6c. MAKING an INSOLUBLE SALT BY A PRECIPITATION REACTION

Doc Brown's Chemistry GCSE/IGCSE/O level Science–Chemistry Revision Notes

The pH scale of acidity and alkalinity, acids, alkalis, salts and neutralisation

6. Methods of making salts

6c. How to make an insoluble salt by a precipitation reaction


Method (a) Making a salt by neutralising a soluble acid with a soluble base (alkali) – neutralisation reaction

Method (b) preparing a salt by reacting an acid with a metal or an insoluble base – oxide, hydroxide or carbonate

Method (c) Preparing an insoluble salt by mixing solutions of two soluble compounds (this page)

Method (d) Making a salt by directly combining its constituent elements

GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids

See also Advanced Level Chemistry Students Acid–Base Revision Notes – use index

6. METHODS of MAKING SALTS – salt preparation procedures

6c. Method (c) Preparing an Insoluble Salt

Procedure for making an insoluble salt my mixing solutions of soluble compounds to form a precipitate. The two soluble compounds must each provide one of the constituent ions of the desired insoluble salt which precipitates out when the solutions are mixed.

NOTE definition: A precipitation reaction is generally defined as 'the formation of an insoluble solid on mixing two solutions of soluble substances or bubbling a gas into a solution'.

Note that several precipitation reactions are used as simple tests for e.g. for sulfate, chloride, bromide, iodide

Salt solubility affects the method you choose to make a salt, the table below will help you decide on the method

A solubility guide for salts and other compounds

Information required to decide on the method used to prepare a salt

salts and other compounds

solubility?

common salts of sodium, potassium and ammonium ions usually soluble in water
common sulfates (sulphates) usually quite soluble except for calcium sulfate (slightly soluble), lead sulfate and barium sulfate are both insoluble
common chlorides (similar rule for bromides and iodides) usually soluble except for insoluble lead(II) chloride and silver chloride
common nitrates all soluble
common carbonates most metal carbonates are insoluble apart from sodium & potassium carbonate.  Ammonium carbonate is also soluble.
common hydroxides most metal hydroxides are insoluble apart from sodium, potassium and ammonium hydroxide

  • How can we make an insoluble salt? How do we prepare an insoluble salt from two soluble compounds?

  • This section describes the preparation of insoluble salts like silver chloride AgCl, lead(II) chloride (lead chloride) PbCl2, lead(II) iodide (lead iodide) PbI2, calcium carbonate CaCO3, barium sulfate (barium sulphate) BaSO4, lead(II) sulfate (lead sulphate, lead sulfate)PbSO4, and 'slightly soluble' calcium sulfate (calcium sulphate) CaSO4, which can all be made by a precipitation reaction.

    • All of the above insoluble salts are white (as in diagram), except lead(II) iodide which is yellow.

  • Many of the salt precipitate are WHITE, but lead iodide is pale yellow.

  • METHOD (c)

  • An insoluble salt can be made by mixing two solutions of soluble salts in a process is called precipitation.

    • The method is quite simple – illustrated above, assuming in this case the insoluble salt is colourless–white.

    • One solution contains the 1st required ion, and the other solution contains the 2nd required ion.

    • STEP 1. So you must prepare two solutions of soluble compounds, each of which provides one of the two ions required to combine and precipitate out as the insoluble salt.

      • Each soluble compound is weighed out into its own beaker and dissolved in a suitable volume of water until the solutions are both quite clear.

      • One solution is then  poured into the other, order doesn't really matter.

    • The two solutions of SOLUBLE compounds must be thoroughly mixed together to ensure all the reactants are used up, so the maximum amount of INSOLUBLE salt precipitate is formed.

      • You see the two original clear solutions on mixing forming a cloudy mixture as the insoluble compound is formed, known as the precipitate.

    • STEP 2.The mixture is then carefully poured into a funnel holding a filter paper.

    • The precipitated salt can then be filtered off with the filter funnel and paper.

    • STEP 3.While still in the filter paper and funnel, the collected solid precipitate is washed with distilled/deionised water to remove any remaining soluble salt impurities and just the damp, but otherwise pure, insoluble salt is left.

    • STEP 4.The precipitate is then carefully removed from the filter paper into a clean dish or basin to be dried e.g. left out in a dry room or warmed in a pre–heated oven.

    • The above four step procedure applies to all the precipitation reactions below, by which you prepare a variety of insoluble salts.

