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

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

Revision Notes KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

Advanced Level Chemistry Acid-Base Revision Notes - use index

GCSE Sub-index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of acid-alkali chemistry : 2. pH scale, indicators, ionic theory of acids-alkali neutralisation : 3. pH examples of acid, neutral or alkaline solutions : 4. Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions : 5. Reactions of bases-alkalis like sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation : 8. Important formulae, salt solubility and water of crystallisation : 9. Further examples of word/symbol equations for salt preparations : 10. More on Acid-Base Theory and Weak and Strong Acids

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

  • Neutralisation with acids is dealt with above.

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

    • e.g. sodium hydroxide + ammonium chloride ==> sodium chloride + water + ammonia

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

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

    • The ionic equation is: NH4+(aq) + OH-(aq) ==> H2O(l) + NH3(l)

    • This reaction can be used to prepare ammonia gas and as a simple chemical test for an ammonium salt (see also the "Chemical Tests" and "Gas Preparation-Collection" pages).

  • 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.

    • e.g. sodium hydroxide + copper(II) sulphate ==> sodium sulphate + 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.

    • 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.

    • 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.

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