MHT-CET Chemistry · Ionic Equilibria

Theories of Acids and Bases

The three definitions of acids and bases (Arrhenius, Bronsted-Lowry, Lewis), how to spot the conjugate acid-base pair in an equilibrium, and which species are amphoteric.

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Why this matters

Twelve PYQs here, every one EASY and pure recall — the opening free marks of the chapter. They cluster three ways: match a definition to its theory (Lewis base donates an electron pair; a Bronsted base accepts a proton), pick the conjugate acid-base pair out of an equilibrium reaction, and name the amphoteric species (almost always water). Memorise the three definitions, learn the one-proton rule for conjugate pairs, and this whole subtopic is guaranteed marks.

Concept 1 of 3: The three theories: Arrhenius, Bronsted-Lowry and Lewis

Chemists defined 'acid' and 'base' three times, each definition wider than the last. Arrhenius talks about ions released in water; Bronsted-Lowry talks about giving and taking a proton; Lewis talks about giving and taking an electron pair. The bank tests these as straight identify-the-species recall, so learn each definition well enough to point at the acid and the base in any list.

Definition

The three definitions of acids and bases, each broader than the last:

Arrhenius — an acid dissociates in water to give $\text{H}^+$ ; a base dissociates in water to give $\text{OH}^-$ . Limited to aqueous solutions.
Bronsted-Lowry — an acid is a proton $(\text{H}^+)$ donor; a base is a proton acceptor. Works beyond water.
Lewis — an acid is an electron-pair acceptor; a base is an electron-pair donor. The most general definition.

So $\text{NH}_3$ (lone pair on N) is a Lewis base, while $\text{BF}_3$ , $\text{AlCl}_3$ , $\text{BCl}_3$ and $\text{Cu}^{2+}$ (electron-deficient) are Lewis acids.

Theory	Acid is	Base is	Example acid / base
Arrhenius	Gives $\text{H}^+$ in water	Gives $\text{OH}^-$ in water	$\text{HCl}$ / $\text{NaOH}$
Bronsted-Lowry	Proton $(\text{H}^+)$ donor	Proton $(\text{H}^+)$ acceptor	$\text{HCl}$ / $\text{NH}_3$ A Bronsted base ACCEPTS a proton — this is why $\text{NH}_3$ 'acts as a base when reacted with water' (it takes an $\text{H}^+$ to become $\text{NH}_4^+$ ).
Lewis	Electron-pair acceptor	Electron-pair donor	$\text{BF}_3$ / $\text{NH}_3$ $\text{BCl}_3$ is a Lewis acid but NOT a Bronsted acid — it accepts an electron pair yet has no proton to donate.

Each later theory contains the earlier one; a Lewis acid is the most general kind.

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 21 April Shift I · Q55Easy

Example 1 · Ionic Equilibria · Theories of Acids and Bases

Which of the following is Lewis base?

Match the activity to the right theory

'Donate a pair of electrons' is the Lewis base definition. Do not confuse it with 'accept

\text{H}^+

' (that is a Bronsted base) or 'donate

\text{OH}^-

' (that is an Arrhenius base). A Lewis base gives an electron pair, not a proton or a hydroxide ion.

Lewis acid vs Bronsted acid

\text{BCl}_3

is a Lewis acid but not a Bronsted acid — it accepts an electron pair but has no

\text{H}^+

to donate.

\text{HNO}_3

and

\text{HSO}_4^-

are both Lewis and Bronsted acids because they can donate a proton too.

Drill 5 more on the three theories: arrhenius, bronsted-lowry and lewis

Concept 2 of 3: Conjugate acid-base pairs

When a Bronsted acid donates its proton, what is left is its conjugate base; when a base accepts a proton, it becomes its conjugate acid. So a conjugate acid-base pair is any two species that differ by exactly one

\text{H}^+

— the acid being the one with the extra proton.

Definition

A conjugate acid-base pair differs by exactly one proton $(\text{H}^+)$ :

The conjugate base of an acid = the acid minus one $\text{H}^+$ (and its charge drops by one).
The conjugate acid of a base = the base plus one $\text{H}^+$ (charge rises by one).
In an equilibrium such as $\text{HCl} + \text{NH}_3 \rightleftharpoons \text{NH}_4^+ + \text{Cl}^-$ , the two pairs are $\text{HCl}/\text{Cl}^-$ and $\text{NH}_4^+/\text{NH}_3$ .

Conjugate base from an acid

\text{Acid} \;\rightleftharpoons\; \text{Conjugate base} + \text{H}^+

$\text{Acid}$ proton donor (the species with the extra H+)
$\text{Conjugate base}$ what remains after the acid loses one H+
$\text{H}^+$ the single proton that distinguishes the pair

Worked example

For the equilibrium

\text{H}_2\text{SO}_3 + \text{H}_2\text{O} \rightleftharpoons \text{HSO}_3^- + \text{H}_3\text{O}^+

, identify a conjugate acid-base pair.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 11th May Shift 2 · Q82Easy

Example 2 · Ionic Equilibria · Theories of Acids and Bases

Which among the following is the conjugate base of

\text{HClO}_4

Conjugate base of a strong acid is weak

The conjugate base of a strong acid like

\text{HClO}_4

\text{ClO}_4^-

, which is a very weak base. In general, the stronger the acid, the weaker its conjugate base — don't expect

\text{ClO}_4^-

to behave like a strong base.

