Predicting the solubility of salts

Predicting the solubility of salts

Lessons

In this lesson, we will learn:

  • To examine the key terms in solution chemistry and define low solubility.
  • How to predict the solubility of a given ionic compound.
  • How to use a solubility table to suggest soluble and insoluble compounds.

Notes:

  • In chemistry the words soluble and solubility are normally used quite loosely:
    • Two substances might both be “soluble in water”, but one may be many times more soluble.
    • We might say a substance is insoluble in another substance, but technically, all substances are soluble in other substances – extremely slightly!
    • For some substances, being extremely slightly soluble is still important. They might be toxic compounds where very small quantities are still harmful.
    To clear this up, we have a definition of low solubility. Low solubility describes any substance that makes a saturated solution with a concentration of less than 0.1 M.
    When you study reactions between ionic compounds, a product with solubility less than 0.1 M has low solubility – it is probably a solid precipitate in the reaction mixture.

  • Using a “solubility of common ions” data sheet reveals some general patterns of solubility of ionic compounds. These patterns can be used to predict whether a compound will be soluble in water or have low solubility:
    • Compounds containing alkali metal ions (Li, Na, K, Rb, Cs) are soluble in water.
    • Compounds containing hydrogen and ammonium ions (H+, NH4+) are soluble in water.
    • Most other cations (positive ions) have low solubility.
    • Compounds containing the nitrate ion, NO3- are soluble in water.
    • Compounds containing halide ions except for fluoride, F-, are generally soluble, but there are some exceptions (such as AgCl).

  • If two ions combine to make a compound of low solubility then it will form a precipitate product. With a solubility table and the points shown above, an important conclusion with some consequences can be drawn:
    • Compounds containing alkali metal ions, H+, NH4+ and NO3- ions do not form precipitates.
      • Therefore, if you have to suggest a soluble compound with a particular anion (negatively charged ion), make the cation (the positively charged ion) an alkali metal such as Li or Na. Do not suggest H+ as the cation, this would make the compound an acid, not a salt!
      • If you have to suggest a soluble compound with a particular cation, make the anion NO3-, the nitrate ion which is soluble in water.
  • Introduction
    What does soluble mean?
    a)
    Examining key terms in solution chemistry.

    b)
    General rules for ion solubility.

    c)
    Predicting solubility in ionic compounds.


  • 1.
    Predict the solubility of ionic compounds using rules of solubility.
    Which of the following salts in the list are soluble?

    NaCl, AgCl, NH4OH, FeCO3, Na2CO3, K2S

  • 2.
    Predict the solubility of ionic compounds as products of a reaction.
    Equal amounts of 0.2 M NaBr and 0.2M Pb(NO3)2 are mixed in a container and a reaction takes place.

    1. What are the products of this reaction?
    2. One of the products of this reaction forms as a precipitate. Which compound is this? Does this precipitate have low solubility?

  • 3.
    Apply your knowledge of solubility to explain practical problems.
    Eutrophication is an environmental problem caused by plant fertilizers being 'run-off' by rain into lakes and rivers. These fertilizers cause algae to grow unchecked, which depletes rivers and lakes of their oxygen.

    Ammonium nitrate and diammonium phosphate are two widely used fertilizers that cause this problem. Why would these two compounds be particularly prone to run-off?