Difficulty: Introductory | Prerequisites: Basic chemical formula reading, ions in solution
There are two frameworks for defining acids and bases. The Arrhenius model says acids produce H⁺ in water and bases produce OH⁻. The Brønsted-Lowry model is broader: acids donate protons (H⁺) and bases accept them, which means water itself can act as either. Every acid-base reaction produces a conjugate pair that differs by one proton.
Arrhenius acid
A substance that produces hydrogen ions (H⁺) when dissolved in water. Example: HCl(aq) → H⁺(aq) + Cl⁻(aq).
In simple terms, this means anything that releases H⁺ into a water solution counts as an Arrhenius acid.
Arrhenius base
A substance that produces hydroxide ions (OH⁻) when dissolved in water. Example: NaOH(aq) → Na⁺(aq) + OH⁻(aq).
Think of it as: if it puts OH⁻ into water, it is an Arrhenius base.
Brønsted-Lowry acid
A proton donor. Any species that gives away an H⁺ ion to another species, whether or not it is in water.
In simple terms, if it hands off a proton, it is a Brønsted-Lowry acid.
Brønsted-Lowry base
A proton acceptor. Any species that takes an H⁺ ion from another species.
Think of it as: if it grabs a proton, it is a Brønsted-Lowry base.
Hydronium ion (H₃O⁺)
The ion formed when water accepts a proton: H₂O + H⁺ → H₃O⁺. In aqueous acid-base chemistry, H₃O⁺ is the form H⁺ actually takes in solution.
In simple terms, "H⁺ in water" really means H₃O⁺. They are used interchangeably in many textbooks.
Conjugate acid-base pair
Two species that differ by exactly one proton (H⁺). When an acid donates its proton it becomes its conjugate base, and when a base accepts a proton it becomes its conjugate acid.
Think of it as: the "before and after" versions of any species that gained or lost one H⁺.
Amphoteric
A substance that can act as either an acid or a base depending on what it reacts with. Water (H₂O) and bicarbonate (HCO₃⁻) are common examples.
In simple terms, amphoteric means it can go either way, donating a proton in one reaction and accepting one in another.
Acid: produces H⁺ in aqueous solution
HCl(aq) → H⁺(aq) + Cl⁻(aq)
Base: produces OH⁻ in aqueous solution
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
Limitation: only works for water-based (aqueous) reactions. Does not cover species like NH₃ that act as bases without containing OH⁻ in their formula.
Acid: donates a proton (H⁺) to another species
HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). Here HCl donates its proton to water.
Base: accepts a proton (H⁺) from another species
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Here NH₃ accepts a proton from water.
This model is broader than Arrhenius: it explains why NH₃ is a base (it accepts a proton) even though it contains no OH⁻.
Every Brønsted-Lowry reaction produces two conjugate pairs.
A conjugate pair differs by exactly one H⁺ in their chemical formulae.
Example: NH₃ / NH₄⁺ is one pair (NH₃ gained a proton). H₂O / OH⁻ is the other pair (H₂O lost a proton).
Water (H₂O) is the classic amphoteric substance. It acts as a base when it accepts a proton from HCl, and as an acid when it donates a proton to NH₃.
Bicarbonate (HCO₃⁻) is also amphoteric:
Acting as a base: HCO₃⁻(aq) + H₂O(l) ⇌ OH⁻(aq) + H₂CO₃(aq)
Acting as an acid: HCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq)
Arrhenius acid dissociation
HCl(aq) → H⁺(aq) + Cl⁻(aq)
Arrhenius base dissociation
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
Brønsted-Lowry acid donating a proton to water
HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
Brønsted-Lowry base accepting a proton from water
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
Amphoteric behaviour of HCO₃⁻
As a base: HCO₃⁻(aq) + H₂O(l) ⇌ OH⁻(aq) + H₂CO₃(aq)
As an acid: HCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq)
Note the arrow types: a single arrow (→) indicates the reaction goes to completion (strong acid/base). A double arrow (⇌) indicates equilibrium (weak acid/base or amphoteric reactions).
