Pattern Recognition

What Dissolves and What Does Not: Some General Rules. Charts similar to this appear in most chemistry texts.

What Dissolves and What Does Not: Some General Rules

After making a large number of observations on the solubility of compounds, chemists have devised some helpful “rules” to determine whether an ionic compound is soluble or insoluble in water. These generalizations are listed in the following chart. The chart is easy to read. For example, it tells us that compounds containing ammonium ion (NH4 +) are soluble (no exceptions). This means that NH4Cl, (NH4)2S, (NH4)3PO4, and so on are all soluble. It also tells us that compounds containing hydroxide (OH-) are insoluble, except the alkali metals, calcium, barium, and strontium. For example, magnesium hydroxide, Mg(OH)2, is insoluble, but calcium hydroxide, Ca(OH)2, is soluble. With a little practice, students can make predictions about solubility of ionic compounds.

Compounds Containing Are Except

Alkali metal ions (Li+, Na+, K+, Rb+, Cs+)

Soluble

  

Ammonium (NH4+)

Soluble

  

Nitrate (NO3-)

Soluble

  

Chlorate (ClO3-)

Soluble

  

Perchlorate (ClO4-)

Soluble

  

Acetate (C2H3O2-)

Soluble

  

Chloride (Cl -), Bromide (Br -), Iodine(I -)

Soluble

Lead (Pb2+), Silver (Ag+), and Mercury (I) (Hg22+),
NH4+

Sulfate (SO42-)

Soluble

Strontium (Sr2+), Barium (Ba2+), Mercury (I)
(Hg22+), Mercury (II) (Hg2+), Lead (Pb2+), NH4+

Hydroxide (OH-)

Insoluble

Alkali Metals, Ca2+, Ba2+, and Sr2+

Sulfite (SO32-), Chromate (CrO42-)

Insoluble

Alkali metals and NH4+

Carbonate (CO32-), and Phosphate (PO43-)

Insoluble

Alkali metals and NH4+

Sulfide (S2-)

Insoluble

Alkali metals, Beryllium (Be2+),
Magnesium (Mg2+), Calcium (Ca2+),
Strontium (Sr2+), Barium (Ba2+), and NH4+

Common Student Misconceptions

  1. “Like always dissolves like.”
    This is only a generalization and there are exceptions. For example, acetic acid is totally miscible in water due to the formation of hydrogen bonds, but it is also soluble in nonpolar solvents such as benzene and carbon tetrachloride.
  2. “The solubility of a solid in a liquid increases with an increase in temperature.”
    This is not always true. For example, sodium sulfite, calcium acetate, and lithium sulfate have solubilities that decrease with an increase in temperature.
  3. “Insoluble compounds do not dissolve.”
    This is not really true. All ionic compounds, even those that we classify as “insoluble” dissolve to some slight extent in water.
  4. “Hydration means the attachment of any solvent molecules to solute molecules.”
    Hydration is used only when the solvent is water. Solvation is the term used to describe any solvent molecule that surrounds and becomes attracted to solute molecules.

(page 14, & 15)

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