  • Examples ...

    • (i) Silver chloride is made by mixing solutions of solutions of silver nitrate and sodium chloride.

      • The silver nitrate provides the silver ion and the sodium chloride provides the chloride ion to prepare the insoluble salt silver chloride which forms as a white precipitate.

      • silver nitrate + sodium chloride ==> silver chloride + sodium nitrate

      • AgNO3(aq) + NaCl(aq) ==> AgCl(s) + NaNO3(aq)

      • in terms of ions it could be written as

      • Ag+NO3(aq) + Na+Cl(aq) ==> AgCl(s) + Na+NO3(aq)

      • or: Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq) ==> AgCl(s) + Na+(aq) + NO3(aq)

      • but the spectator ions are nitrate NO3 and sodium Na+ which do not change at all,

        • The 'active ions' and resulting precipitate are highlighted in yellow.

      • so the proper ionic equation is simply: Ag+(aq) + Cl(aq) ==> AgCl(s)

        • Note (i) the use of state symbols (aq) and (s) AND

        • (ii) that ionic equations omit ions that do not change there chemical or physical state.

          • You must NOT include the spectator ions in an ionic equation!

        • In this case the nitrate, NO3(aq) and sodium Na+(aq) ions do not change physically or chemically and are called spectator ions,

        • BUT the aqueous silver ion, Ag+(aq), combines with the aqueous chloride ion, Cl(aq), to form the insoluble salt silver chloride, AgCl(s), thereby changing their states both chemically and physically.

        • More Ionic equations explained with all spectator ions indicated

      • You can just use dilute hydrochloric acid and silver nitrate solution to make insoluble silver chloride by precipitation.

      • The silver nitrate provides the silver ion and the hydrochloric acid provides the chloride ion to prepare the insoluble salt silver chloride which forms as a white precipitate.

      • silver nitrate + hydrochloric acid ==> silver chloride + nitric acid

      • AgNO3(aq) + HCl(aq) ==> AgCl(s) + HNO3(aq)

      • in terms of ions it could be written as

      • Ag+NO3(aq) + H+Cl(aq) ==> AgCl(s) + H+NO3(aq)

      • or: Ag+(aq) + NO3(aq) + H+(aq) + Cl(aq) ==> AgCl(s) + H+(aq) + NO3(aq)

      • but the spectator ions are nitrate NO3 and hydrogen H+ which do not change at all,

        • The 'active ions' and resulting precipitate are highlighted in yellow.

      • so the proper ionic equation is simply: Ag+(aq) + Cl(aq) ==> AgCl(s)

      • If you use barium chloride the word and symbol equations are ...

      • barium chloride + silver nitrate ==> silver chloride + barium nitrate

      • BaCl2(aq) + 2AgNO3(aq) ==> 2AgCl(s) + Ba(NO3)2(aq)

      • which can be written as

      • Ba2+(aq) + 2Cl(aq) + 2Ag+(aq) + 2NO3(aq) ==> 2AgCl(s) + Ba2+(aq) + 2NO3(aq)

      • the spectator ions are Ba2+ and NO3

      • so the ionic equation is: Ag+(aq) + Cl(aq) ==> AgCl(s)

        • You can prepare the insoluble salts silver bromide and silver iodide in a similar way by precipitation.

        • You can make silver bromide by mixing solutions of silver nitrate and potassium bromide.

        • The silver nitrate provides the silver ion and the potassium bromide provides the bromide ion to prepare the insoluble salt silver bromide which forms as a cream precipitate.

        • silver nitrate + potassium bromide ==> silver bromide + potassium nitrate

        • AgNO3(aq) + KBr(aq) ==> AgBr(s) + KNO3(aq)

        • the spectator ions are nitrate NO3 and potassium K+ which do not change at all,

        • so the proper ionic equation is simply: Ag+(aq) + Br(aq) ==> AgBr(s)

        • Similarly you can make silver iodide by mixing solutions of silver nitrate and potassium iodide.

        • The silver nitrate provides the silver ion and the potassium iodide provides the iodide ion to prepare the insoluble salt silver iodide which forms as a pale yellow precipitate.

