Precipitation Reactions and Solubility Rules

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions

HSC Chemistry Syllabus

  • Conduct investigations to predict and identify the products of a range of reactions, for example:
    • Precipitation

    Precipitation Reactions and Solubility Rules

    This video discusses precipitation reactions and the solubility rules.

    What is Precipitation?

    Precipitation is a process where a solid forms when solutions mix. The solution turns cloudy, and is sometimes coloured, like for example: the yellow formation of lead iodide


    Precipitation reactions follow the general formula:

    $$\text{Solution A} + \text{Solution B} \rightarrow \text{insoluble salt in Solution C}$$

    For instance:

    Solution 1: Contains `A^+` and `B^-` ions

    Solution 2: Contains `C^+` and `D^-` ions

    Mix them together and observe that `A^+` and `D^-` combine to form the fully solid ionic substance AD. However `C^+` and `B^-` remain dissolved. 

    The reason for this can be explained by exploring solubility rules.  

    Solubility Rules:

    The solubility rules help determine which ions react to form an insoluble salt. The rules predict the outcome of precipitation reactions.

    Soluble Compounds:

    • All Group 1 metal cations (e.g. lithium, sodium, potassium).
    • Acetates and nitrates: `C_2H_3O_2` and `NO_3^-` respectively
    • Halides are mostly soluble. Exceptions: Silver, lead halides, and group 2 metal fluorides. 
    • Sulfates are soluble. Exceptions: Silver, barium, calcium, lead. 


    Mostly Insoluble Compounds: 

    • Carbonate, phosphate, and sulfide. Exceptions: Group 1 cations and ammonium.
    • Hydroxides. Exception: Group 1 hydroxides. 


    Practice Questions (Answered in Video): 

    1. Write a balanced chemical equation demonstrating what happens when a solution of Silver nitrate is added to a solution of Potassium chloride

    2. Write a chemical equation demonstrating what happens when a solution of Barium hydroxide is added to a solution of Magnesium sulfate.

    3. What solution can be added to Potassium sulphate to produce a precipitate?


    Precipitation Reaction Expressions:

    Precipitation reactions can be expressed as either Neutral Species, Complete Ionic, or Net Ionic equations. We will use the precipitation reaction between silver nitrate and sodium chloride to illustrate them.

    • Neutral Species Equation:

      A neutral species equation gives the complete neutral chemical formulae of both reactant and products without breaking them down into individual ions. 

      $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$$


    • Complete Ionic Equation:

      A complete ionic equation shows all of the ions present in a solution. When writing a complete ionic equation - ionic compounds that are soluble in water are broken down into their individual cations and anions

      For the same reaction:

      $$Na^+(aq) + Cl^-(aq) + Ag^+(aq) + NO_3^-(aq) \rightarrow AgCl(s) + Na^+(aq) + NO_3^-(aq)$$

    • Net Ionic Equations:

      The net ionic equation represents only the chemical species that are involved in the reaction. This means that "spectator ions" are eliminated and only ions or molecules which undergo chemical change are represented. 

      Continuing with our example

      From complete ionic equation: 

      $$Na^+(aq) + Cl^-(aq) + Ag^+(aq) + NO_3^-(aq) \rightarrow AgCl(s) + Na^+(aq) + NO_3^-(aq)$$

      To Net Ionic Equation

      $$ Cl^-(aq) + Ag^+(aq)  \rightarrow AgCl(s)$$