Chemical changes · GCSE Chemistry
Strong and weak acids
GCSE Chemistry revision on strong and weak acids (Higher): complete versus partial ionisation, pH at the same concentration, and why strength is not the same as concentration.
Strength is how fully the acid ionises. Concentration is moles per dm³. HCl is strong even when dilute. Ethanoic acid is weak even when concentrated.
The important bits
What you need to know
- 1
A strong acid ionises completely in water. Hydrochloric, sulfuric and nitric acids are strong: HCl(aq) → H⁺(aq) + Cl⁻(aq).
- 2
A weak acid ionises only partially. Ethanoic acid is in equilibrium: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq). Most molecules remain un-ionised.
- 3
Concentration is the amount of acid dissolved per dm³ (mol/dm³ or g/dm³). You can have dilute strong acid or concentrated weak acid.
- 4
At the same concentration, a strong acid has a lower pH than a weak acid because it produces more H⁺(aq). 0.1 mol/dm³ HCl is about pH 1; 0.1 mol/dm³ CH₃COOH is about pH 3.
- 5
Both strong and weak acids can be fully neutralised by an alkali. A weak acid keeps producing extra H⁺ as the equilibrium shifts when OH⁻ removes H⁺.
- 6
For the same concentration and volume, moles of acid molecules can be equal, so they can need the same volume of NaOH to reach the end-point — even though pH values differed at the start (Higher).
- 7
Strong alkalis such as NaOH and KOH ionise completely to OH⁻. Ammonia is a weak alkali: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
- 8
This distinction is Higher tier on Combined and Chemistry papers. Foundation may only need pH colours, not the equilibrium arrow.
Quotations worth analysing
Short evidence. Real method.
“HCl(aq) → H⁺(aq) + Cl⁻(aq) CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)”
A full arrow means complete ionisation. An equilibrium arrow means partial ionisation. That is strength, not how many grams you dissolved.
“Strength ≠ concentration”
Concentrated means lots of solute per dm³. Strong means fully ionised. Lab bottles labelled “dilute hydrochloric acid” are still strong acids.
“H⁺(aq) + OH⁻(aq) → H₂O(l)”
Weak acids still donate H⁺ until they are used up. The ionic equation at the end-point is the same idea; the pH curve is just a different shape.
Go deeper
Same concentration, different pH — that is the exam contrast
Prepare two solutions both 0.10 mol/dm³. Hydrochloric acid has essentially 0.10 mol/dm³ of H⁺ because every HCl splits. Ethanoic acid has far fewer H⁺ because the equilibrium lies to the left. Universal indicator may look red in both if you are not careful; a pH probe or a comparison with a colour chart shows the weak acid is less acidic. Electrical conductivity of the strong acid is higher because there are more mobile ions. Those two measurements — pH and conductivity — are the practical evidence of strength when concentration is controlled.
Go deeper
Titration volumes can match even when pH does not
This is the Higher-tier twist. 25.0 cm³ of 0.10 mol/dm³ HCl and 25.0 cm³ of 0.10 mol/dm³ CH₃COOH contain the same number of acid molecules (0.0025 mol). Both can require 25.0 cm³ of 0.10 mol/dm³ NaOH if the mole ratio is 1:1, because as H⁺ is removed from the weak acid, more CH₃COOH ionises. Students think a weak acid needs less alkali. That would be true only if you stopped at the same pH without letting the equilibrium shift, which is not how a full titration to the phenolphthalein end-point works. Strength affects pH and rate; it does not reduce the moles of acid you weighed out.
Go deeper
Rate with a metal is another way to see strength
Equal concentrations of HCl and CH₃COOH with magnesium: hydrochloric acid produces hydrogen faster because [H⁺] is higher, so successful collisions with the metal are more frequent. The final volume of hydrogen can still be the same if the moles of acid (and the limiting reagent) match and the metal is in excess. That links this page to collision theory. Do not say the weak acid “has fewer molecules”. It has fewer ions at any instant, not necessarily fewer molecules overall. A pH probe would still show the weak acid starting higher, even when the gas volumes finish equal.
See the idea in action
Two acids have concentration 0.10 mol/dm³. Acid A has pH 1.0; acid B has pH 3.0. Acid A is strong: fully ionised, [H⁺] ≈ 0.10 mol/dm³. Acid B is weak: only partly ionised. Each pH unit is a ten-fold [H⁺] change, so pH 1 has about 100 times the [H⁺] of pH 3. If 25.0 cm³ of each is titrated with 0.10 mol/dm³ NaOH and both acids are monoprotic, both mean titres are 25.0 cm³. Strength explained the pH; concentration explained the titre.
Exam technique
Turn knowledge into marks
Define strength as complete or partial ionisation, and concentration as mol/dm³, in the first sentence. Then compare pH at the same concentration. On Higher, mention that full neutralisation can still need the same volume of alkali.
Common mistakes
Do not give these marks away
- 01
Using strong to mean concentrated, or weak to mean dilute.
- 02
Saying a weak acid cannot be neutralised, or needs less alkali because pH is higher.
- 03
Writing a full arrow for ethanoic acid ionisation, or an equilibrium arrow for HCl.
Hydrochloric acid and ethanoic acid are both 0.10 mol/dm³. Why is the pH of HCl lower?
AHCl is more concentrated
BHCl ionises completely so it produces more H⁺(aq); ethanoic acid ionises only partially
CEthanoic acid is an alkali
DHCl has a higher relative formula mass
Show the answer
HCl ionises completely so it produces more H⁺(aq); ethanoic acid ionises only partially. Concentration is the same. Strength is different. Complete ionisation means more free hydrogen ions and a lower pH.
Quick questions
If this is the bit you searched
What is the difference between a strong acid and a weak acid GCSE?
A strong acid ionises completely in water. A weak acid ionises only partially, shown with an equilibrium arrow. This is not the same as concentrated versus dilute.
Is dilute hydrochloric acid a weak acid?
No. Hydrochloric acid is strong even when dilute, because the HCl that is dissolved still ionises fully. Dilute only means a small amount per dm³.
Why can a weak acid still need the same volume of alkali in a titration?
If the concentrations of acid molecules are equal and the mole ratio is 1:1, the moles to be neutralised are equal. The weak acid’s equilibrium supplies more H⁺ as existing H⁺ is removed.
Give an example of a strong acid and a weak acid.
Strong: hydrochloric, sulfuric or nitric acid. Weak: ethanoic acid (vinegar), citric acid or carbonic acid.