Pick species differing by ONE proton — not a random pair

\text{HCl}+\text{NH}_3 \rightleftharpoons \text{NH}_4^+ + \text{Cl}^-

, the pair is

\text{NH}_4^+/\text{NH}_3

, NOT

\text{NH}_4^+/\text{HCl}

\text{Cl}^-/\text{NH}_4^+

. A conjugate pair must be the same core species before and after losing one

\text{H}^+

Drill 3 more on conjugate acid-base pairs

Concept 3 of 3: Amphoteric species

Some species can play either role — donating a proton when faced with a base, or accepting one when faced with an acid. Water is the textbook example, and it is nearly always the bank's answer to 'which is amphoteric'.

Definition

An amphoteric (or amphiprotic) species can act as both an acid and a base:

Water $(\text{H}_2\text{O})$ donates a proton to become $\text{OH}^-$ (acting as an acid) and accepts a proton to become $\text{H}_3\text{O}^+$ (acting as a base). It is the classic amphoteric compound.
Hydrogencarbonate $(\text{HCO}_3^-)$ is also amphoteric — it can lose an $\text{H}^+$ to give $\text{CO}_3^{2-}$ or gain one to give $\text{H}_2\text{CO}_3$ .
Compounds like $\text{HCl}$ (acid only), $\text{NaOH}$ (base only) and $\text{CH}_3\text{COOH}$ (acid only) are not amphoteric.

Species	Amphoteric?	Why
$\text{H}_2\text{O}$	Yes	Gives $\text{OH}^-$ (acid) and takes $\text{H}^+$ to form $\text{H}_3\text{O}^+$ (base) Water is the bank's default answer for 'which species is amphoteric'.
$\text{HCO}_3^-$	Yes	Loses $\text{H}^+$ to $\text{CO}_3^{2-}$ or gains $\text{H}^+$ to $\text{H}_2\text{CO}_3$
$\text{HCl}$	No	Only donates a proton (acid only)
$\text{NaOH}$	No	Only gives $\text{OH}^-$ (base only)
$\text{CH}_3\text{COOH}$	No	Acts only as an acid (donates a proton)

Amphoteric = can be either acid or base; water and

\text{HCO}_3^-

are the standard examples.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 3rd May Shift 2 · Q66Easy

Example 3 · Ionic Equilibria · Theories of Acids and Bases

Which of the following compounds amphoteric in nature?

Water is amphoteric — acetic acid is not

\text{H}_2\text{O}

is amphoteric because it both donates and accepts protons.

\text{CH}_3\text{COOH}

is a distractor here: it can lose a proton to a stronger base, but it does not readily accept one, so it acts only as an acid and is not amphoteric.

Drill 1 more on amphoteric species

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Conjugate acid-base pairs
Conjugate base from an acid
$\text{Acid} \;\rightleftharpoons\; \text{Conjugate base} + \text{H}^+$

Reference tables (2)

The three theories: Arrhenius, Bronsted-Lowry and Lewis3 rows

Theory	Acid is	Base is	Example acid / base
Arrhenius	Gives $\text{H}^+$ in water	Gives $\text{OH}^-$ in water	$\text{HCl}$ / $\text{NaOH}$
Bronsted-Lowry	Proton $(\text{H}^+)$ donor	Proton $(\text{H}^+)$ acceptor	$\text{HCl}$ / $\text{NH}_3$ A Bronsted base ACCEPTS a proton — this is why $\text{NH}_3$ 'acts as a base when reacted with water' (it takes an $\text{H}^+$ to become $\text{NH}_4^+$ ).
Lewis	Electron-pair acceptor	Electron-pair donor	$\text{BF}_3$ / $\text{NH}_3$ $\text{BCl}_3$ is a Lewis acid but NOT a Bronsted acid — it accepts an electron pair yet has no proton to donate.

Each later theory contains the earlier one; a Lewis acid is the most general kind.

Amphoteric species5 rows

Species	Amphoteric?	Why
$\text{H}_2\text{O}$	Yes	Gives $\text{OH}^-$ (acid) and takes $\text{H}^+$ to form $\text{H}_3\text{O}^+$ (base) Water is the bank's default answer for 'which species is amphoteric'.
$\text{HCO}_3^-$	Yes	Loses $\text{H}^+$ to $\text{CO}_3^{2-}$ or gains $\text{H}^+$ to $\text{H}_2\text{CO}_3$
$\text{HCl}$	No	Only donates a proton (acid only)
$\text{NaOH}$	No	Only gives $\text{OH}^-$ (base only)
$\text{CH}_3\text{COOH}$	No	Acts only as an acid (donates a proton)

Amphoteric = can be either acid or base; water and

\text{HCO}_3^-

are the standard examples.

Watch out for (5)

Match the activity to the right theory
→ The three theories: Arrhenius, Bronsted-Lowry and Lewis
Lewis acid vs Bronsted acid
→ The three theories: Arrhenius, Bronsted-Lowry and Lewis
Conjugate base of a strong acid is weak
→ Conjugate acid-base pairs
Pick species differing by ONE proton — not a random pair
→ Conjugate acid-base pairs
Water is amphoteric — acetic acid is not
→ Amphoteric species

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Related notes

Acid-Base Theory, Oxides and Electrolytes (NDA Chemistry) →