15.3 Precipitation Reactions
Learning Objectives
By the end of this section, you will be able to:
- Predict the solubility of common inorganic compounds by using solubility rules
- Define precipitation reactions
- Recognize and identify examples of precipitation reactions
- Apply the solubility rules of common inorganic compounds to predict the products formed when two aqueous solutions are mixed
Scientists have found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This section of this chapter will focus on a specific type of double displacement reaction called a precipitation reaction.
Precipitation Reactions and Solubility Rules
Solubility of Inorganic Compounds
The idea of solubility was introduced in the solutions chapter. The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble and are found as dissolved ions in aqueous solution. A substance will precipitate when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution to form a solid (s). For purposes of predicting the identities of solids formed by precipitation reactions, one may simply refer to the solubility guidelines for many ionic compounds in Table 15.3a to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. First, it is important to become familiar with using the solubility table to determine if a given salt will dissolve (aq), or not (s) in aqueous solution. If a salt is said to be insoluble, or has low solubility, or is slightly soluble, it will form a precipitate – a solid – and the symbol (s) will be used to represent that observation. If a salt is soluble, the salt will dissociate (ionize) in aqueous solution, and the symbol (aq) will be used to show that chemistry. It is important to mention that Table 15.3a does not include every possible soluble, insoluble salt combination. Since there are other possibilities, Table 14.2a and Table 14.2b can also be referenced to determine solubility of inorganic compounds.
| Negative Ion (Anion) | Positive Ion (Cation) | Solubility | Phase, Phase Symbol |
|---|---|---|---|
| All | Li+, Na+, K+, Rb+, Cs+ NH4+ | Soluble | aqueous, (aq) |
| Chloride (Cl–), Bromide (Br–), Iodide (I–) | Ag+, Pb2+, Hg22+, Cu+ | Low solubility | solid, (s) |
| Chloride (Cl–), Bromide (Br–), Iodide (I–) | All others | Soluble | aqueous, (aq) |
| F– | compounds with group 2 metal cations, Li+, Al3+, Pb2+, Fe2+and Fe3+ | Low solubility | solid, (s) |
| F– | All others | Soluble | aqueous, (aq) |
| Hydroxide (OH–) | Li+, Na+, K+, Rb+, NH4+, Sr2+, Ba2+ | Soluble | aqueous, (aq) |
| Hydroxide (OH–) | All others | Low solubility | solid, (s) |
| NO3–, NO2– | All (exception: AgNO2 is insoluble) | Soluble | aqueous, (aq) |
| Phosphate (PO43-), Carbonate (CO32-) | Na+, K+, Rb+, NH4+ | Soluble | aqueous, (aq) |
| Phosphate (PO43-), Carbonate (CO32-) | All others | Low solubility | solid, (s) |
| Sulphate (SO42-) | Ca2+, Sr2+, Ba2+, Ag+, Pb2+ | Low solubility | solid, (s) |
| Sulphate (SO42-) | All others | Soluble | aqueous, (aq) |
Source: “Table 15.3a Solubility Guidelines for Inorganic Compounds in Water at 25°C” was created by Jackie MacDonald, CC BY-NC-SA 4.0.
Using the Solubility Table Guidelines to Determine Solubility
To qualitatively determine the solubility of a salt in aqueous solution, reference Table 15.3a and follow the steps outline below:
- Identify the negative ion in the salt; locate the ion in the first column of Table 15.3a.
- Determine the positive ion in the salt by moving into the second column, same row as your anion.
- Finally, move horizontally in that row into the solubility column (column 3) to determine whether that compound is soluble or insoluble.
- Identify the symbol used to represent its solubility
Example 15.3a
Use the solubility rules to determine the solubility of the following substances. Categorize each as soluble or insoluble and also write the symbol used to show its solubility.
- Na2S
- AgBr
- Mg(Cl)2
Solution
- Soluble, (aq) – Since S is not listed as an anion, it falls into the “all” option. It’s cation is a Na+ cation, so it is soluble, aq.
- Insoluble, (s) – When Br– forms a salt with Ag+, it forms an insoluble salt, s.
- Soluble, (aq) – When Cl– forms a salt with Mg2+, it fall into the “all others” category for cations, and it forms a soluble salt, aq.
Source: Example 15.3a created by Jackie MacDonald and David McCuaig, CC BY-NC-SA 4.0.
Exercise 15.3a
Check Your Learning Exercise (Text Version)
From the options provided, identify the salts that will form a precipitate in aqueous solution at 25 degrees Celsius.
-
- Fe(NO3)2
- BaSO4
- AgF
- Mg(OH)2
- AgNO2
- ZnCl2
- PbI2
- KBr
- LiClO3
- AgBr
- KOH
- (NH4)PO4
- CaCO3
Check Your Answers[1]
Source: “Exercise 15.3a” created by Jackie MacDonald and David McCuaig, licensed under CC BY-NC-SA 4.0.
