Precipitation Reactions

As you have seen earlier in this lesson, solubility rules can be used to figure out whether a certain combination of ions will come apart and dissolve in water. Some will and some won't.

Solubility rules can also be used to figure out whether ions that are already in solution will remain apart or come together.

For example, the rule for hydroxide says that sodium hydroxide is soluble. That means that if you start with solid sodium hydroxide, sodium ions and hydroxide ions will separate and go into solution. It also means that if you start with dissolved sodium chloride, the ions will not come together out of solution to form a solid material. As another example, the rule for chlorides says that lead(II) chloride is insoluble. That means that if you start with solid lead(II) chloride, the ions will not separate and go into solution. That also means that if you start with lead(II) ions and chloride ions already in solution, they will come together to form a solid material that we say precipitates out of solution.

Predicting Precipitates

In exercise 9 you will use the solubility rules to figure out whether certain combinations of ions will remain in solution when they are mixed or come together and form insoluble precipitates.

A complicating factor in figuring this out is that each ionic solution contains both positive ions and negative ions. Consequently, there are two combinations that must be considered. For example, when barium nitrate is mixed with copper(II) sulfate, one possible combination is copper(II) nitrate and the other is barium sulfate. According to the nitrate rule, all nitrates, including copper(II) nitrate, are soluble. According to the sulfate rule, barium sulfate is insoluble. Therefore this combination of ions will form a precipitate and that precipitate will be barium sulfate. We can test that prediction by mixing the solutions and seeing what happens.

Now do exercise 9. First use the solubility rules to determine whether a precipitate will occur for each combination. Then, when you are in the lab, mix the solutions on a spot plate to see which combinations actually do form precipitates.

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