Predicting Precipitation Reactions

Solubility rules can be used to figure out whether ions that are already in solution will come together to form an insoluble compound, that is, precipitate.

Let's briefly consider two examples. Because the rule for "hydroxides" says that sodium hydroxide is soluble, sodium ions and hydroxide ions will not come together out of solution to form a solid material.	Na⁺(aq) + OH^-(aq) no reaction
On the other hand, the rule for "chlorides" says that lead(II) chloride is insoluble. Therefore lead(II) ions and chloride ions already in solution will come together to form a solid material that we say "precipitates out of solution."	Pb²⁺(aq) + 2 Cl^-(aq) PbCl₂(s)

To see what that reaction looks like, move your mouse over the image to the right. (You do not need to click.) The test tube contains lead(II) ions and the dropper contains chloride ions.

Practice

Take a moment now to determine whether the combinations of ions listed below will result in the formation of insoluble precipitates. In each case where they do, give the name and the formula of the compound that is formed. Then check your answers before you continue with the lesson.

NH₄⁺ and Cl^-

Pb²⁺ and OH^-

Ba²⁺ and PO₄^3-

K⁺ and S^2-

Mixing Actual Solutions

When working with actual solutions, things are not as simple as in the examples and exercises above. The complicating factor is that each ionic solution contains both positive ions and negative ions. Consequently, even in the simplest cases, there are two combinations that must be considered and checked for solubility.

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 "nitrates" rule, all nitrates, including copper(II) nitrate, are soluble. According to the "sulfates" rule, barium sulfate is insoluble. Therefore, we can predict that this combination of ions will form a precipitate and that the precipitate will be barium sulfate. We can test that prediction by mixing the solutions and seeing what happens.

That is what you have done (or will do) in exercise 9. First use the solubility rules to determine whether a precipitate will occur for each combination. That should be done now. Then, when you are in the lab, mix the solutions on a spot plate to see which combinations actually do form precipitates. If there are discrepancies between your predictions and your observations, check with the instructor in the lab to find out why.

Top of Page

E-mail instructor: Eden Francis

Clackamas Community College
Š1998, 2003 Clackamas Community College, Hal Bender