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Cambridge IGCSE Chemistry · 0620

Chapter 7: Acids, Bases and Salts — Part 1

Topic 7.1 · Characteristic properties of acids and bases

Definitions and the pH Scale

  • Acids are defined as proton donors, and bases are defined as proton acceptors.
  • An acid is a substance with a pH value less than 7, while a base that is soluble in water is called an alkali and has a pH value greater than 7.
  • A pH scale (0–14) is used to measure acidity and alkalinity: 0 is the most acidic, 14 is the most alkaline, and pH 7 is neutral (e.g., pure water).
  • Aqueous solutions of acids contain H+ ions, while aqueous solutions of alkalis contain OH- ions.
  • Universal indicator paper can compare hydrogen ion concentration and neutrality: red, orange, and yellow indicate acidity, green is neutral, and blue or violet indicate alkalinity.

Exam Traps

  • Do not call every base an alkali — an alkali must be a soluble base (e.g. CuO is a base but not an alkali).
  • Do not say pH 7 is slightly acidic — it is neutral.

Indicators and Colour Changes

Indicators change colour according to the acidity or alkalinity of a solution:

Indicator Colour in Acid Colour in Alkali
Litmus Red Blue
Thymolphthalein Colourless Blue
Methyl orange Red Yellow

Exam Traps

  • Do not say methyl orange turns blue in alkali — it turns yellow.

Chemical Reactions of Acids

  • With Metals: Only metals above hydrogen in the reactivity series react with dilute acids to form a salt and hydrogen gas.
    • Example: 2Na(s) + 2HCl(aq) ? 2NaCl(aq) + H2(g).
  • With Bases: Acids react with metal oxides or hydroxides in a neutralisation reaction to form a salt and water.
    • Example: MgO(s) + H2SO4(aq) ? MgSO4(aq) + H2O(l).
  • With Carbonates: Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas.
    • Example: MgCO3(s) + 2HCl(aq) ? MgCl2(aq) + H2O(l) + CO2(g).

Exam Traps

  • Do not say all metals react with dilute acids — copper and silver do not produce hydrogen.
  • Do not forget CO2 as a product from acid + carbonate — it is not just salt and water.

Chemical Reactions of Bases and Alkalis

  • With Acids: Bases neutralise acids to form salt and water. The ionic equation for this reaction is: H+(aq) + OH-(aq) ? H2O(l).
  • With Ammonium Salts: When an ammonium salt is warmed with a base, it undergoes thermal decomposition to produce a salt, water, and ammonia gas.

Exam Traps

  • Do not write HCl + NaOH → NaCl when asked for the ionic equation — cancel spectator ions.
  • Do not confuse the ammonia test (red litmus → blue) with the acid test (blue litmus → red).

Strong and Weak Acids

  • A strong acid (e.g., hydrochloric acid) completely dissociates in aqueous solution: HCl(aq) ? H+(aq) + Cl-(aq).
  • A weak acid (e.g., ethanoic acid) only partially dissociates in aqueous solution, shown by a reversible sign: CH3COOH(aq) ? H+(aq) + CH3COO-(aq).

Exam Traps

  • Do not say weak acids contain no H+ ions — they do, but at lower concentration than strong acids of the same concentration.
  • Do not use a single arrow for ethanoic acid dissociation — it is an equilibrium (⇔).

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