Lesson focus: Acid strength describes ionisationWhen a substance splits into charged particles (ions) as it dissolves in water., while concentrationThe amount of solute dissolved in a given volume of solution, such as in g/dm³; a higher concentration has more solute particles in the same volume. describes the amount of acid per volume; either can influence pH and hazard.
Distinguish strong from concentrated and use particle models to compare acid solutions.
Build on earlier learning
Indicators and pHThe pH scale provides evidence that must now be separated from acid identity and concentration.
You will be successful when you can…
- Define acid strength.
- Define concentration.
- Compare particle models.
- Explain why pH depends on both.
Words that carry the science
- strong acid
- An acid that ionises almost completely in water.
- weak acid
- An acid that ionises only partially in water.
- concentration
- Amount of dissolved substance per unit volume.
- dilute
- Having a relatively small amount of solute per volume.
- ionisation
- Formation of ions from dissolved particles.
Strength describes proportion ionised
01Strength describes proportion ionised
A strong acidAn acid that completely splits into ions in water, such as hydrochloric acid. ionises almost completely in water; a weak acidAn acid that only partly splits into ions in water, such as vinegar. ionises partially and establishes an equilibrium between molecules and ions.
Strength is a property of the acid under stated conditions. Hydrochloric acid is strong; ethanoic acid is weak.
02Concentration describes amount per volume
A concentrated solution contains more dissolved acid per unit volume than a diluteContaining only a small amount of a substance dissolved in a lot of water, like weak squash. solution of the same acid.
Adding water dilutes an acid but does not change a strong acidAn acid that completely splits into ions in water, such as hydrochloric acid. into a weak one. It changes particle amount per volume, not acid identity.
03pH reflects the resulting ion conditions
pH depends on hydrogen-ion concentrationThe amount of solute dissolved in a given volume of solution, such as in g/dm³; a higher concentration has more solute particles in the same volume., which is influenced by both acid concentration and extent of ionisationWhen a substance splits into charged particles (ions) as it dissolves in water..
A dilute strong acid and a more concentrated weak acid can have similar pH, so pH alone does not identify acid strength without concentration information.
Strong and weak, concentrated and dilute
A strong acid splits up (ionises) completely in water, releasing all its hydrogen ions (H⁺). A weak acid only partly ionises. Concentration is different: it is how many acid particles there are in a given volume.
Red H⁺: hydrogen ion. Blue A⁻: the rest of the acid particle. HA: an acid molecule that has not split up. Particle counts are illustrative.
Controls
A concentrated weak acid can have a higher pH than a dilute strong acid. Strong does not mean concentrated.
Compare four acid descriptions
Classify concentrated HCl, dilute HCl, concentrated ethanoic acid and dilute ethanoic acid.
- HCl is strong in both solutions because it ionises almost completely.
- Ethanoic acid is weak in both because it ionises partially.
- Concentrated means more acid particles per volume.
- Dilute means fewer acid particles per volume.
How to make the reasoning visible: Strength and concentration form two independent descriptions.
Interpret acid particle models
Each row describes equal solution volumes.
| Model | Acid particles present | Fraction ionised | Description |
|---|---|---|---|
| A | many | almost all | concentrated strong |
| B | few | almost all | dilute strong |
| C | many | small fraction | concentrated weak |
| D | few | small fraction | dilute weak |
Which pairs share strength?
Why can C contain more H⁺ than B despite being weak?
Check your understanding
Answer from memory, then check against the suggested answer.
What defines strong acid?
What defines concentration?
Does dilution change strength?
A bottle is labelled “dilute hydrochloric acid”. Is it weak?
Why is pH alone insufficient to compare acid strength?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Dilute hydrochloric acid can have a higher pH than concentrated ethanoic acid.
Explain the difference between “strong” and “dilute”.
(3)Explain why the dilute strong acid can have a higher pH.
(2)Total for this question: 5 marks
Command word: Keep strength, concentration and pH as separate linked ideas.