Acid and Base Definitions

Over the past century, chemists have developed three definitions to classify compounds as acids or bases — Arrhenius, Brønsted-Lowry, and Lewis — and each one is more inclusive than the last. The MCAT expects you to know all three, how they relate to each other, what it means for a species to be amphoteric or amphiprotic, and how acids get their names.

Key Takeaways

• Arrhenius: acids produce H⁺ (as H₃O⁺) and bases produce OH⁻ in water — the most restrictive definition, identifiable by formula (H... for acids, ...OH for bases).

• Brønsted-Lowry: acids donate protons, bases accept protons — works outside water and generates conjugate acid-base pairs.

• Lewis: acids accept electron pairs, bases donate electron pairs, forming coordinate covalent bonds — the most inclusive definition (e.g., BF₃, AlCl₃).

• Inclusiveness hierarchy: every Arrhenius acid/base is Brønsted-Lowry, and every Brønsted-Lowry acid/base is Lewis — not the reverse.

• Amphoteric: can act as acid or base depending on environment. Amphiprotic: a subset of amphoteric that specifically gains or loses protons. Water is the classic example of both; Al(OH)₃ and ZnO are amphoteric but not amphiprotic.

• Binary acids: "hydro-" + root + "-ic acid" (from an -ide anion). Oxyacids: -ite → -ous acid, -ate → -ic acid, with hypo-/per- prefixes retained.

The Arrhenius Definition

An Arrhenius acid is a compound that dissociates in water to produce hydrogen ions (H⁺), which associate with water molecules to form hydronium ions (H₃O⁺):

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq)

Here, HA is the Arrhenius acid, and A⁻ is its conjugate base. An Arrhenius base is a compound that dissociates in water to produce hydroxide ions (OH⁻):

B(aq) + H₂O(l) ⇌ OH⁻(aq) + HB⁺(aq)

Arrhenius acids and bases are easy to spot from their formula: acids start with H (HCl, HNO₃, H₂SO₄), and bases end in OH (NaOH, Ca(OH)₂, Fe(OH)₃). This makes the Arrhenius definition useful for quick identification, but it's also the most restrictive of the three — it only applies to aqueous solutions and only to compounds that literally contain H or OH. On Test Day, Arrhenius acids and bases are usually mentioned in comparison to the broader definitions below.

The Brønsted-Lowry Definition

A Brønsted-Lowry acid is a species that donates a proton (H⁺); a Brønsted-Lowry base is a species that accepts one:

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq) (HA donates a proton to water)

B(aq) + H₂O(l) ⇌ BH⁺(aq) + OH⁻(aq) (B accepts a proton from water)

This definition improves on Arrhenius in two ways. First, it isn't limited to aqueous solutions — ammonia (NH₃) and fluoride ion (F⁻) are Brønsted-Lowry bases because they can accept a proton, even though neither one produces hydroxide ions in water. Second, it covers more species: water itself can act as either a Brønsted-Lowry acid (donating a proton) or a base (accepting one), depending on what it's reacting with.

Brønsted-Lowry acids and bases always come in conjugate acid-base pairs. When an acid donates a proton, what's left behind is its conjugate base; when a base accepts a proton, what results is its conjugate acid. The clearest example is the autoionization of water:

H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)

One water molecule donates a proton (acting as the acid) and the other accepts it (acting as the base). H₂O and H₃O⁺ are one conjugate pair; H₂O and OH⁻ are the other.

The Lewis Definition

A Lewis acid is an electron pair acceptor; a Lewis base is an electron pair donor. Proposed by Gilbert Lewis around the same time as the Brønsted-Lowry concept, this definition shifts the focus from proton transfer to electron pair transfer — the acid and base form a coordinate covalent bond, in which the base supplies both electrons.

This is the key contrast between the two frameworks: Brønsted-Lowry tracks where a proton goes, while Lewis tracks where an electron pair goes. In a typical Brønsted-Lowry reaction the acid donates a proton and the base accepts it; described in Lewis terms, the base is donating an electron pair to the acid, and the two descriptions are really pointing at the same event. Compounds like BF₃ and AlCl₃ are Lewis acids — they accept an electron pair readily — but they have no hydrogen to donate, so they don't qualify as Brønsted-Lowry or Arrhenius acids at all. On the MCAT, Lewis acids show up often in organic chemistry, where they're frequently used as catalysts.

How the Three Definitions Relate

The Lewis definition is the most inclusive of the three, and the relationship is a strict hierarchy:

• Every Arrhenius acid is also a Brønsted-Lowry acid, and every Brønsted-Lowry acid is also a Lewis acid.

• Every Arrhenius base is also a Brønsted-Lowry base, and every Brønsted-Lowry base is also a Lewis base.

• The reverse isn't true — BF₃ and AlCl₃ above are Lewis acids that fail both narrower definitions.

Each definition simply builds on the one before it, letting the Lewis definition capture reactions — like those involving coordinate covalent bond formation without any hydrogen ions at all — that the other two can't describe.

Amphoteric and Amphiprotic Species

An amphoteric species reacts like an acid when it's in a basic environment and like a base when it's in an acidic environment. A species that is amphoteric specifically because it can either gain or lose a proton (the Brønsted-Lowry sense) is also called amphiprotic.