        • silver nitrate + potassium iodide ==> silver iodide + potassium nitrate

        • AgNO3(aq) + KI(aq) ==> AgI(s) + KNO3(aq)

        • the spectator ions are nitrate NO3 and potassium K+ which do not change at all,

        • so the proper ionic equation is simply: Ag+(aq) + I(aq) ==> AgI(s)

    • (ii) Lead(II) iodide, a yellow precipitate (insoluble in water!) can be made by mixing lead(II) nitrate solution with e.g. potassium iodide solution.

      • The

      • lead nitrate supplies the lead ion and the potassium iodide the iodide ion to make the insoluble salt lead iodide.
      • lead(II) nitrate + potassium iodide ==> lead(II) iodide + potassium nitrate

      • Pb(NO3)2(aq) + 2KI(aq) ==> PbI2(s) + 2KNO3(aq)

      • which can be written as

      • Pb2+(aq) + 2NO3(aq) + 2K+(aq) + 2I(aq) ==> PbI2(s) + 2K+(aq) + 2NO3(aq)

      • the ionic equation is: Pb2+(aq) + 2I(aq) ==> PbI2(s)

      • because the spectator ions are nitrate NO3 and potassium K+.

      • In a similar way you can make lead(II) chloride by e.g. using dilute hydrochloric acid

        • The hydrochloric acid supplies the chloride ion.

        • lead(II) nitrate + hydrochloric acid ==> lead(II) chloride + nitric acid

        • Pb(NO3)2(aq) + 2HCl(aq) ==> PbCl2(s) + 2HNO3(aq)

        • Pb2+(aq) + 2NO3(aq) + 2H+(aq) + 2Cl(aq) ==> PbCl2(s) + 2H+(aq) + 2NO3(aq)

        • the proper ionic equation is: Pb2+(aq) + 2Cl(aq) ==> PbCl2(s)

        • because the spectator ions are nitrate NO3 and hydrogen H+.

      • Similarly you can make lead(II) chloride by e.g. using dilute sodium chloride solution

        • The silver nitrate provides the silver ion and the sodium chloride provides the chloride ion to prepare the insoluble salt lead chloride which forms as a white precipitate.

        • lead(II) nitrate + sodium chloride ==> lead(II) chloride + sodium nitrate

        • Pb(NO3)2(aq) + 2NaCl(aq) ==> PbCl2(s) + 2NaNO3(aq)

        • Pb2+(aq) + 2NO3(aq) + 2Na+(aq) + 2Cl(aq) ==> PbCl2(s) + 2Na+(aq) + 2NO3(aq)

        • the proper ionic equation is: Pb2+(aq) + 2Cl(aq) ==> PbCl2(s)

        • because the spectator ions are nitrate NO3 and sodium Na+.

      • You can make lead(II) bromide by e.g. using sodium bromide solution

        • lead(II) nitrate + sodium bromide ==> lead(II) bromide + sodium nitrate

        • Pb(NO3)2(aq) + 2NaBr(aq) ==> PbBr2(s) + 2NaNO3(aq)

        • Pb2+(aq) + 2NO3(aq) + 2Na+(aq) + 2Br(aq) ==> PbBr2(s) + 2Na+(aq) + 2NO3(aq)

        • the proper ionic equation is: Pb2+(aq) + 2Br(aq) ==> PbBr2(s)

        • because the spectator ions are nitrate NO3 and sodium Na+.

        • The silver nitrate provides the silver ion and the sodium bromide provides the bromide ion to prepare the insoluble salt silver bromide which forms as a cream precipitate.

        • You can also use potassium bromide, just swap the Na for a K.

      • and you can make lead(II) chloride by e.g. using sodium chloride solution

        • lead(II) nitrate + sodium chloride ==> lead(II) chloride + sodium nitrate

        • Pb(NO3)2(aq) + 2NaCl(aq) ==> PbCl2(s) + 2NaNO3(aq)

        • Pb2+(aq) + 2NO3(aq) + 2Na+(aq) + 2Cl(aq) ==> PbCl2(s) + 2Na+(aq) + 2NO3(aq)

        • the proper ionic equation is: Pb2+(aq) + 2Cl(aq) ==> PbCl2(s)

        • because the spectator ions are nitrate NO3 and sodium Na+.

    • (iii) Calcium carbonate, a white precipitate, forms on e.g. mixing calcium chloride solution and sodium carbonate solutions ...

