"Strong acids and bases" is a list question in Class 10 and GCSE, a definition question in IGCSE and MYP, and a calculation question at A-level, IB and AP, and the same misunderstanding runs through all of them: that strong means concentrated or dangerous. It means neither. This page gives the two lists with uses, the definition in terms of ionisation, the pH and conductivity evidence, the equations, the acid dissociation constant for the older students, and the printable one-page version.
Download the strong acids and bases list as a PDF (both lists with formulas and uses, the definition and the pH comparison; A4, one page, free).
The seven strong acids
| Acid | Formula | Where you meet it |
|---|---|---|
| Hydrochloric acid | HCl | Stomach acid, lab reagent |
| Hydrobromic acid | HBr | Lab reagent |
| Hydroiodic acid | HI | Lab reagent, strongest of the three |
| Nitric acid | HNO3 | Fertilisers, explosives |
| Sulfuric acid | H2SO4 | Car batteries, industry (first proton only) |
| Perchloric acid | HClO4 | Analytical chemistry |
| Chloric acid | HClO3 | Rarely met at school |
Sulfuric acid is strong for its first proton (H2SO4 → H+ + HSO4−, complete) and weak for its second (HSO4− ⇌ H+ + SO42−, partial). At GCSE and Class 10 it is simply "strong".
Everything not on this list is a weak acid: ethanoic (acetic) acid in vinegar, citric acid in lemons, carbonic acid in fizzy drinks, phosphoric acid in cola, methanoic acid in ant stings, hydrofluoric acid (dangerous, but chemically weak), and every other organic acid.
The 7 strong acids ranked: approximate pKa
| Rank | Acid | Approximate pKa | Note |
|---|---|---|---|
| 1 | Perchloric acid, HClO4 | about −10 | the strongest common acid |
| 2 | Hydroiodic acid, HI | about −10 | strongest of the three hydrohalic acids |
| 3 | Hydrobromic acid, HBr | about −9 | |
| 4 | Hydrochloric acid, HCl | about −6 | |
| 5 | Sulfuric acid, H2SO4 | about −3 (first proton); 1.99 (second) | strong for the first proton only |
| 6 | Nitric acid, HNO3 | about −1.4 | |
| 7 | Chloric acid, HClO3 | about −1 | rarely met at school |
| for comparison | hydrofluoric acid, HF | 3.2 | weak |
| ethanoic (acetic) acid, CH3COOH | 4.76 | weak | |
| carbonic acid, H2CO3 | 6.35 | weak |
A negative pKa means the acid is more than fully ionised in water in the sense that water cannot tell the strong acids apart (the levelling effect), which is why the values for the top four are measured in other solvents and differ between textbooks by a unit or two; the order is what matters. The formulas and molar masses of all of these, and seventy other common compounds, are on the chemical formulas chart.
The eight strong bases
| Base | Formula | Where you meet it |
|---|---|---|
| Lithium hydroxide | LiOH | Batteries, CO2 scrubbers |
| Sodium hydroxide | NaOH | Caustic soda, drain cleaner, soap |
| Potassium hydroxide | KOH | Caustic potash, alkaline batteries |
| Rubidium and caesium hydroxide | RbOH, CsOH | Rare |
| Calcium hydroxide | Ca(OH)2 | Limewater, slaked lime (strong but only slightly soluble) |
| Strontium and barium hydroxide | Sr(OH)2, Ba(OH)2 | Lab reagents |
Calcium hydroxide is the one that confuses: it is a strong base (what dissolves is fully dissociated) but only slightly soluble, so limewater has a lower pH (about 12.4) than sodium hydroxide solution of the same nominal concentration would. Strong and soluble are different properties.
The weak bases: ammonia (NH3), the amines, sodium carbonate and hydrogencarbonate, and the metal oxides that react with water only partly.
What strong means: complete ionisation
An acid is a substance that releases hydrogen ions (H+, more precisely H3O+) in water. A strong acid ionises completely: every molecule gives up its H+.
HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq), or in the short form HCl → H+ + Cl−. A one-way arrow.
A weak acid ionises partly and sets up an equilibrium, with most molecules still intact:
CH3COOH ⇌ H+ + CH3COO−. A two-way arrow, lying well to the left; in 0.1 mol/dm3 ethanoic acid about 1.3 % of the molecules are ionised.
The same for bases: NaOH → Na+ + OH− (strong, complete) against NH3 + H2O ⇌ NH4+ + OH− (weak, partial).
The arrow is the definition. In an exam, a strong acid written with an equilibrium arrow, or a weak acid written with a one-way arrow, loses the mark even if the formula is right.
