2.10 Solubility of Inorganics

Let us now examine the aqueous solubility of inorganic compounds (mass % data from⁴). Compare the data to the solubility rules

2.10.1 Solubility Rules

Ionic compounds may or may not be soluble in water. Solubility rules tell us which compounds are soluble in water and to what extent they are soluble.

Soluble Ions	Exception
Alkali metals (Group I) Li⁺, Na⁺, K⁺, etc.	none
Ammonium Ions NH₄⁺	none
Nitrates, acetates, chlorates, and perchlorate NO₃^–, C₂H₃O₂^–, ClO₃^–, ClO₄^–	none
Binary compounds of halogens (X) with metals (M) MCl, MBr, MI, etc.	F^–, Ag⁺, Pb²⁺, Hg²⁺
Sulfates SO₄^2–	Ba²⁺, Sr²⁺, Ca²⁺, Pb²⁺, Ag⁺, and Hg²⁺
Slightly Soluble Ions	Exception
Sulfates of lead, silver, and mercury SO₄^2– with Pb²⁺, Ag⁺, and Hg²⁺	none
Hydroxides of alkaline earth metals (Group II) OH^– with Ca²⁺, Sr²⁺, etc.	Ba²⁺
Insoluble Ions	Exception
Sulfides S^2–	Ca²⁺, Ba²⁺, Sr²⁺, Mg²⁺, Na⁺, K⁺, and NH₄⁺
Hydroxides OH^–	Alkali metals (Group I), transition metals, Al³⁺, and NH₄⁺
Carbonates, oxalates, chromates, and phosphates CO₃^2–, C₂O₄^2–, CrO₄^2–, and PO₄^3–	Alkali metals (Group I) and NH₄⁺

2.10.2 Alkali compounds

Alkali metals (Group I) are soluble without exception. Notice that hydroxides (OH^–) are soluble when paired with alkali metals.

Figure 2.15: Aqueous solubility of some alkali compounds at various temperatures.

2.10.3 Ammonium

Ammonium (NH₄⁺) ions are soluble without exception.

Figure 2.16: Aqueous solubility of some ammonium compounds at various temperatures.

2.10.4 Binary Metal/Halogen Compounds

Binary compounds of halogens (X) with metals (M) are soluble except F^–, Ag⁺, Pb²⁺, and Hg²⁺.

Figure 2.17: Aqueous solubility of some binary metal/halogen compounds at various temperatures.

2.10.5 Binary Alkaline Compounds

Sulfates (SO₄^2–) are soluble except when paired with Ba²⁺, Sr²⁺, Ca²⁺, Pb²⁺, Ag⁺, and Hg²⁺.

Figure 2.18: Aqueous solubility of some binary alkaline compounds at various temperatures.

Sodium Sulfate

The solubility of sodium sulfate increases with increasing temperature until 32.384 °C (with a mass % of around 30) after which the solubility begins to decrease. This is due to the salt changing phase.

At low temperature (< 32.384 °C), sodium sulfate is a salt hydrate meaning it is an ionic compound with a number of water molecules that are enclosed within its crystal lattice. The chemical formula for this is Na₂SO₄ · 10H₂O and is called mirabilite. Interestingly, the cations are [Na(OH₂)₆]⁺ in this structure.

Mirabilite ([Wikipedia](https://en.wikipedia.org/wiki/Mirabilite)).

Figure 2.19: Mirabilite (Wikipedia).

At higher temperatures, the waters are released (“melt away”) from the crystal lattice to give anhydrous thenardite (Na₂SO₄). This process is given below.
\[ \begin{align*} \mathrm{Na_2SO_4\cdot10H_2O}(aq) &\longrightarrow \mathrm{Na_2SO_4}(aq) + 10\mathrm{H_2O}(l) \\[1.25ex] \mathrm{low~}T ~~~~~~~~~~~~~~~ &\phantom{\longrightarrow} ~~~~~~~~~~~~~~~~~~~ \mathrm{high~}T \end{align*} \]

Thernadite ([Wikipedia](https://en.wikipedia.org/wiki/Thenardite)).

Figure 2.20: Thernadite (Wikipedia).

2.10.6 Some Sulfates

Sulfates of lead, silver, and mercury are slightly soluble.

Figure 2.21: Aqueous solubility of some sulfate compounds at various temperatures.

2.10.7 Some Hydroxides

Hydroxides of alkaline earth metals (Group II) are slightly soluble with the exception of Ba²⁺ which is highly soluble.

Figure 2.22: Aqueous solubility of some hydroxide compounds at various temperatures.

References

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CRC Handbook of Chemistry and Physics, 97th ed.; William M. Haynes, T. J. B., David R. Lide, Ed.; CRC Press, 2016.