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Rules for Learning the Solubility of Ionic Compounds in Water

Negative Ions
(Anions)
+ Positive Ions
(Cations)
= Solubility of compounds in water Example
any anion + alkali ions
(Li+,Na+,K+,Rb+,Cs+,Fr+)
= soluble sodium fluoride, NaF, is soluble
any anion + hydrogen ion
[H+(aq)]
= soluble hydrogen chloride, HCl, is soluble
any anion + amonium ion
(NH4+)
= soluble ammonium chloride, NH4Cl, is soluble
nitrate
NO3-
+ any cation = soluble potassium nitrate, KNO3, is soluble
acetate
(CH3COO-)
+ any cation = soluble sodium acetate, CH3COONa, is soluble
Chloride (Cl-), Bromide (Br-), Iodide (I-) + silver (Ag+), lead (Pb2+), mercury (Hg2+), copper (Cu+), thallium (Tl+) = low solubility (insoluble) silver chloride, AgCl, forms a white precipitate (a white solid)
+ any other cation = soluble potassium bromide, KBr, is soluble
Sulphate
(SO42-)
+ calcium (Ca2+), strontium (Sr2+), barium (Ba2+), silver (Ag+), lead (Pb2+), radium (Ra2+) = low solubility (insoluble) barium sulphate, BaSO4, forms a white precipitate (a white solid)
+ any other cation = soluble copper sulphate, CuSO4, is soluble
Sulfide
S2-
+ alkali ions (Li+,Na+,K+,Rb+,Cs+,Fr+), alkali earth metals (Be2+,Mg2+,Ca2+,Sr2+,Ba2+,Ra2+),
and H+(aq), NH4+
= soluble magnesium sulfide, MgS, is soluble
+ any other cation = low solubility (insoluble) zinc sulfide, ZnS, is insoluble
Hydroxide
OH-
+ alkali ions (Li+,Na+,K+,Rb+,Cs+,Fr+), H+(aq),NH4+,Sr2+,Ba2+,Ra2+,Tl+ = soluble strontium hydroxide,
Sr(OH)2, is soluble
+ any other cation = low solubility (insoluble) silver hydroxide, AgOH, is insoluble (forms a precipitate)
Phosphate, PO43-, Carbonate, CO32-, sulphite, SO32- + alkali ions (Li+,Na+,K+,Rb+,Cs+,Fr+), H+(aq),NH4+ = soluble ammonium phosphate,
(NH4)3PO4, is soluble
+ any other cation = low solubility (insoluble) magnesium carbonate, MgCO3, is insoluble

  • All compounds of the ammonium ion (NH4+), and of Alkali metal (Group IA) cations, are soluble.

  • All nitrates and acetates (ethanoates) are soluble.

  • All chlorides, bromides and iodides are soluble EXCEPT those of silver, lead and mercury(I).

  • All sulphates are soluble EXCEPT those of silver, lead, mercury(I), barium, strontium and calcium.

  • All carbonates, sulfites and phosphates are insoluble EXCEPT those of ammonium and Alkali metal (Group IA) cations.

  • All hydroxides are insoluble EXCEPT those of ammonium, barium and alkali metal (Group I) cations.

  • All sulfides are insoluble EXCEPT those of ammonium, Alkali metal (Group I) cations and Alkali earth metal (Group II) cations.

  • All oxides are insoluble EXCEPT those of calcium, barium and Alkali metal (Group I) cations; these soluble ones actually react with the water to form hydroxides (hydrolyse).


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