Strong is not concentrated
| Dilute | Concentrated | |
|---|---|---|
| Strong | 0.01 mol/dm3 HCl: fully ionised, few ions per litre, pH 2 | 10 mol/dm3 HCl: fully ionised, many ions per litre, very corrosive |
| Weak | 0.01 mol/dm3 ethanoic acid: partly ionised, very few ions, pH about 3.4 | Glacial ethanoic acid (17 mol/dm3): partly ionised, still corrosive |
Strong/weak is about the fraction ionised. Concentrated/dilute is about the amount per litre. All four boxes exist. The sentence to write: strength is the degree of ionisation; concentration is the number of moles of acid per dm3 of solution.
The evidence: pH, conductivity, reaction rate
Take a strong and a weak acid at the same concentration, 0.1 mol/dm3:
| Measurement | 0.1 mol/dm3 HCl (strong) | 0.1 mol/dm3 CH3COOH (weak) | Why |
|---|---|---|---|
| pH | 1 | about 2.9 | Strong gives 0.1 mol/dm3 H+; weak gives about 0.0013 |
| Electrical conductivity | High (bulb bright) | Low (bulb dim) | More ions to carry the current |
| Rate with magnesium | Fast fizzing | Slow fizzing | Rate depends on H+ concentration |
| Volume of 0.1 mol/dm3 NaOH to neutralise 25 cm3 | 25 cm3 | 25 cm3 | Same amount of acid: the weak acid keeps ionising as its H+ is used up |
The last row is the one that separates understanding from recitation. A weak acid needs the same volume of alkali as a strong acid of the same concentration, because neutralisation pulls the equilibrium to the right until every molecule has reacted. Strength changes the pH and the rate, not the amount of base needed.
For A-level, IB and AP: Ka and pKa
The strength of a weak acid is measured by its acid dissociation constant, Ka = [H+][A−] / [HA], and pKa = −log10 Ka. The bigger Ka (the smaller pKa), the stronger the acid.
| Acid | Ka (mol/dm3) | pKa | Class |
|---|---|---|---|
| Hydrochloric acid | about 107 | about −7 | Strong |
| Sulfuric acid (first proton) | about 103 | about −3 | Strong |
| Nitric acid | about 25 | about −1.4 | Strong |
| Hydrofluoric acid | 6.3 × 10−4 | 3.2 | Weak |
| Methanoic acid | 1.8 × 10−4 | 3.75 | Weak |
| Ethanoic acid | 1.75 × 10−5 | 4.76 | Weak |
| Carbonic acid (first proton) | 4.5 × 10−7 | 6.35 | Weak |
| Hydrocyanic acid | 6.2 × 10−10 | 9.2 | Very weak |
For a strong acid, [H+] = the acid concentration, so pH = −log10(c). For a weak acid, [H+] = √(Ka × c), so 0.1 mol/dm3 ethanoic acid has [H+] = √(1.75 × 10−5 × 0.1) = 1.3 × 10−3 and pH = 2.88. That one calculation, done from memory, is the A-level version of this whole topic.
A strong acid has a weak conjugate base (Cl− has no tendency to take a proton back); a weak acid has a stronger conjugate base (ethanoate does). This is why sodium ethanoate solution is slightly alkaline and sodium chloride solution is neutral.
The mistakes that lose marks
- Writing "strong acid = concentrated acid" or "weak acid = dilute acid".
- Using ⇌ for a strong acid or → for a weak one.
- Saying a weak acid needs less alkali to neutralise it.
- Calling hydrofluoric acid strong because it is dangerous (it is weak, Ka 6 × 10−4).
- Calling calcium hydroxide weak because limewater is dilute (it is strong and slightly soluble).
- Forgetting that pH is logarithmic: pH 1 has ten times the H+ of pH 2, a hundred times that of pH 3.
How we teach acids and bases
A 1-on-1 online class, one tutor, one student: the lists first, then the definition with the arrows, then the four-box strong/concentrated grid drawn by the student, then the calculations for the older ones. The topic takes two lessons at Class 10 or GCSE and four at A-level with Ka, buffers and titration curves. The first 30-minute class is free: send the topic and the last test on WhatsApp or the contact page. The definitions of solution and concentration that this topic rests on are in solution, solute and solvent; the subject page is online chemistry tutoring.
Questions parents ask
1What are the seven strong acids?
2What are the strong bases?
3What is the difference between a strong acid and a concentrated acid?
4Why does a strong acid have a lower pH than a weak acid of the same concentration?
See how we teach this, 1-on-1 online →
Shobha
Founder and lead tutor, Science with Shobha
Teaching maths, science and English to children online since 2018. 500+ students, 20+ tutors, families in nine countries.