The Brønsted-Lowry framework is how biochemists describe proton transfers in enzyme active sites and how environmental scientists explain ocean acidification (CO₂ dissolving in seawater acts as an acid, donating protons). Buffer systems in blood rely on conjugate acid-base pairs (carbonic acid / bicarbonate) to keep pH stable.
Students often think that H⁺ floats around freely in water. It does not. In aqueous solution, H⁺ is always bonded to water as H₃O⁺ (hydronium). The two notations are interchangeable, but exams sometimes test whether you know they are the same thing.
Students often assume that a base must contain OH⁻ in its chemical formula. Under the Brønsted-Lowry definition it does not. NH₃ is a base because it accepts a proton, despite having no hydroxide group.
Students sometimes confuse "amphoteric" with "amphiprotic." For this course they are effectively the same, but strictly, amphiprotic refers to species that can donate or accept a proton, while amphoteric is the broader term for anything that reacts with both acids and bases.
Students often forget that the conjugate of a strong acid is a negligibly weak base (Cl⁻ from HCl, for instance, does not meaningfully accept protons back).
⚠️ Exams frequently ask you to identify every conjugate acid-base pair in a given reaction. Practise labelling both pairs.
⚠️ You may be asked to write two reactions for an amphoteric species (one as an acid, one as a base). HCO₃⁻ and H₂O are the most commonly tested.
⚠️ Know the difference between Arrhenius and Brønsted-Lowry definitions cold. A typical question: "Which definition explains why NH₃ is a base?" Answer: Brønsted-Lowry, because it accepts a proton.
⚠️ Arrow direction matters. A single forward arrow means complete dissociation (strong). A double equilibrium arrow means partial (weak or amphoteric).
True or false: An Arrhenius base must contain OH⁻ in its chemical formula. (True)
True or false: Under the Brønsted-Lowry model, water can act as both an acid and a base. (True)
Fill in the blank: A conjugate acid-base pair differs by exactly one ______. (proton, or H⁺)
True or false: H⁺ and H₃O⁺ refer to the same species in aqueous solution. (True)
Fill in the blank: A substance that can act as either an acid or a base is called ______. (amphoteric)
Q: In the reaction NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), identify the two conjugate acid-base pairs.
A: Pair 1: NH₃ (base) and NH₄⁺ (conjugate acid). Pair 2: H₂O (acid) and OH⁻ (conjugate base).
Q: Explain why NH₃ is classified as a base under Brønsted-Lowry but not under the Arrhenius definition.
A: Brønsted-Lowry defines a base as a proton acceptor. NH₃ accepts a proton from water to form NH₄⁺, so it qualifies. The Arrhenius definition requires a base to produce OH⁻ directly from its own formula, and NH₃ does not contain OH⁻.
Q: Write two reactions showing that HCO₃⁻ is amphoteric.
A: As a base: HCO₃⁻(aq) + H₂O(l) ⇌ OH⁻(aq) + H₂CO₃(aq). As an acid: HCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq).
Q: What is the hydronium ion, and why is it more accurate to write H₃O⁺ than H⁺ in aqueous reactions?
A: The hydronium ion (H₃O⁺) is formed when a bare proton bonds to a water molecule. A free H⁺ does not exist in water on its own; it is always associated with H₂O. Writing H₃O⁺ reflects what is physically present in solution.
This material connects directly to pH and pOH calculations (Ch. 12), because pH measures the concentration of the hydronium ion you have just learnt about. It also connects to strong vs. weak acids and bases: whether a reaction goes to completion or reaches equilibrium depends on the strength of the acid or base donating/accepting the proton. Buffer systems, covered later in Ch. 12, rely on conjugate acid-base pairs to resist pH changes.
Arrhenius acid, Arrhenius base, Brønsted-Lowry acid, Brønsted-Lowry base, proton donor, proton acceptor, hydronium ion, H₃O⁺, conjugate acid, conjugate base, conjugate acid-base pair, amphoteric, amphiprotic, HCO₃⁻ bicarbonate, water as acid and base, acid-base definitions, CHEM 2, general chemistry, Chapter 12