Water is the MCAT's go-to example of both. When water reacts with a base, it behaves as an acid, donating a proton. When water reacts with an acid, it behaves as a base, accepting a proton. This dual behavior is what makes water central to acid-base chemistry.

Other amphoteric/amphiprotic species worth knowing:

• Amino acids — the carboxyl group (–COOH) can donate a proton, while the amino group (–NH₂) can accept one, giving amino acids both acidic and basic character.

• Partially deprotonated polyprotic acids, such as bicarbonate (HCO₃⁻) and bisulfate (HSO₄⁻) — each still has a proton it can lose but can also accept one to re-form the fully protonated acid.

• Certain metal oxides and hydroxides, such as aluminum hydroxide (Al(OH)₃) and zinc oxide (ZnO) — these react with both acids and bases, making them amphoteric, but the reactions don't necessarily involve direct proton transfer, so they aren't classified as amphiprotic.

That last point is the key distinction to keep straight: every amphiprotic species is amphoteric, but not every amphoteric species is amphiprotic — amphiprotic specifically requires proton donation and acceptance.

Naming Acids: Binary Acids and Oxyacids

Acid nomenclature follows the name of the parent anion.

Binary acids contain hydrogen and one other element. When the parent anion's name ends in -ide, the acid name takes the prefix "hydro-" and the suffix "-ic acid":

• Fluoride (F⁻) → hydrofluoric acid (HF)

• Chloride (Cl⁻) → hydrochloric acid (HCl)

• Bromide (Br⁻) → hydrobromic acid (HBr)

Oxyacids contain hydrogen, oxygen, and another element, and are named from their parent oxyanion (a polyatomic ion containing oxygen):

• If the oxyanion ends in -ite (fewer oxygen atoms), the acid ends in "-ous acid."

• If the oxyanion ends in -ate (more oxygen atoms), the acid ends in "-ic acid."

• Any hypo- or per- prefix on the oxyanion carries over unchanged into the acid name.

Common MCAT Mistakes

• Assuming Arrhenius, Brønsted-Lowry, and Lewis are three unrelated definitions. They're nested: every Arrhenius acid/base is Brønsted-Lowry, and every Brønsted-Lowry acid/base is Lewis. The definitions expand outward, they don't compete.

• Forgetting a species can be a Lewis acid without any hydrogen at all. BF₃ and AlCl₃ are Lewis acids (electron pair acceptors) but fail the Arrhenius and Brønsted-Lowry definitions entirely, since neither one has an acidic proton to donate.

• Treating "amphoteric" and "amphiprotic" as synonyms. Every amphiprotic species is amphoteric, but not every amphoteric species is amphiprotic — Al(OH)₃ and ZnO react with both acids and bases without necessarily transferring a proton directly, so they're amphoteric but not amphiprotic.

• Mixing up -ite/-ate and -ous/-ic when naming oxyacids. The pattern is parallel, not identical: an oxyanion ending in -ite (fewer oxygens) gives an -ous acid, and one ending in -ate (more oxygens) gives an -ic acid — hypo-/per- prefixes just carry straight over.

MCAT-Style Concept Check

Question: BF₃ readily accepts an electron pair from ammonia (NH₃) to form a coordinate covalent bond. Based on this behavior alone, which classification applies to BF₃?

• A) Arrhenius acid only

• B) Brønsted-Lowry acid only

• C) Lewis acid only

• D) Lewis base only

Answer: C

Explanation: BF₃ has no hydrogen to donate, so it cannot be an Arrhenius acid (no H⁺ released in water) or a Brønsted-Lowry acid (no proton to donate). But because it accepts an electron pair from NH₃, it fits the Lewis definition of an acid — an electron pair acceptor. This is the classic example of a species that's a Lewis acid without qualifying under either narrower definition.

FAQ

What's the difference between the Arrhenius, Brønsted-Lowry, and Lewis definitions of acids and bases?

Arrhenius acids/bases produce H⁺ or OH⁻ in water; Brønsted-Lowry acids/bases donate or accept a proton, in or out of water; Lewis acids/bases accept or donate an electron pair, the broadest definition. Each definition includes everything the previous one covers, plus more.

Why is BF₃ considered a Lewis acid but not a Brønsted-Lowry acid?

BF₃ has no hydrogen atom to donate as a proton, so it can't satisfy the Brønsted-Lowry (or Arrhenius) definition of an acid. But it readily accepts an electron pair to form a coordinate covalent bond, which is exactly what makes something a Lewis acid.

What's the difference between amphoteric and amphiprotic?

Amphoteric means a species can act as either an acid or a base depending on its environment. Amphiprotic is a narrower term for species that are amphoteric specifically because they can gain or lose a proton — every amphiprotic species is amphoteric, but species like Al(OH)₃ are amphoteric without being classified as amphiprotic.

How do you name an oxyacid from its oxyanion?

Identify the oxyanion, then swap its suffix: -ite becomes -ous acid (e.g., chlorite → chlorous acid), and -ate becomes -ic acid (e.g., chlorate → chloric acid). Any hypo- or per- prefix on the oxyanion stays the same in the acid name.