Topic 09 · Salts

Salts

1

Salts and Solubility

1.1What a salt is

A salt is an ionic compound made of a cation (positive ion) and an anion (negative ion).
  • A salt forms when the hydrogen ions of an acid are replaced by metal ions or ammonium ions, e.g. in the reactions of acids with metals, bases and carbonates.
  • Sulfuric acid gives sulfates, nitric acid gives nitrates and hydrochloric acid gives chlorides.

1.2Solubility rules

SaltsSoluble in waterInsoluble in water
sodium, potassium and ammonium saltsallnone
nitratesallnone
chloridesmostsilver chloride AgCl and lead(II) chloride PbCl2
sulfatesmostbarium sulfate BaSO4, lead(II) sulfate PbSO4 and calcium sulfate CaSO4 (sparingly soluble)
carbonatessodium, potassium and ammonium carbonates onlyall others, e.g. CaCO3, CuCO3, ZnCO3
hydroxidessodium, potassium and ammonium hydroxides (calcium hydroxide is slightly soluble)all others, e.g. Cu(OH)2, Fe(OH)3
Exam tipCheck the solubility of the salt and of each starting material before you choose a method to prepare a salt.
46Topic 09 · Salts
2

Preparing Salts

2.1Choosing a method

Is the salt soluble in water?Is the metal, base or carbonate soluble?Method 3: precipitationmix a solution of the cationwith a solution of the anionMethod 1: excess solidacid + excess metal,insoluble base orinsoluble carbonateMethod 2: titrationacid + alkali orsoluble carbonateyesnonoyes

2.2Method 1: an acid and an excess insoluble substance

acid + metal → salt + hydrogenacid + insoluble base → salt + wateracid + insoluble carbonate → salt + water + carbon dioxide
1234add excess zincto the acidfilter off theexcess zincheat the filtrateuntil saturatedcool to crystallise, thenfilter, wash and dry
  • Add the solid in excess, so that all the acid is used up and the salt is not contaminated with acid. The excess solid is insoluble, so it is removed by filtration.
  • Heat the filtrate until saturated: a glass rod dipped in and removed shows small crystals forming as it cools.
  • Do not use very reactive metals (they react violently) or unreactive metals such as copper (they do not react with dilute acids). Use the metal oxide or carbonate instead.
H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)2HNO3(aq) + CuO(s) → Cu(NO3)2(aq) + H2O(l)2HCl(aq) + MgCO3(s) → MgCl2(aq) + CO2(g) + H2O(l)
Topic 09 · Salts47
2

Preparing Salts (continued)

2.3Method 2: titration

dilutenitric acidburetteaqueous NaOH+ methyl orange
acid + alkali → salt + wateracid + soluble carbonate → salt + water + carbon dioxide
  • Used for sodium, potassium and ammonium salts, where all the starting materials are soluble.
  • An excess of either reactant cannot be filtered off, so the volumes must be exact.
  1. Pipette 25.0 cm3 of the alkali into a conical flask and add a few drops of indicator, e.g. methyl orange.
  2. Add the acid from a burette until the indicator just changes colour (the end-point). Record the volume of acid used.
  3. Repeat with the same volumes but without the indicator, which would make the salt impure.
  4. Heat the solution until saturated, cool to crystallise, then filter, wash with a little cold distilled water and dry the crystals.
HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)

2.4Method 3: precipitation

An insoluble salt is made by mixing two solutions: one contains its cation and the other contains its anion. The salt forms as a precipitate.
mix: a white precipitate formsfilter: the precipitate is the residue
  1. Mix the two solutions and stir until no more precipitate forms.
  2. Filter to collect the precipitate as the residue.
  3. Wash it with distilled water to remove soluble impurities, then dry it between sheets of filter paper.
Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq)ionic equation: Ba2+(aq) + SO42−(aq) → BaSO4(s)
48Topic 09 · Salts
2

Preparing Salts (continued)

2.5Planning a preparation

SaltSoluble?MethodStarting materials
zinc sulfateyesexcess insoluble substancedilute sulfuric acid + excess zinc, zinc oxide or zinc carbonate
copper(II) nitrateyesexcess insoluble substancedilute nitric acid + excess copper(II) oxide or carbonate (not copper)
sodium nitrateyestitrationdilute nitric acid + aqueous sodium hydroxide or sodium carbonate (not sodium)
barium sulfatenoprecipitationaqueous barium nitrate + dilute sulfuric acid or any soluble sulfate

Worked exampleDescribe how you would prepare lead(II) chloride from lead(II) carbonate, dilute nitric acid and aqueous potassium chloride.

Lead(II) chloride is insoluble, so it is made by precipitation. Lead(II) carbonate is also insoluble, so first turn it into a soluble lead(II) salt. Add excess lead(II) carbonate to dilute nitric acid: PbCO3(s) + 2HNO3(aq) → Pb(NO3)2(aq) + CO2(g) + H2O(l). Filter off the unreacted lead(II) carbonate. The filtrate is aqueous lead(II) nitrate. Add excess aqueous potassium chloride to the filtrate: Pb(NO3)2(aq) + 2KCl(aq) → PbCl2(s) + 2KNO3(aq). Filter to collect the lead(II) chloride, wash it with distilled water and dry it between sheets of filter paper.
Exam tipIn a description, name the reagents, say which one is in excess and why, and give every step: filter, heat to saturation, cool, filter, wash and dry.

2.6Common mistakes

MistakeWhy it is wrong
Using sodium or potassium metal with an acidVery reactive metals react violently with acids.
Using copper or silver with a dilute acidUnreactive metals do not react with dilute acids.
Not adding the solid in excess (method 1)Some acid is left, so the salt is contaminated with acid.
Leaving the indicator in (method 2)The indicator makes the salt impure.
Using method 1 for a sodium saltSodium compounds are soluble, so an excess cannot be filtered off.
Washing crystals with a lot of waterUse a little cold distilled water, or the crystals dissolve.
Topic 09 · Salts49