anyway? Well, you have two ionic compounds (e.g. lead (II) nitrate and magnesium iodide, Pb(NO3)2 and MgI2, respectively), and you put them into water. Depending on the compounds, the ions that make the compound may split up (in our example: goes to Pb+2, 2 NO3-1, Mg+2, and 2 I-1) and a precipitate will form (PbI2, or lead(II) iodide)

So what exactly is happening? Well, some ionic compounds are, by their very nature, soluble (in other words, they dissolve in water). When an ionic compound dissolves in water, it splits up into its component ions. They are happy little ions, floating around in the water. Other ionic compounds are not soluble. If their component ions are floating around in a solution, they'll form a precipitate, which is basically their solid ionic compound. Still others are only "slightly soluble." In this case, only a small amount of the compound dissociates, and the precipitate and the ions in the solution are in equilibrium (expressed via a solubility product constant, which is just another equilibrium constant). (Why certain types of compounds are soluble and others aren't, I really don't know. An explanation of this would be helpful).

And yes, as the name of this node implies, there are rules to determining which ionic compounds are soluble and which aren't. I'll give you what rules I know (/msg me if you know of any more).

The following rules tell more of which ionic compounds are INsoluble. Note that each of the following rules doesn't apply IF it's the salt of sodium, potassium, or ammonium (as per the first rule in above). Any such sodium, potassium, or ammonium salts with the following compounds are still soluble compounds. In addition, some rules have more exceptions as well.

SOURCE: My chemistry textbook (when I have access to my book again, I'll put in exact information)

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