The solubility rules, and the exceptions that matter
Definition
Solubility
A measure of how much of a substance will dissolve in a given amount of solvent.
Definition
Salt
The compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion.
- All common sodium, potassium and ammonium salts are soluble.
- All nitrates are soluble.
- Common chlorides are soluble, except silver chloride and lead chloride.
- Common sulfates are soluble, except lead sulfate, barium sulfate and calcium sulfate.
- Common carbonates are insoluble, except those of sodium, potassium and ammonium.
- Common hydroxides are insoluble, except those of sodium, potassium and calcium, and calcium hydroxide is only slightly soluble.
- The exceptions are the part worth learning, because the general rules follow a pattern.
Common Mistake
- Silver and lead break the chloride rule, and lead, barium and calcium break the sulfate rule.
- Calcium hydroxide is the awkward one, because it is only slightly soluble rather than fully soluble.
Predicting whether a precipitate forms
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed.
- Write down the ions present in each of the two solutions.
- Pair each positive ion with the negative ion that came from the other solution.
- Check both of those possible products against the solubility rules.
- If one of them is insoluble it forms a precipitate, and it should be named.
- If both are soluble, no precipitate forms and no reaction takes place.
- Silver nitrate and sodium chloride: AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)
- Barium chloride and sodium sulfate: BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq)\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq)
- Potassium nitrate and sodium chloride give no reaction, because every possible product is soluble and all the ions simply stay in solution.
Example
- A precipitate: AgCl\text{AgCl}AgCl is the exception to the chloride rule, so it drops out as a white solid.
- No precipitate: mixing potassium nitrate with sodium chloride leaves every ion dissolved.
Ionic equations show only the ions that react
Definition
Ionic equation
An equation that shows only the ions and substances that change during a reaction, with the spectator ions left out.
Definition
Spectator ion
An ion that is unchanged by the reaction and appears on both sides of the full ionic equation, so it is cancelled out.
- Only the ions that form the precipitate actually change.
- The rest stay in solution throughout and are spectator ions.
- For silver chloride the ionic equation is: Ag+(aq)+Cl−(aq)→AgCl(s)\text{Ag}^{+}(aq) + \text{Cl}^{-}(aq) \rightarrow \text{AgCl}(s)Ag+(aq)+Cl−(aq)→AgCl(s)
- For barium sulfate it is: Ba2+(aq)+SO42−(aq)→BaSO4(s)\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)Ba2+(aq)+SO42−(aq)→BaSO4(s)
- The state symbols carry the meaning here, because (aq)(aq)(aq) and (s)(s)(s) are what show the precipitation.
Note
- Cancel the spectator ions, which are the ones appearing unchanged on both sides.
- Balance charge as well as atoms, because an ionic equation has to balance both.
Preparing a pure, dry insoluble salt
- Choose two soluble solutions that between them contain the ions you want.
- Mix them and stir, so that the insoluble salt precipitates out.
- Filter the mixture, keeping the residue on the filter paper.
- Wash that residue with distilled water, which rinses away the soluble substances clinging to it.
- Dry the washed solid between filter papers or in a warm oven.
- The product is pure because everything else was soluble and has been washed away.
Exam technique
- Naming both starting solutions is part of the answer, because choosing them is what the question asks.
- Washing is the step most often missed, and without it soluble salt is left on the crystals.
- Check the rules in both directions, since one soluble product is not enough to settle the question.
Self review
- Which salts of sodium, potassium and ammonium are soluble?
- Which chlorides and which sulfates are the insoluble exceptions?
- What precipitate forms when silver nitrate is mixed with sodium chloride?
- Why is the residue washed with distilled water?
- What is the ionic equation for the formation of barium sulfate?
