Strong and Weak Electrolytes and Electrical Conductivity – Electrolytes and Non-Electrolytes (Senior 2 Chemistry) | Ganzaa

Electrolytes and Non-Electrolytes

Lessons

Senior 2 · Chemistry

Electrolytes and Non-Electrolytes

3 lessons

Lesson 2 of 3

Strong and Weak Electrolytes and Electrical Conductivity

20 min

Why this matters

In the last lesson, dilute hydrochloric acid lit the bulb brightly while vinegar only made it glow. Both are acids and both conduct, so why the difference? The answer, how completely a substance forms ions, explains why hydrochloric acid burns skin while vinegar goes on your food. It is also one of the most common sources of confusion in O-level chemistry: the difference between strong and concentrated.

Strong electrolytes

A strong electrolyte is almost completely ionised or dissociated in solution: practically every dissolved particle becomes ions. Its solution therefore contains many ions and conducts well (the bulb is bright).

Strong acids: hydrochloric acid, HCl; sulfuric acid, H₂SO₄; nitric acid, HNO₃.
Strong bases (alkalis): sodium hydroxide, NaOH; potassium hydroxide, KOH.
Almost all soluble salts: NaCl, KNO₃, CuSO₄, NH₄Cl.

A single arrow shows that the change is complete: HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq).

Weak electrolytes

A weak electrolyte is only partly ionised: most of the dissolved particles stay as whole molecules and only a small fraction form ions. Its solution conducts poorly (the bulb is dim).

Weak acids: ethanoic acid (in vinegar), carbonic acid (in fizzy drinks), citric acid (in lemons and oranges).
Weak bases: ammonia solution.
Water itself is an extremely weak electrolyte.

A double arrow (⇌) shows that the ionisation is incomplete and reversible: CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq). In a typical vinegar solution only about 1 ethanoic acid molecule in every 100 is ionised at any moment.

Strong is not the same as concentrated

Strong or weak describes what fraction of the dissolved substance forms ions.
Concentrated or dilute describes how much substance is dissolved in a given volume of water.

You can have a dilute solution of a strong acid (a few drops of hydrochloric acid in a beaker of water) or a concentrated solution of a weak acid (pure vinegar essence).

What decides how well a solution conducts?

The concentration of ions, which depends on both the strength of the electrolyte and its concentration: the more ions per cm³, the larger the current.
The charge on the ions: a Cu²⁺ ion carries twice the charge of a Na⁺ ion.
Temperature: warm solutions conduct better because the ions move faster.

Worked example: counting ions

Two solutions each contain 0.1 mol of acid in 1 dm³ of water. One is hydrochloric acid (strong). The other is ethanoic acid, of which about 1 molecule in 100 ionises. Compare the number of hydrogen ions in each.

Hydrochloric acid: almost every molecule ionises, so the solution contains about 0.1 mol of H₃O⁺ ions.
Ethanoic acid: only 1 in 100 molecules ionises, so it contains about 0.1 ÷ 100 = 0.001 mol of H₃O⁺ ions.
0.1 ÷ 0.001 = 100, so the hydrochloric acid has about 100 times as many hydrogen ions. That is why it conducts much better, reacts much faster with magnesium and is far more corrosive, even though both solutions have the same concentration.

Try it on a computer

Open the Acid-Base Solutions simulation. On the Intro screen, choose "Strong Acid" and then "Weak Acid", and select the conductivity tester (the light-bulb tool) to compare how brightly the bulb glows. Switch on the molecule view to see how many molecules have ionised. On the My Solution screen, change the strength and the concentration separately, and notice that both affect the brightness.

Tip

Strong = splits completely; concentrated = lots dissolved

Examiners test this distinction every year. If a question says "strong", think about the percentage ionised. If it says "concentrated", think about the amount per dm³. For conductivity, what matters in the end is how many mobile ions there are.

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Practice

What is the difference between a strong electrolyte and a weak electrolyte?

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Solution: A strong electrolyte is almost completely ionised in solution; a weak electrolyte is only partly ionised, so most of it stays as molecules.

Classify as strong or weak: potassium hydroxide, citric acid, sodium nitrate, ammonia solution.

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Solution: Potassium hydroxide: strong. Citric acid: weak. Sodium nitrate: strong (a soluble salt). Ammonia solution: weak.

Why is a double arrow used in the equation for ethanoic acid ionising?

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Solution: The ionisation is incomplete and reversible: at any moment most of the molecules are un-ionised, and the ions keep recombining into molecules.

A solution of sodium chloride is diluted with an equal volume of water. What happens to the brightness of the bulb, and why?

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Solution: The bulb becomes dimmer. The salt is still a strong electrolyte, but there are now half as many ions in each cm³, so the solution carries less current.

If 1 molecule in every 100 ionises, how many of 5 000 dissolved ethanoic acid molecules form ions?

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Solution: 5 000 ÷ 100 = 50 molecules.

Summary

Strong electrolytes (strong acids, strong alkalis, soluble salts) are almost completely ionised and conduct well.
Weak electrolytes (weak acids such as ethanoic, carbonic and citric acids, ammonia solution and water) are only partly ionised and conduct poorly; their equations use ⇌.
Strength means the fraction ionised; concentration means the amount dissolved per dm³.
Conductivity rises with the concentration of ions, the charge on the ions and the temperature.

Sources and credits

Chemistry 2e, section 11.2: Electrolytes (strong and weak electrolytes; conductance and ion concentration) by Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson (OpenStax), hosted by LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Chemistry 2e, section 4.2: Classifying Chemical Reactions (common strong acids; ethanoic acid and ammonia as weak electrolytes about 1% ionised) by Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson (OpenStax), hosted by LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Acid-Base Solutions (version 1.3.13). Simulation by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC BY 4.0 (https://phet.colorado.edu).
Everything else in this lesson, including the worked examples, the results tables and the Practice questions, was written by Ganzaa.

1Why this matters

2Strong electrolytes

3Weak electrolytes

4Strong is not the same as concentrated

5What decides how well a solution conducts?

Worked example: counting ions

6Try it on a computer

Practice

Why this matters

Strong electrolytes

Weak electrolytes

Strong is not the same as concentrated

What decides how well a solution conducts?

Try it on a computer