The characteristic properties of acids and bases

O Level / IGCSE Chemistry · Acids, bases and salts. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

An acid is a proton donor: in water it gives H+ ions. A base is a proton acceptor. Most bases are the oxides and hydroxides of metals. A base that dissolves in water is an alkali, and its solution contains OH− ions.

An acid and a base neutralise each other to form a salt and water. Acids also react with reactive metals and with carbonates.

A strong acid is completely dissociated into ions in water. A weak acid is only partially dissociated, so at the same concentration it has fewer H+ ions and a higher pH. The lower the pH, the higher the concentration of H+ ions.

Rules to remember

Common mistake

Students use strong to mean concentrated. Strong and weak describe how fully the acid dissociates into ions; concentrated and dilute describe how much acid is dissolved in the water.

Worked example

Hydrochloric acid and ethanoic acid, both 0.1 mol/dm3, are each tested with universal indicator and with magnesium ribbon. How do the results differ?

  1. Hydrochloric acid is strong, so it is completely dissociated: HCl → H+ + Cl−.
  2. Ethanoic acid is weak, so it is only partially dissociated and has a lower concentration of H+ ions.
  3. A higher H+ concentration gives a lower pH and more frequent collisions with the magnesium.

Answer: Hydrochloric acid gives the lower pH (red, about pH 1) and fizzes faster. Ethanoic acid gives a higher pH (orange, about pH 3) and fizzes more slowly. Both produce hydrogen.

Practice questions

Try each one, then open the answer.

1. Which ion is present in all aqueous acids?

  1. A
    Na+
  2. B
    OH−
  3. C
    Cl−
  4. D
    H+
Show answer

Answer: D. An acid is a substance that produces hydrogen ions, H+, when dissolved in water. Alkalis produce OH−.

2. Four colourless solutions have these pH values: W 1, X 6, Y 8, Z 13. Which solution turns red litmus paper blue and has the highest concentration of hydroxide ions?

  1. A
    W
  2. B
    X
  3. C
    Y
  4. D
    Z
Show answer

Answer: D. Alkalis (pH above 7) turn red litmus blue. Both Y and Z are alkaline, but the higher the pH the greater the concentration of OH− ions, so Z, at pH 13, has the most.

3. Which metal does not react with dilute hydrochloric acid?

  1. A
    copper
  2. B
    magnesium
  3. C
    zinc
  4. D
    iron
Show answer

Answer: A. Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from an acid.

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