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Precipitation reactions

Mix the four reagents and look at the precipitate, spectator ions, and precipitate mass.

    C13 · Precipitation reactionsReady
    Mix the four reagents and look at the precipitate, spectator ions, and precipitate mass. You can also experiment with the numbers and controls below.

    Chemistry · Reaction formula · Quantitative relationships / Principles and exploration

    Precipitation reactions — principles and investigation

    The reason a solid is formed when two clear solutions are mixed is because certain ions create substances that do not dissolve well in water. Change the type, concentration, and volume of the reagent and compare the presence and amount of precipitate. First, distinguish between the number of moles of reagent and the number of moles of ions supplied by the reagent.

    How does it work?

    Only ions that meet in a given reaction table form solids. Multiply the number of moles of reagent by the number of ions in the chemical formula and find the limiting ion. There are 2 moles of Cl⁻ in 1 mol of BaCl₂.

    Key relationship

    n precipitation = min(n positive ion, n negative ion) m = n precipitation M

    Try this investigation

    1. Determine the reagent, concentration, and volume and mix. Compare the production of AgCl or BaSO₄ with the remaining ions. Even if you select the same reagent twice, no precipitate will form.
    2. Run and record AgNO₃+NaCl and BaCl₂+Na₂SO₄ respectively. Set the first reagent to 0.01 mol and the second reagent to 0.02 mol.
    3. Record the case where only the AgNO₃ concentration is changed to 0.20 mol/L or when only the first reagent is changed to Na₂SO₄ in the standard conditions.

    Interpreting your results

    Why are Na⁺ and NO₃⁻ called spectator ions?

    AgCl(s) is made by mixing only Ag⁺ and Cl⁻ 1:1. Na⁺ and NO₃⁻ are omitted from the net ionic equation because they remain in aqueous solution.

    Assumptions and limitations

    We only use fixed reaction tables of four reagents. Ag⁺+Cl⁻ and Ba²⁺+SO₄²⁻ precipitate completely, and other combinations are treated as no precipitation. In particular, the actual dissolution equilibrium of Ag⁺+SO₄²⁻ is also excluded from this introductory model. Ksp, activity, particle growth, and temperature effects are not calculated. The molar mass is rounded to 143.32 AgCl, 233.39 g/mol BaSO₄. The dots and solid pieces are for illustrative purposes only.