6b. Making a soluble salt from an acid and an insoluble base (can be an oxide, hydroxide, carbonate) or metal

Sub-index of salt preparation methods

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 with an insoluble base - oxide, hydroxide or carbonate (this page)

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

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

One important point is to recognise that one of the reactants is insoluble here, which is why you can't use a titration procedure to work out how much of the acid is to be added to a given amount of the solid. However, you can add the solid reactant to the acid until no more reacts and dissolves and then filter off the excess solid leaving a solution of the desired salt.

e.g. an insoluble base such as a metal oxide, metal hydroxide or a metal carbonate, often of a Group 2 metal like calcium, magnesium or a Transition Metal like nickel, copper or zinc. Copper metal won't dissolve in acids, but its oxide and carbonate will. Using the same procedure you can also start with a metal that has a low reactivity towards water e.g. magnesium, zinc or iron.

Typical common insoluble bases used for preparing soluble salts:

MgO magnesium oxide, MgCO₃ magnesium carbonate, CaO Calcium oxide, CaCO₃ calcium carbonate, Ca(OH)₂ calcium hydroxide, NiO nickel(II) oxide, ZnO zinc oxide, Zn(OH)₂, zinc hydroxide, ZnCO₃ zinc carbonate, CuO copper(II) oxide, CuCO₃ copper(II) carbonate, PbCO₃ lead(II) carbonate (with nitric acid to make lead(II) nitrate), FeCO₃ iron(II) carbonate (to make iron(II) salts), MnCO₃ manganese(II) carbonate

Typical examples shown by the word and symbol equations below include ...

copper(II) oxide + sulfuric acid ==> copper(II) sulfate + water

CuO + H₂SO₄ ==> CuSO₄ + H₂O

CuO_(s) + H₂SO_4(aq) ==> CuSO_4(aq) + H₂O_(l)

magnesium hydroxide + sulfuric acid ==> magnesium sulfate + water

Mg(OH)₂ + H₂SO₄ ==> MgSO₄ + 2H₂O

Mg(OH)_2(s) + H₂SO_4(aq) ==> MgSO_4(aq) + 2H₂O_(l)

 

magnesium hydroxide + hydrochloric acid ==> magnesium chloride + water

Mg(OH)_2(s) + 2HCl_(aq) ==> MgCl_2(aq) + 2H₂O_(l)

Zinc carbonate + nitric acid ==> zinc nitrate + water + carbon dioxide

ZnCO₃ + 2HNO₃ ==> Zn(NO₃)₂ + H₂O ₊ CO₂

ZnCO_3(s) + 2HNO_3(aq) ==> Zn(NO₃)_2(aq) + H₂O_{(l) +} CO_{2 (g)}

 

zinc oxide + hydrochloric acid ==> zinc chloride + water

ZnO_(s) + 2HCl_(aq) ==> ZnCl_2(aq) + H₂O_(l)

Similar for many other Group 2 and Transition metal oxides  e.g. Mg, Ca, Ba and Co, Ni, Cu instead of Zn

 

zinc + sulfuric acid ==> zinc sulfate + hydrogen

Zn + H₂SO₄ ==> ZnSO₄ + H₂

Zn_(s) + H₂SO_4(aq) ==> ZnSO_4(aq) + H_2(g)

 

calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

CaCO_3(s) + 2HCl_(aq) ==> CaCl_2(aq)+ H₂O_(l) + CO_2(g)

Similar for many other Group 2 and Transition metal carbonates e.g. Mg, Sr, Ba and Ni, Co, Zn instead of Ca

 

• Carbonates are frequently used in this method of salt making, e.g. using copper carbonate to make copper salts

  • The equations are give with, and without sate symbols.

• copper(II) carbonate + hydrochloric acid ==> Copper(II) chloride + water + carbon dioxide

  • CuCO₃ + 2HCl ==> CuCl₂ + H₂O ₊ CO₂

  • CuCO₃(s) + 2HCl(aq) ==> CuCl₂(aq) + H₂O(l) ₊ CO₂(g)

  • and with sulfuric acid a blue solution of copper(II) sulfate is formed.

