Strength describes how far an acid ionises in water. Concentration describes how much acid is dissolved in a given volume. The two words answer different questions, so one cannot stand in for the other.
This lesson is part of acids, bases and salts in SPM Chemistry. It prepares you for writing neutralisation equations.
How do the two ideas fit on a grid?
Every acid solution has a place on two separate scales. The grid below shows one example for each box.
| Dilute | Concentrated | |
|---|---|---|
| Strong | 0.001 mol dm⁻³ hydrochloric acid | 2.0 mol dm⁻³ hydrochloric acid |
| Weak | 0.001 mol dm⁻³ ethanoic acid | 2.0 mol dm⁻³ ethanoic acid |
Each row keeps the strength the same and changes the concentration, and each column does the opposite. If you change only one word in your answer, you should move only one way on the grid.
Worked example: two solutions, one pH
Solution A is 0.001 mol dm⁻³ hydrochloric acid. Solution B is 0.10 mol dm⁻³ ethanoic acid. A student says, “B is stronger because it has a higher concentration.”
Check the evidence. Hydrochloric acid ionises completely, so A has a hydrogen ion concentration of about 0.001 mol dm⁻³ and a pH of about 3. Ethanoic acid ionises only slightly, so B’s hydrogen ion concentration is also small, and its pH is also around 3.
The two solutions have a similar pH even though B contains about one hundred times more acid. That means B needs far more acid to provide the same number of hydrogen ions, which is exactly what “weak” means. The student’s claim confused the amount of acid with the extent of ionisation.
What evidence shows strength?
Use evidence only when the concentrations are equal.
- Measure the pH of both acids at the same concentration. The lower pH belongs to the stronger acid.
- Compare electrical conductivity. More ions give higher conductivity.
- React equal amounts with magnesium. The stronger acid produces bubbles faster at first.
If the concentrations differ, say so in your answer and do not conclude anything about strength. A sentence such as “Both acids have a concentration of 0.10 mol dm⁻³, so the lower pH shows that hydrochloric acid ionises more” earns the reasoning mark.
The mistake to avoid
The common mistake is to describe a high concentration as a strong acid in an explanation of a test result. Compare the wrong and the right sentences.
| Wrong | Right |
|---|---|
| The acid is strong because it has a high concentration. | The acid is concentrated because it contains more solute per dm³. |
| The acid is weak because it is dilute. | The acid is dilute because it contains less solute per dm³. |
Match “strong” and “weak” to ionisation, and “concentrated” and “dilute” to amount per volume. The concentration and dilution tutor lets you change one variable at a time to see the difference.
Check yourself
A student tests two acids of the same concentration, 0.10 mol dm⁻³. Acid P has pH 1 and acid Q has pH 3. Which is stronger, and what is your evidence?
Answer
Acid P is stronger. At equal concentration, a lower pH means a higher hydrogen ion concentration, so P ionises more than Q. Q is a weak acid because only a small fraction of its molecules ionise.
What to study next
Continue to separating concentration from strength using particle models to see the same idea drawn particle by particle. Then write balanced equations in writing neutralisation equations.
If you want a teacher to test your understanding with you, see online one-to-one Chemistry tuition.