we know that a precipitation reaction is one in which components interact in solution and a solid will crash out of solution collecting at the bottom of the flask this is typically the result of a double displacement or double replacement reaction in the form of a b plus c d yields a d plus c b where the reactants are both ionic compounds that are soluble in a particular solvent while one of the products is not soluble and will thus crash out of solution to form a lattice such reactions are commonplace in both nature and industry and the key thing to understand in predicting such a reaction is the concept of solubility this is the amount of some substance that is capable of dissolving in a particular volume of a particular solvent or the maximum concentration of a substance that can be achieved if a substance will dissolve in a particular solvent it is said to be soluble if it will not dissolve in any appreciable amount it is said to be insoluble when the concentration of a substance exceeds its solubility a precipitate will form let's look at the water solubilities of various ionic compounds starting with ones involving group 1 metals these include lithium sodium potassium and the rest down the group we will also lump in the ammonium ion with this group ionic solids with these cations are always water soluble and there are no notable exceptions to this trend next there are the larger halide ions chloride bromide and iodide compounds with these will typically be water soluble with the exceptions of halides formed with silver plus mercury poly cations and lead 2 plus those will be water insoluble next is the remaining halide fluoride this will be soluble except for when combined with group two metal cations as well as lead two plus and iron three plus next are compounds made with acetate bicarbonate nitrate and chlorate these are always water soluble and lastly the sulfates these tend to be water soluble except when joined with any of these six cations next we have the rules for insolubility these say that carbonates chromates phosphates and sulfides are insoluble except when formed with group one metal cations or ammonium and then hydroxides will also be insoluble except when formed with group 1 metal cations or barium if any of these insoluble compounds are generated in solution a precipitate will form here is an example of such a situation if lead 2 plus ions and iodide ions meet in solution this will form lead iodide and lead is an exception to the solubility of iodides so a precipitate will indeed form which will have a yellow tint in order to tell that a precipitate will form we simply write out a net ionic equation such as this one here where sodium and nitrate are identified as spectator ions and the formation of silver fluoride is all that matters and that's what we need to know about solubility and precipitation