• copper(II) carbonate + sulfuric acid ==> Copper(II) sulfate + water + carbon dioxide

  • CuCO₃ + H₂SO₄ ==> CuSO₄ + H₂O ₊ CO₂

  • CuCO₃(s) + H₂SO₄(aq) ==> CuSO₄(aq) + H₂O(l) ₊ CO₂(g)

• copper(II) carbonate + nitric acid ==> Copper(II) nitrate + water + carbon dioxide

  • CuCO₃ + 2HNO₃ ==> Cu(NO₃)₂ + H₂O ₊ CO₂

  • CuCO₃(s) + 2HNO₃(aq) ==> CuSO₄(aq) + H₂O(l) ₊ CO₂(g)

• Similar equations for other carbonates to give soluble salts which can be crystallised from solution e.g.

  • calcium carbonate CaCO₃, to make two salts - calcium chloride/nitrate (calcium sulfate is not very soluble)

  • iron(II) carbonate FeCO₃, to make three salts - iron(II) chloride/sulfate/nitrate

  • magnesium carbonate MgCO₃, to make three salts - magnesium chloride/sulfate/nitrate

  • manganese(II) carbonate MnCO₃, to make three salts - manganese(II) chloride/sulfate/nitrate

  • zinc carbonate ZnCO₃, to make three salts - zinc chloride/sulfate/nitrate

  • lead(II) carbonate PbCO₃, only nitric acid to make lead(II) nitrate, lead(II) chloride and lead(II) sulfate are insoluble and must be prepared by method (c)

More examples of neutralization equations are given in section 4.

(1) The required volume of acid is measured out into the beaker with a measuring cylinder. The excess of insoluble metal, oxide, hydroxide or carbonate is weighed out (*) and the solid added in small portions to the acid in the beaker with stirring. Doing a weighing will minimise trial and error especially if the reaction is slow, as long as you know how to do the theoretical calculation and add on a little excess!

You need to be able to calculate the quantities required.

(*) You can avoid doing a calculation and weighing of the insoluble solid reactant by adding small quantities to the hot acid until no more apparently dissolves.

(2) The mixture may be heated to speed up the reaction. When no more of the solid dissolves it means ALL the acid is neutralised and there should be a little excess solid.

You should see a residue of the solid (oxide, hydroxide, carbonate) left at the bottom of the beaker.

(3)  The hot solution (with care!) is filtered to remove the excess solid metal/oxide/carbonate, into an evaporating dish.

On filtration, only a solution of the salt is left i.e. the liquid that passes through the filter paper.

(4) You may need to carefully heat the solution to evaporate some of the water.

This evaporation can be done safely with an electric heater of a hot water bath.

Then hot concentrated solution is left to cool and crystallise.

After crystallisation, you collect and dry the crystals with a filter paper.

Extra guidance notes

(i) Apparatus used: (1) balance, measuring cylinder, beaker and glass stirring rod. (2) beaker/rod, bunsen burner, tripod and gauze; (3)-(4) filter funnel and filter paper, evaporating (crystallising) dish.

(ii) A measuring cylinder is adequate for measuring the acid volume, you do not need the accuracy of a pipette or burette required in method (a).

(iii) How to calculate amounts required and % yield is dealt with in Chemical Calculations Part 14

Salt solubility affects the method you choose to make a salt and so  section 8. contains tables of information-data on salt solubility which will help you decide on the method to prepare a salt.

• Multiple choice revision quizzes and other worksheets

• GCSE/IGCSE foundation-easier multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE higher-harder multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE Structured question worksheet on Acid Reaction word equations and symbol equation questions

•  Word equation answers and symbol equation answers)

• GCSE/IGCSE word-fill worksheet on Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE matching pair quiz on Acids, Bases, Salts and pH

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