PRECIPITATION REACTIONS
Okay, so now you know that when ionic solids are dissolved in water, they
dissociate. But what happens when soluble ions in separate solutions are mixed
together, and they form an insoluble compound? Well, the product of this type of
reaction will result in a solid substance that settles out of solution, called a
precipitate. One example of this is the reaction between lead nitrate and
potassium iodide.
2KI(aq) + Pb(NO 3 ) 2 (aq) → 2KNO 3 (aq) + PbI(s)Each of the reactants in this reaction is an ionic compound that’s colorless in
solution, but when they’re combined, they react to form a product, lead iodide,
which precipitates out of solution as a yellow solid. In this reaction, the anions
and cations of the reactants are exchanged in a double replacement reaction,
which typically results in the formation of a precipitate. But how do you know
when a precipitation reaction will proceed? The solubility rules tell you which
ionic compounds are soluble in water and which are not and enable you to make
predictions about whether certain ions will react with one another to form a
precipitate.
The Solubility Rules
• Most silver, lead, and mercury salts are INSOLUBLE except for their
nitrates and perchlorates.• Most hydroxides (OH−) are INSOLUBLE except those of alkali metals
and barium.• All nitrates (NO 3 −) and perchlorates (ClO 4 −) are SOLUBLE.• All alkali metal and ammonium (NH 4 +) compounds are SOLUBLE.Enough said! Now review what we’ve said about solutions, and try the following
set of questions. Answers can be found in Part III.