Dilute and concentrated describe how much acid is dissolved
Definition
Dilute acid
An acid solution that contains a small amount of acid dissolved in a given volume.
Definition
Concentrated acid
An acid solution that contains a large amount of acid dissolved in a given volume.
- The concentration of an acid is the amount of acid in each cubic decimetre of solution, measured in mol/dm3\text{mol/dm}^3mol/dm3.
- Hydrochloric acid of 2.0 mol/dm32.0\,\text{mol/dm}^32.0mol/dm3 holds twice as much acid in each dm3\text{dm}^3dm3 as hydrochloric acid of 1.0 mol/dm31.0\,\text{mol/dm}^31.0mol/dm3.
- Adding water dilutes an acid, because the same amount of acid is spread through a larger volume.
- Making 100 cm3100\,\text{cm}^3100cm3 of acid up to 1000 cm31000\,\text{cm}^31000cm3 with water makes its concentration ten times smaller.
- Dilute and concentrated are comparative words, so 0.1 mol/dm30.1\,\text{mol/dm}^30.1mol/dm3 acid is dilute compared with 5 mol/dm35\,\text{mol/dm}^35mol/dm3 acid.
Note
Concentration tells you how much acid is dissolved, but not how the acid behaves once it is in water, which is described by its strength.
Strong and weak describe how fully an acid ionises
Definition
Strong acid
An acid that is fully ionised when it dissolves in water.
Definition
Weak acid
An acid that is only partially ionised when it dissolves in water.
- When an acid dissolves, its molecules split up to release hydrogen ions, and this process is called ionisation.
- Every HCl molecule in hydrochloric acid ionises, so it is a strong acid:
HCl(aq)→H+(aq)+Cl−(aq)\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}HCl(aq)→H+(aq)+Cl−(aq)
- Sulfuric acid and nitric acid are also fully ionised in water.
- Only a small fraction of the molecules in ethanoic acid, the acid in vinegar, ionise, so it is a weak acid:
CH3COOH(aq)⇌CH3COO−(aq)+H+(aq)\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}CH3COOH(aq)⇌CH3COO−(aq)+H+(aq)
- The reversible arrow shows that most ethanoic acid stays as whole molecules, with ions forming and joining back together all the time.
- Citric acid and carbonic acid are other examples of weak acids.
Common Mistake
Do not describe a strong acid as one with lots of hydrogen ions, because a dilute strong acid can contain fewer hydrogen ions than a concentrated weak acid.
An acid's strength and concentration are separate ideas
- A concentrated strong acid, such as 5 mol/dm35\,\text{mol/dm}^35mol/dm3 hydrochloric acid, has a lot of acid in each dm3\text{dm}^3dm3 and all of it is ionised.
- A dilute strong acid, such as 0.01 mol/dm30.01\,\text{mol/dm}^30.01mol/dm3 hydrochloric acid, has little acid in each dm3\text{dm}^3dm3 but all of it is still ionised.
- A concentrated weak acid, such as 5 mol/dm35\,\text{mol/dm}^35mol/dm3 ethanoic acid, has a lot of acid in each dm3\text{dm}^3dm3 but only a small fraction of it is ionised.
- A dilute weak acid, such as vinegar, has little acid in each dm3\text{dm}^3dm3 and only a small fraction of it is ionised.
- Diluting an acid lowers its concentration, but a weak acid stays weak and a strong acid stays strong.
Key Idea
Concentration answers how much acid is dissolved, while strength answers how much of that acid has ionised.
At equal concentration, strong acids release more hydrogen ions
- Hydrochloric acid and ethanoic acid, both at 1 mol/dm31\,\text{mol/dm}^31mol/dm3, contain the same amount of acid in each dm3\text{dm}^3dm3.
- Hydrochloric acid is fully ionised, so it has a much higher concentration of hydrogen ions.
- Its pH is therefore lower, about pH 0 compared with about pH 2 for the ethanoic acid.
- Magnesium ribbon fizzes more vigorously in the hydrochloric acid, because more hydrogen ions are available to react with it.
- These differences come from the degree of ionisation, since the two acids have the same concentration.
Exam technique
- When a question asks you to compare two acids, say whether they differ in concentration, meaning the amount of acid per dm3\text{dm}^3dm3, or in strength, meaning the degree of ionisation.
- Use the phrases fully ionised and partially ionised, rather than describing a strong acid as having lots of hydrogen ions.
Self review
- What does concentrated mean when it describes an acid?
- What is the difference between a strong acid and a weak acid?
- Can a strong acid be dilute, and why?
- Why does 1 mol/dm31\,\text{mol/dm}^31mol/dm3 hydrochloric acid have a lower pH than 1 mol/dm31\,\text{mol/dm}^31mol/dm3 ethanoic acid?
- What happens to the strength of an acid when water is added to it?
