GCSE Chemistry notes: Other reactions of soluble bases (alkalis) & insoluble bases

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5. More on other important chemical reactions of insoluble bases and soluble bases (alkalis) e.g. sodium hydroxide and ammonia

Index of all my GCSE notes on acids, bases and salts

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Part 5 Apart from the reactions with acids, this section also covers in detail other reactions of bases (acid reactions are covered in Part 4) with other chemicals e.g. the reaction of the alkali sodium hydroxide with ammonium salts, carbon dioxide and metals salts to give hydroxide precipitates. Word and symbol equations are given for the reaction of the base sodium hydroxide with ammonium salts. Word and symbol equations are given for the formation of metal hydroxide precipitates when sodium hydroxide is added to solutions of soluble metal salts.  These revision notes on the reactions of soluble bases (alkalis) and insoluble bases should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9, 10 school science courses for ~14-16 year old science students for national examinations in chemistry topics including acids bases alkalis salts preparations reactions

5. Some important reactions of Bases (alkali = soluble base)

Bases, both soluble and insoluble, react with acids to form salts. Such base may be metal oxides, metal hydroxides, metal carbonates and ammonia. Alkalis (soluble bases) include soluble metal hydroxides, soluble metal carbonates and ammonia.

  • Neutralisation of bases with acids is dealt with Part 4, so Part 5 mainly concentrates on other reactions of bases.

    • BUT a few 'reminder' examples wouldn't go amiss here e.g

      • (soluble base) potassium hydroxide + hydrobromic acid ==> potassium bromide + water

        • KOH(aq) + HBr(aq) ==> KBr(aq) + H2O(l)

      • (insoluble base) magnesium hydroxide + hydrochloric acid ==> magnesium chloride + water

        • Mg(OH)2(s) + 2HCl(aq) ==> MgCl2(aq) + 2H2O(l)

      • (soluble, alkali) ammonia + nitric acid ==> ammonium nitrate

        • NH3 + HNO3 ==> NH4NO3

          • NH3(aq) + HNO3(aq) ==> NH4NO3(aq)

          Similarly ...

          ammonia + hydrochloric acid ==> ammonium chloride

          • NH3 + HCl ==> NH4Cl

            • NH3(aq) + HCl(aq) ==> NH4Cl(aq)


          ammonia + sulfuric acid ==> ammonium sulfate

          • 2NH3 + H2SO4 ==> (NH4)2SO4

            • 2NH3(aq) + H2SO4(aq) ==> (NH4)2SO4(aq)

      • (insoluble base) zinc oxide + hydrochloric acid ==> zinc chloride + water

        • ZnO(s) + 2HCl(aq) ==> ZnCl2(aq) + H2O(l)

      • (insoluble) calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

        • CaCO3(s) + 2HCl(aq) ==> CaCl2(aq)+ H2O(l) + CO2(g)

      • (soluble) sodium carbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide

        • Na2CO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + CO2(g)

      • (slightly soluble) sodium hydrogencarbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide

        • NaHCO3(s) + HCl(aq) ==> NaCl(aq) + H2O(l) + CO2(g)

  • Reaction of sodium hydroxide with ammonium salts

  • Ammonium salts are decomposed when mixed with a strong base e.g. the alkali sodium hydroxide produces ammonia gas  ...

    • (i) sodium hydroxide + ammonium chloride ==> sodium chloride + water + ammonia

      • NaOH(aq) + NH4Cl(aq) ==> NaCl(aq) + H2O(l) + NH3(g)

    • (ii) sodium hydroxide + ammonium nitrate ==> sodium nitrate + water + ammonia

      • NaOH(aq) + NH4NO3(aq) ==> NaNO3(aq) + H2O(l) + NH3(g)

    • (iii) sodium hydroxide + ammonium sulfate ==> sodium sulfate + water + ammonia

      • 2NaOH(aq) + (NH4)2SO4(aq) ==> Na2SO4(aq) + 2H2O(l) + 2NH3(g)

    • The ammonia is readily detected by its pungent odour (strong smell) and by turning damp red litmus blue.

    • The ionic equation for all three reactions is:

      • NH4+(aq) + OH(aq) ==> H2O(l) + NH3(g)

    • This reaction can be used to prepare ammonia gas and as a simple chemical test for an ammonium salt.

  • Use of limestone and lime to control soil acidity is dealt with in the 1st section on the Extra Industrial Chemistry page.

  • Alkali's (soluble bases) are used to produce the insoluble hydroxide precipitates of many metal ions from their soluble salt solutions.

    • (aq) indicates soluble, (s) indicates insoluble - precipitate formed e.g.

    • sodium hydroxide + copper(II) sulfate ==> sodium sulfate + copper(II) hydroxide

    • 2NaOH(aq) + CuSO4(aq) ==> Na2SO4(aq) + Cu(OH)2(s) a blue precipitate

    • ionically: Cu2+(aq) + 2OH(aq) ==>  Cu(OH)2(s) 

    • This reaction can be used as a simple test to help identify certain metal ions.

  • Aqueous solutions of alkalis like sodium hydroxide ('caustic soda') and calcium hydroxide ('limewater') react with the acidic gas carbon dioxide to form carbonate compounds if the gas is bubbled into their solutions.

    • There are two possible reactions with sodium hydroxide and carbon dioxide

    • (i) sodium hydroxide + carbon dioxide ==> sodium hydrogen carbonate + water

      • NaOH(aq) + CO2(g)  ==> NaHCO3(aq)

    • (ii) sodium hydroxide + carbon dioxide ==> sodium carbonate + water

      • 2NaOH(aq) + CO2(g)  ==> Na2CO3(aq) + H2O(l)

      • This reaction can be used to remove carbon dioxide gas from a mixture of gases.

    • The reaction with 'limewater' is ...

    • calcium hydroxide + carbon dioxide ==> calcium carbonate + water

      • Ca(OH)2(aq) + CO2(g) ==> CaCO3(s) + H2O(l)

      • The formation of the white precipitate of calcium carbonate is used as a test for the gas carbon dioxide.


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

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