Skip to main content
Chemistry LibreTexts

5.2: Precipitation Reactions

  1. Last updated
  2. Save as PDF
  • Page ID
    366437

    • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Learning Objectives
    • Classify chemical reactions as precipitation reactions
    • Predict the the products of precipitation reactions

    Humans interact with one another in various and complex ways, and we classify these interactions according to common patterns of behavior. When two humans exchange information, we say they are communicating. When they exchange blows with their fists or feet, we say they are fighting. Faced with a wide range of varied interactions between chemical substances, scientists have likewise found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This module will provide an introduction to three of the most prevalent types of chemical reactions: precipitation, acid-base, and oxidation-reduction.

    Precipitation Reactions and Solubility Rules

    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 (see the last module of this chapter).

    This unbalanced equation of a precipitation reaction has the general form of a double displacement:

    \[ \overbrace{\ce{AC}}^{\text{soluble}} + \overbrace{\ce{BD}}^{\text{soluble}} \rightarrow \underbrace{\ce{AD}}_{\text{insoluble}} + \overbrace{\ce{BC}}^{\text{soluble}} \label{4.2.2}\]

    A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:

    \[\ce{2KI}(aq)+\ce{Pb(NO3)2}(aq)\rightarrow \ce{PbI2}(s)+\ce{2KNO3}(aq)\]

    This observation is consistent with the solubility guidelines in Table \(4.5.1\): 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 \(\PageIndex{1}\)). The properties of pure PbI2 crystals make them useful for fabrication of X-ray and gamma ray detectors.

    A photograph is shown of a yellow green opaque substance swirled through a clear, colorless liquid in a test tube.
    Figure \(\PageIndex{1}\): A precipitate of PbI2 forms when solutions containing Pb2+ and I− are mixed. (credit: Der Kreole/Wikimedia Commons)

    The solubility guidelines in Table \(4.5.1\) 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 solutions of silver nitrate and sodium fluoride will yield a solution containing Ag+, \(\ce{NO3-}\), Na+, and F− ions. Aside from the two ionic compounds originally present in the solutions, AgNO3 and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO3 and AgF. The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equations:

    \[\ce{NaF}(aq)+\ce{AgNO3}(aq)\rightarrow \ce{AgF}(s)+\ce{NaNO3}(aq)\hspace{20px}\ce{(molecular)}\]

    Determining the Products for Precipitation Reactions: https://youtu.be/r0kYeZVuTAM

    Example \(\PageIndex{1}\): Predicting Precipitation Reactions

    Predict the result of mixing reasonably concentrated solutions of the following ionic compounds. If precipitation is expected, write a balanced equation for the reaction.

    1. potassium sulfate and barium nitrate
    2. lithium chloride and silver acetate
    3. lead nitrate and ammonium carbonate

    Solution

    (a) The two possible products for this combination are KNO3 and BaSO4. The solubility guidelines indicate BaSO4 is insoluble, and so a precipitation reaction is expected. The balance chemical reaction is,

    \[\ce{K2SO4}(aq)+\ce{Ba(NO3)2}(aq)\rightarrow \ce{BaSO4}(s)+\ce{2KNO3}(aq)\]

    (b) The two possible products for this combination are LiC2H3O2 and AgCl. The solubility guidelines indicate AgCl is insoluble, and so a precipitation reaction is expected. The balance chemical reaction is,

    \[\ce{LiCl}(aq)+\ce{AgC2H3O2}(aq)\rightarrow \ce{AgCl}(s)+\ce{LiC2H3O2}(aq)\]

    (c) The two possible products for this combination are PbCO3 and NH4NO3. The solubility guidelines indicate PbCO3 is insoluble, and so a precipitation reaction is expected. The balance chemical reaction is,

    \[\ce{Pd(NO3)2}(aq)+\ce{(NH4)2CO3}(aq)\rightarrow \ce{PbCO3}(s)+\ce{2NH4NO3}(aq)\]
    Exercise \(\PageIndex{1}\)

    Which solution could be used to precipitate the barium ion, Ba2+, in a water sample: sodium chloride, sodium hydroxide, or sodium sulfate? What is the formula for the expected precipitate?

    Answer

    sodium sulfate, BaSO4

    Summary

    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.

    Glossary

    precipitate
    insoluble product that forms from reaction of soluble reactants
    precipitation reaction
    reaction that produces one or more insoluble products; when reactants are ionic compounds, sometimes called double-displacement or metathesis
    soluble
    of relatively high solubility; dissolving to a relatively large extent
    solubility
    the extent to which a substance may be dissolved in water, or any solvent

    Contributors and Attributions

    5.2: Precipitation Reactions is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

    1. Back to top
    • Was this article helpful?

    Recommended articles

    1. Article type
      Section or Page
    2. Tags
      This page has no tags.