Predicting solubility - Solubility Equilibria

Predicting solubility

Lessons

Notes:

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.
  • Intro Lesson
    What does soluble mean?
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Predicting solubility

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