Watch the video Precipitation Reactions starting at 6min 31sec until 9min05sec. It demonstrates how to use a similar solubility table to determine solubility.
Video Source: Angles and Acid (2020, May 16). Precipitation Reactions [Video]. YouTube.
For a more in depth solubility table that is online and printable, link to “Solubility Rules Chart” by MilliporeSigma.
Source: Section titled “How to Use the Solubility Table Guidelines to Determine Solubility” was created by Jackie MacDonald and David McCuaig and is licensed under CC BY-NC-SA 4.0.
Precipitation Reactions
A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a number of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and gravimetric methods for determining the composition of matter.
A vivid example of precipitation is observed when aqueous solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:
2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)
This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts. Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (Figure 15.3a). The properties of pure PbI2 crystals make them useful for fabrication of X-ray and gamma ray detectors.
The solubility table may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound.
For example, mixing aqueous solutions of silver nitrate and sodium chloride will yield a solution containing Ag+, NO3−, Na+, and Cl− ions. Aside from the two ionic compounds originally present in the solutions, AgNO3 and NaCl, two additional ionic compounds may be derived from this collection of ions: NaNO3 and AgCl. The solubility table can be used to determine if either of these salt combinations are insoluble in aqueous solution. Insoluble salts will precipitate out of the solution to form a solid (s).
The solubility table indicates all nitrate salts are soluble, so sodium nitrate (NaNO3) will remain ions in solution. However, silver chloride, AgCl, is one of the exceptions to the general solubility rules of chloride salts, and this combination of ions will form a solid in aqueous solution. Therefore, a precipitation reaction is predicted to occur, as described by the following molecular equation:
NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
Watch Precipitation Reactions (10mins 13sec).
Video Source: Angles and Acid (2020, May 16). Precipitation reactions [Video]. YouTube.
Example 15.3b
Predict the result of mixing reasonably concentrated solutions of the following ionic compounds. If a reaction occurs and a precipitation is expected, write a balanced molecular equation for the following two reactions.
- potassium sulfate and barium nitrate
- lithium chloride and silver acetate)
Solution
- The two possible products for this combination are KNO3 and BaSO4. The solubility guidelines indicate KNO3 is soluble and BaSO4 is insoluble, so a precipitation reaction is expected. The balanced molecular equation for this reaction is K2SO4(aq) + Ba(NO3)2 (aq) → BaSO4(s) + 2KNO3(aq)
- The two possible products for this combination are LiC2H3O2 and AgCl. The solubility guidelines indicate LiC2H3O2 is soluble and AgCl is insoluble, and so a precipitation reaction is expected. The balanced molecular equation for this reaction is LiCl (aq) + AgC2H3O2(aq) → AgCl(s) + LiC2H3O2(aq)
Exercise 15.3b
Predict the result of mixing reasonably concentrated aqueous solutions of the following ionic compounds. If a reaction occurs and a precipitation is expected, write a balanced molecular equation for the following reaction between lead(II) nitrate and ammonium carbonate.
Check Your Answer[2]
Exercise 15.3c
Which solution(s) could be used to precipitate the barium ion, Ba2+, in a aqueous solution barium nitrate: sodium chloride, sodium hydroxide, or sodium sulfate? If a reaction occurs and a precipitation is expected, write a balanced molecular equation for that reaction
Check Your Answer[3]
Links to Interactive Learning Tools
Explore Precipitation Reactions from the Physics Classroom.
Attribution & References
Except where otherwise noted, this page is adapted by Jackie MacDonald from “6.2 Precipitation Reactions” In CHEM 1114 – Introduction to Chemistry (BCcampus, Pressbooks) by Shirley Wacowich-Sgarbi and Langara Chemistry Department is licensed under CC BY-NC-SA 4.0. / Adaptations and additions to content in this section were made for student comprehension.
- The salts that form precipitates are PbI2, AgBr, CaCO3, Mg(OH)2, BaSO4, AgNO2 ↵
- The two possible products for this combination are PbCO3 and NH4NO3. The solubility guidelines indicate NH4NO3 is soluble and PbCO3 is insoluble, and so a precipitation reaction is expected. The balanced molecular equation for this reaction is Pb(NO3)2(aq) + (NH4)2CO3(aq) → PbCO3(s) + 2NH4NO3(aq) ↵
- The solubility table can be used to determine which ionic compounds containing barium will be insoluble in aqueous solution. There are three possibilities to consider: BaCl2, Ba(OH)2, and BaSO4. The solubility guidelines indicate BaCl2 and Ba(OH)2 are soluble in water and will not form a precipitate; however, BaSO4 is insoluble, so this combination will result in a precipitation reaction. The balanced molecular equation for this reaction is
Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq) ↵
extent to which a solute may be dissolved in water, or any solvent
of relatively high solubility; dissolving to a relatively large extent
insoluble product that forms from reaction of soluble reactants
of relatively low solubility; dissolving only to a slight extent
reaction that produces one or more insoluble products; when reactants are ionic compounds, sometimes called double-displacement or metathesis