      • calcium chloride + sodium carbonate ==> calcium carbonate + sodium chloride

      • CaCl2(aq) + Na2CO3(aq) ==> CaCO3(s) + 2NaCl(aq)

      • Ca2+(aq) + 2Cl(aq) + 2Na+(aq) + CO32–(aq) ==> CaCO3(s) + 2Na+(aq) + 2Cl(aq)

      • the ionic equation is: Ca2+(aq) + CO32–(aq) ==> CaCO3(s)

      • because the spectator ions are chloride Cl and sodium Na+.

    • (iv) Barium sulphate (barium sulphate), a white precipitate, forms on mixing e.g. barium chloride solution and dilute sulphuric acid ...

      • barium chloride + sulphuric acid ==> barium sulphate + hydrochloric acid

      • BaCl2(aq) + H2SO4(aq) ==> BaSO4(s) + 2HCl(aq)

      • Ba2+(aq) + 2Cl(aq) + 2H+(aq) + SO42–(aq) ==> BaSO4(s) + 2H+(aq) + 2Cl(aq)

      • so the ionic equation is: Ba2+(aq) + SO42–(aq) ==> BaSO4(s)

      • because the spectator ions are chloride Cl and hydrogen H+.

        • Or you can use sulphate salts like sodium sulphate, so the word and symbol equations are ..

        • barium chloride + sodium sulfate ==> barium sulfate + sodium chloride

        • BaCl2(aq) + Na2SO4(aq) ==> BaSO4(s) + 2NaCl(aq)

        • The ionic equation is the same: Ba2+(aq) + SO42–(aq) ==> BaSO4(s)

        • because the spectator ions are sodium Na+ and chloride Cl

        • You can do exactly the same preparation using potassium sulfate and barium nitrate or barium chloride, basically any solutions of a soluble barium salt and a soluble sulfate salt can be used to prepare barium sulfate.

        • e.g.     barium nitrate + potassium sulfate ==> barium sulfate + potassium nitrate

        • or        barium chloride + potassium sulfate ==> barium sulfate + potassium chloride

        • See at the end of the page the uses of barium sulfate.

    • (v) Lead(II) sulphate (lead sulphate), a white precipitate, forms in a similar way e.g.

      • lead(II) nitrate + sodium sulphate ==> lead(II) sulphate + sodium nitrate

      • Pb(NO3)2 (aq) + Na2SO4(aq) ==> PbSO4(s) + 2NaNO3 (aq)

      • ionic equation: Pb2+(aq) + SO42–(aq) ==> PbSO4(s)

      • because the spectator ions are sodium Na+ and nitrate NO3

      • Lead(II) sulphate can also be precipitated using dilute sulfuric acid.

        • lead(II) nitrate + sodium sulphate ==> lead(II) sulphate + nitric acid

        • Pb(NO3)2 (aq) + H2SO4(aq) ==> PbSO4(s) + 2HNO3 (aq)

        • ionic equation: Pb2+(aq) + SO42–(aq) ==> PbSO4(s)

        • because the spectator ions are hydrogen H+ and nitrate NO3 ions

    • (vi) Calcium sulphate (calcium sulphate), a white precipitate, forms on mixing e.g. calcium chloride solution and dilute sulphuric acid ...

      • calcium chloride + sulphuric acid ==> calcium sulphate + hydrochloric acid

      • CaCl2(aq) + H2SO4(aq) ==> CaSO4(s) + 2HCl(aq)

      • Ca2+(aq) + 2Cl(aq) + 2H+(aq) + SO42–(aq) ==> CaSO4(s) + 2H+(aq) + 2Cl(aq)

      • so the ionic equation is: Ca2+(aq) + SO42–(aq) ==> CaSO4(s)

      • because the spectator ions are chloride Cl and hydrogen H+.

        • Or you can use sulphate salts like sodium sulphate, so the word and symbol equations are ..

        • calcium chloride + sodium sulfate ==> calcium sulfate + sodium chloride

        • CaCl2(aq) + Na2SO4(aq) ==> CaSO4(s) + 2NaCl(aq)

        • The ionic equation is the same: Ca2+(aq) + SO42–(aq) ==> CaSO4(s)

        • because the spectator ions are sodium Na+ and chloride Cl

        • You can do exactly the same preparation using potassium sulfate and calcium nitrate or calcium chloride, basically any solutions of a soluble calcium salt and a soluble sulfate salt can be used to prepare and precipitate calcium sulfate.

          • The yield is a little lower than the theoretical because calcium sulfate is slightly soluble in water.

