Strong Acids and Bases: The Short List You Have to Memorize

Why Strong and Weak Matters

In aqueous solution, an acid donates a proton (H⁺) and a base accepts one. The difference between a strong and a weak acid or base has nothing to do with how dangerous it is — it is about how completely the species dissociates in water.

• A strong acid dissociates essentially 100% in water. Every HX molecule you add produces one H₃O⁺ and one X⁻.
• A weak acid only partially dissociates. Most of the molecules stay intact, and an equilibrium sits between the protonated and deprotonated forms.

The same distinction applies to bases. Strong bases fully dissociate into their cation and hydroxide (OH⁻). Weak bases accept protons reversibly, setting up an equilibrium.

Because there are very few strong acids and strong bases, and many weak ones, chemistry courses simply ask you to memorize the strong list. Anything not on the list is weak.

The Strong Acids

There are seven commonly taught strong acids. Every intro chemistry course uses this same list.

| Name | Formula | |---|---| | Hydrochloric acid | HCl | | Hydrobromic acid | HBr | | Hydroiodic acid | HI | | Nitric acid | HNO₃ | | Sulfuric acid | H₂SO₄ (first proton only) | | Perchloric acid | HClO₄ | | Chloric acid | HClO₃ |

A few notes on this list:

• HF is not a strong acid, despite the rest of HCl, HBr, HI being strong. The H–F bond is unusually strong, so HF only partially dissociates. This is the single most common trick question in intro acid-base chemistry.
• Sulfuric acid is a diprotic acid — it has two ionizable hydrogens. The first dissociation is strong. The second (HSO₄⁻ → H⁺ + SO₄²⁻) is weak. For most intro-chemistry problems, treating H₂SO₄ as strong is fine.
• Chloric acid (HClO₃) is sometimes included and sometimes left out depending on the textbook. Perchloric acid (HClO₄) is always included.

The Strong Bases

Strong bases are essentially the soluble hydroxides of Group 1 (alkali metals) and the heavier members of Group 2 (alkaline earth metals).

| Name | Formula | |---|---| | Lithium hydroxide | LiOH | | Sodium hydroxide | NaOH | | Potassium hydroxide | KOH | | Rubidium hydroxide | RbOH | | Cesium hydroxide | CsOH | | Calcium hydroxide | Ca(OH)₂ | | Strontium hydroxide | Sr(OH)₂ | | Barium hydroxide | Ba(OH)₂ |

A few notes:

• All Group 1 hydroxides are strong bases.
• Magnesium hydroxide (Mg(OH)₂) is sometimes listed as a strong base and sometimes not. It is a strong electrolyte — what dissolves is fully dissociated — but it has very low solubility in water, so the concentration of OH⁻ stays small. Different textbooks make different calls; read your course's convention.
• Ammonia (NH₃) is a weak base, not a strong one. Students sometimes expect it on the strong list because it is so common. It is not.

A Mnemonic for the Strong Acids

A common mnemonic to memorize the strong acids:

"Bring No Clorox Here, It Is So Bad"

• Bring → HBr
• No → HNO₃
• Clorox → HCl, HClO₃, HClO₄
• Here → (placeholder)
• Is → HI
• So → H₂SO₄
• Bad → (placeholder)

There are many variations. The specific mnemonic matters less than picking one and drilling it until the seven acids come up in seconds.

How to Tell a Weak Acid at a Glance

If a compound looks like it could be an acid but is not on the strong list, assume it is weak. Common weak acids you will encounter:

• Hydrofluoric acid (HF) — the famous exception
• Acetic acid (CH₃COOH) — vinegar
• Carbonic acid (H₂CO₃) — CO₂ in water
• Phosphoric acid (H₃PO₄) — in cola
• Citric acid (C₆H₈O₇) — in citrus fruits
• Hydrocyanic acid (HCN) — cyanide in water
• Boric acid (H₃BO₃) — mild antiseptic

Common weak bases:

• Ammonia (NH₃)
• Methylamine (CH₃NH₂) and other amines
• Pyridine (C₅H₅N)
• Aniline (C₆H₅NH₂)

Why Classification Affects Calculations

• For a strong acid, [H₃O⁺] equals the initial acid concentration (for monoprotic acids). You can calculate pH directly.
• For a weak acid, you need the acid dissociation constant Kₐ and an equilibrium calculation. The [H₃O⁺] is much smaller than the initial concentration.
• For a strong base, [OH⁻] comes from full dissociation. Use [OH⁻] to find pOH, then pH = 14 − pOH at 25 °C.
• For a weak base, use K_b and an equilibrium calculation.

If you misclassify the acid or base, every subsequent calculation is wrong. That is why this short list is worth memorizing early and well.

A Quick Self-Test

Cover the formulas and try to list all seven strong acids from memory. Then do the same for the eight strong bases (or nine if you count Mg(OH)₂). If you can write both lists in under a minute, you are set for every acid-base problem you will see at the intro level.