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Chemistry · Practice book · Senior 2 · Unit 8 of 9

Electrolytes and non-electrolytes

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 8 Electrolytes and non-electrolytes

Key unit competence: Distinguish between non-electrolytes, weak electrolytes and strong electrolytes.

Key ideas

  • An electrolyte is a substance that conducts electricity when molten or dissolved in water and is decomposed by it; the current is carried by moving ions. A non-electrolyte does not conduct in any state (sugar, ethanol, urea, paraffin).
  • Strong electrolytes are completely ionised in solution and conduct well: strong acids (HCl, H2SO4, HNO3), strong alkalis (NaOH, KOH) and most salts (NaCl, CuSO4).
  • Weak electrolytes are only partly ionised and conduct poorly: weak acids (ethanoic acid, carbonic acid), weak bases (ammonia solution) and water itself.
  • Conductivity can be tested with a circuit of a battery, bulb or ammeter and two electrodes (carbon or copper). A bright bulb means a strong electrolyte; dim bulb, weak; no light, non-electrolyte.
  • Metals and graphite conduct by moving electrons and are not decomposed (conductors, not electrolytes). Solid ionic compounds do not conduct because their ions cannot move.
  • Applications: batteries and car accumulators (sulfuric acid), electrolysis and electroplating, oral rehydration salts and sports drinks (to replace lost body electrolytes such as Na+ and K+), and nerve impulses in the body.

Common misconceptions

  • "Pure water is a good conductor of electricity." No: Pure water is a very weak electrolyte; tap water conducts because of dissolved salts.
  • "Copper is an electrolyte because it conducts." No: Metals conduct by electrons and are not decomposed; electrolytes conduct by ions.
  • "A concentrated weak acid is a strong electrolyte." No: Strength depends on how much the acid ionises, not on its concentration.

Worked examples

Example 1. Describe an experiment to classify solutions as strong electrolytes, weak electrolytes or non-electrolytes.

Solution

Set up a circuit with a battery, a bulb (or ammeter) and two carbon electrodes.

Dip the electrodes in each solution of the same concentration, rinsing between tests.

Bright bulb/large current: strong electrolyte (e.g. NaCl); dim bulb/small current: weak electrolyte (e.g. ethanoic acid); no light/no current: non-electrolyte (e.g. sugar).

Example 2. Explain the difference between conduction in a copper wire and in copper(II) sulfate solution.

Solution

In copper wire, delocalised electrons move; the copper is not changed.

In copper(II) sulfate solution, ions move: Cu2+ to the cathode and SO42− to the anode.

The solution is decomposed at the electrodes (electrolysis), so it is an electrolyte.

Practice questions

  1. Define a non-electrolyte and give two examples.

    Show answer

    A substance that does not conduct electricity in any state; e.g. sugar, ethanol, paraffin.

    Common mistake: Giving distilled water as a strong non-electrolyte without explanation.

  2. Name the particles that carry current in an electrolyte.

    Show answer

    Ions.

    Common mistake: Saying electrons.

  3. Give two examples of strong electrolytes.

    Show answer

    HCl, H2SO4, NaOH, KOH, NaCl (any two).

    Common mistake: Naming ethanoic acid.

  4. Why is ethanoic acid called a weak electrolyte?

    Show answer

    It is only partly ionised in water.

    Common mistake: Saying it is dilute.

  5. Explain why solid KCl does not conduct.

    Show answer

    Its ions are fixed in the lattice and cannot move.

    Common mistake: Saying it has no ions.

  6. Write the ionisation equation of NaOH in water.

    Show answer

    NaOH(aq) → Na+(aq) + OH−(aq).

    Common mistake: Using a reversible arrow for a strong electrolyte.

  7. State one difference between a metallic conductor and an electrolyte.

    Show answer

    Metal: electrons move, no chemical change. Electrolyte: ions move, decomposed.

    Common mistake: Saying metals contain ions that move.

  8. Why do doctors give oral rehydration salts?

    Show answer

    To replace water and electrolytes lost through diarrhoea or vomiting.

    Common mistake: Saying to add vitamins.

  9. A solution makes a bulb glow dimly. Classify it and give an example.

    Show answer

    Weak electrolyte, e.g. ethanoic acid or ammonia solution.

    Common mistake: Calling it a non-electrolyte.

  10. Explain why lightning is dangerous for people standing in floodwater.

    Show answer

    Floodwater contains dissolved salts that conduct electricity, so current spreads through the water.

    Common mistake: Saying water is an insulator.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which is a weak electrolyte?a. a strong electrolyte
    2. Which is a non-electrolyte?b. Ethanoic acid (vinegar)
    3. A bulb glows brightly in a solution. The solution is most likely:c. NaOH and H2SO4
    4. Which pair are both strong electrolytes?d. Sugar solution
    Show answer

    1 → b (Ethanoic acid (vinegar)) · 2 → d (Sugar solution) · 3 → a (a strong electrolyte) · 4 → c (NaOH and H2SO4)

    • Ethanoic acid is only partly ionised.
    • Sugar dissolves as molecules, not ions.
    • Many ions carry a large current.
    • Both ionise completely.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    is only partly ionised  ·  provide protein  ·  ions  ·  replace electrolytes lost in sweat
    1. An athlete drinks a sports drink containing NaCl and KCl after training. The main purpose is to .
    2. In an electrolyte, electricity is carried by .
    3. Ammonia solution is a weak electrolyte because it .
    Show answer

    1: replace electrolytes lost in sweat · 2: ions · 3: is only partly ionised

    • Step 1. Sweat removes Na+, K+ and Cl−.
      Step 2. These ions are needed for nerves and muscles.
    • Positive and negative ions move to the electrodes.
    • Only a small fraction forms NH4+ and OH−.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Oral rehydration salts help a patient with diarrhoea by:a. conducts electricity when molten or in solution and is decomposed
    2. Which is a strong electrolyte?b. Dilute hydrochloric acid
    3. When glucose dissolves in water, the solution is a non-electrolyte because glucose:c. replacing lost ions such as sodium and potassium
    4. An electrolyte is a substance that:d. dissolves as neutral molecules
    Show answer

    1 → c (replacing lost ions such as sodium and potassium) · 2 → b (Dilute hydrochloric acid) · 3 → d (dissolves as neutral molecules) · 4 → a (conducts electricity when molten or in solution and is decomposed)

    • The body loses electrolytes and water during diarrhoea.
    • HCl is completely ionised in water.
    • No ions are formed.
    • Ions carry the current and the substance is broken down.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    a dim bulb  ·  contains dissolved salts (ions)  ·  is colder  ·  dilute sulfuric acid
    1. A weak electrolyte gives .
    2. Tap water conducts better than distilled water because tap water .
    3. The liquid in a car battery is .
    Show answer

    1: a dim bulb · 2: contains dissolved salts (ions) · 3: dilute sulfuric acid

    • Few ions are present.
    • Dissolved minerals provide ions.
    • It is an electrolyte for the lead–acid cell.

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