        • See at the end of the page the uses of barium sulfate.

    • NOTE reminder definition: A precipitation reaction is generally defined as 'the formation of an insoluble solid on mixing two solutions or bubbling a gas into a solution'.

  • General rules which describe the solubility of common types of compounds in water:

    • All common sodium, potassium and ammonium salts are soluble e.g. NaCl, K2SO4, NH4NO3

    • All nitrate salts are soluble e.g. NaNO3, Mg(NO3)2, Al(NO3)3, NH4NO3

    • Some ethanoate salts are soluble e.g. CH3COONa

    • Common chloride salts are soluble except those of silver and lead e.g.

      • soluble: KCl, CaCl2, AlCl3 or insoluble AgCl, PbCl2

    • Common sulfates are soluble except those of lead, barium and calcium: soluble e.g.

      • soluble: Na2SO4, MgSO4, Al2(SO4)3

      • insoluble: PbSO4, BaSO4, CaSO4 is slightly soluble.

    • Common oxides, hydroxides and carbonates are usually insoluble (e.g. Group 2 and Transition Metals) except those of the Group 1 Alkali Metals sodium, potassium etc. and ammonium:

      • soluble bases–alkalis oxides, hydroxides or carbonates: K2O, KOH, NaOH, NH3(aq), Na2CO3, (NH4)2CO3  

      • insoluble bases – basic oxides, hydroxides or insoluble carbonates: MgO, CuO, ZnO, Mg(OH)2, Fe(OH)2, Cu(OH)2, CuCO3, ZnCO3, CaCO3

  • Some uses of insoluble salts (their preparation has been described above)

    • Calcium sulfate CaSO4 (calcium sulphate) is used in plaster of Paris and plaster for domestic wall covering.

    • Barium sulfate BaSO4 (barium sulphate)

      • Barium sulfates quite a dense material is used in with X-rays for particular medical examinations.

      • Barium sulfate, like all barium salts is toxic, BUT, because it is insoluble, non of it is absorbed by the body into the bloodstream and eventually it will pass right through the gut system and be expelled in the normal faeces.

      • X-rays are used to investigate bone structure e.g. fractured or broken bones and the technique works because bone material is too dense to let weak X-rays through, so on shooting an X-ray photograph you get the bone structure as a sort of shadow effect where the X-rays have been absorbed by the bone.

      • Therefore, normally you can't use X-rays to examine soft tissue areas like the gut.

      • However, if the patient takes a 'barium meal', a thick harmless fluid containing a suspension of the insoluble barium sulfate, this can pass through the stomach and into the gut.

      • Therefore it is then possible to X-ray the intestinal gut system because the barium sulfate absorbs the X-rays just like bone.

      • So, because barium sulfate is opaque to X-rays, the X-ray photograph via the barium meal will highlight the structure of the tissue lining of the gut and show up any structural abnormalities and blockages.

 


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 Outline the steps in making an insoluble salt by precipitation how do you make the insoluble salt silver bromide AgBr by precipitation? Revision KS4 Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel Science Chemistry IGCSE Chemistry & OCR 21st Century Science Chemistry, OCR Gateway Science Chemistry, WJEC gcse science chemistry CCEA/CEA gcse science chemistry (revise science chemistry courses equal to US grade 8, grade 9 grade 10. Revision notes on acids, bases, alkalis, salts, solution pH word equations balanced symbol equations science chemistry courses revision guides. These revision notes on making salts using a precipitation reaction, should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses This section describes how to make insoluble salts: insoluble silver chloride salt AgCl, how to make, how to make insoluble lead(II) chloride salt (lead chloride) PbCl2, how to make insoluble lead(II) iodide salt (lead iodide) PbI2, how do you make insoluble lead bromide PbBr2 by a precipitation reaction how to make calcium insoluble carbonate CaCO3, how to make insoluble barium sulfate salt (barium sulphate) BaSO4, how to make insoluble lead(II) sulfate (lead sulphate, how to make insoluble lead sulfate salt PbSO4, and how to make 'slightly soluble' calcium sulfate salt CaSO4, which can all be made by a precipitation reaction how do you prepare insoluble salts? what do need to prepare an insoluble salt? how do you prepare the insoluble salt lead bromide by precipitation? what apparatus do you need to prepare an insoluble salt by precipitation? describe a method of making an insoluble salt? 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