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Is ammonia a base? Yes, but not under the first definition of a base most students learn. Chemistry has three working definitions of acids and bases, and exam questions often turn on which one applies. This post sets out each definition, shows where each one breaks down, works through five reactions under all three, and lists the slips that cost marks.
Where is this in the syllabus?
The JEE Main 2026 syllabus lists "various concepts of acids and bases (Arrhenius, Bronsted - Lowry and Lewis) and their ionization" under ionic equilibrium. The CBSE Chemistry curriculum for 2026-27 covers acids, bases and salts in the Class 11 Equilibrium unit. The textbook treatment is section 6.10 of the NCERT Class 11 chapter on Equilibrium.
What does each definition say?
Arrhenius. An acid dissociates in water to give hydrogen ions, H+(aq). A base produces hydroxyl ions, OH−(aq). NCERT notes that a bare proton cannot exist freely in water. It bonds to a water molecule to form the hydronium ion, H3O+.
Bronsted-Lowry. An acid is a proton (H+) donor and a base is a proton acceptor. Johannes Brønsted and Thomas Lowry published the idea separately in 1923, Brønsted in Recueil des Travaux Chimiques des Pays-Bas and Lowry in the Journal of the Society of Chemical Industry.
Lewis. NCERT credits G.N. Lewis with the 1923 definition: an acid accepts an electron pair and a base donates one.
Where does each definition fall short?
NCERT names two limits of the Arrhenius idea. It works only in water, and it cannot explain why ammonia, which has no OH group, is a base. Bronsted-Lowry fixes both, because a proton can move between any two species. But it still needs a proton. BF3 has no proton, yet it reacts with NH3 exactly as an acid would. Lewis's definition covers that case, because BF3 accepts NH3's lone pair.
The definitions are nested. Every Arrhenius acid is a Bronsted acid, and every Bronsted base, which must have a lone pair to hold the proton, is a Lewis base. The reverse does not hold.
Worked example: classify five reactions
HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq). Arrhenius acid: yes, HCl gives H+ in water. Bronsted: HCl donates a proton (acid) and H2O accepts it (base). Lewis: H2O donates a lone pair to H+. All three apply.
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq). Arrhenius: NCERT says this definition does not account for ammonia's basicity, since NH3 has no OH of its own. Bronsted: H2O is the acid and NH3 the base. The conjugate pairs are NH4+/NH3 and H2O/OH−.
NH3(g) + HCl(g) → NH4Cl(s). No water is involved, so Arrhenius cannot be used. Bronsted: HCl donates a proton to NH3.
BF3 + :NH3 → F3B:NH3. No proton moves. Only Lewis applies: BF3 is the acid (electron-pair acceptor) and NH3 the base.
H+ + OH− → H2O. Lewis: H+ accepts a pair from OH−. NCERT's Problem 6.15 classifies OH− and F− as Lewis bases and H+ and BCl3 as Lewis acids.
How are conjugate pairs and Ka and Kb linked?
A conjugate acid-base pair differs by exactly one proton. NCERT gives the conjugate bases of HF, H2SO4 and HCO3− as F−, HSO4− and CO3²−. Water, HCO3−, HSO4− and NH3 can each act as an acid or a base, depending on what they react with.
For a conjugate pair in water, Ka × Kb = Kw. NCERT lists Ka for acetic acid as 1.74 × 10⁻⁵. At 298 K, with Kw = 1.0 × 10⁻¹⁴, Kb for the acetate ion is:
Kb = Kw/Ka = (1.0 × 10⁻¹⁴)/(1.74 × 10⁻⁵) = 5.75 × 10⁻¹⁰
That is a very weak base, which fits NCERT's rule that a stronger acid has a weaker conjugate base. NCERT's own check uses ammonia: (5.6 × 10⁻¹⁰) × (1.8 × 10⁻⁵) = 1.0 × 10⁻¹⁴.
Why do the three models get blurred?
Drechsler and Schmidt (2005) examined the chemistry textbooks most used in Swedish upper secondary schools and interviewed six chemistry teachers. The textbooks did not describe the differences between the acid-base models or explain why the Brønsted model was introduced. Some teachers had not reflected on the differences at all. If your notes say "an acid is something that gives H+" without saying which model, you have inherited the same blur. The fix is to label the model every time.
Common mistakes and how to avoid them
Calling NH3 an Arrhenius base. Name it a Bronsted or Lewis base.
Writing conjugate pairs that differ by two protons, such as H2SO4 and SO4²−. The conjugate base of H2SO4 is HSO4−.
Forgetting water's two roles. It is a base towards HCl and an acid towards NH3.
Calling every Lewis acid a proton donor. BF3, AlCl3 and metal ions like Co3+ and Mg2+ have no proton to give.
Using Ka × Kb = Kw for an unrelated acid and base. It holds only for a conjugate pair.
A practice routine
Daily, ten minutes: write three reactions and classify each under all three definitions, naming the pairs.
Twice a week: for five acids from NCERT's Ka table, write the conjugate base and calculate Kb.
Weekly: explain aloud why BF3 is an acid, as if to a Class 10 student.
For the next step, using these ideas to calculate pH, see our post on pH, buffers and ionic equilibrium, and for ranking acid strength in organic compounds, inductive and resonance effects. If conjugate pairs still feel shaky, Fermi's Concept Library covers every concept in chemistry, each with its own lesson and its place in the map.
Today, classify AlCl3 + Cl− → AlCl4− under all three definitions, and say which one fits.
