**i.** What are acids and bases according to Arrhenius theory ?

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Q. i. What are acids and bases according to Arrhenius theory ?

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✓ Free question

Step 1. Arrhenius theory (section 3.3.1) defines acids and bases by what ion they release specifically in AQUEOUS solution.

Step 2. An acid is a hydrogen-containing substance that yields H+ ions in water: $HCl(aq)\xrightarrow{water}H^+(aq)+Cl^-(aq)$ , $CH_3COOH(aq)\xrightarrow{water}CH_3COO^-(aq)+H^+(aq)$ .

Step 3. A base is a substance containing an OH group that yields OH- ions in water: $NaOH(aq)\rightleftharpoons Na^+(aq)+OH^-(aq)$ , $NH_4OH(aq)\rightleftharpoons NH_4^+(aq)+OH^-(aq)$ .

Step 4. The theory is limited to aqueous solutions and cannot explain the basicity of OH-group-free substances like NH3 or Na2CO3 -- a limitation resolved by the later Bronsted-Lowry and Lewis theories.

✓Final answer

Arrhenius acids release H+ in water; Arrhenius bases (which must contain an OH group) release OH- in water.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set ANNUAL3 marks
$^{-4}$
›Reveal solutionSolution

$pOH=-\log(2\times10^{-4})=3.7$ ; $pH=14-3.7=10.3$ .

Conjugate acid-base pair: by the Bronsted–Lowry theory, when an acid donates a proton it becomes its conjugate base, and this acid–base pair (differing only by $H^+$ ) is called a conjugate acid-base pair, e.g. $CH_3COOH$ (acid) / $CH_3COO^-$ (its conjugate base).

pH calculation: given $[OH^-] = 2\times10^{-4}\ mol\,dm^{-3}$

$pOH = -\log(2\times10^{-4}) = 4 - \log 2 = 4 - 0.301 = 3.7$

Using $pH + pOH = 14$ :

$pH = 14 - 3.7 = 10.3$

✓Final answer
$CH_3COOH/CH_3COO^-$
CBSE 2024Set ANNUAL3 marks
$Q. Define acids and bases according to Bronsted-Lowry theory. Derive relationship between pH and pOH.$
›Reveal solutionSolution

Bronsted acid donates H $^+$ , base accepts it. From $K_w=[H^+][OH^-]$ : $pH+pOH=pK_w=14$ at 25°C.

Bronsted–Lowry theory: an acid is a species that donates a proton ( $H^+$ ) to another species; a base is a species that accepts a proton. Every acid, on losing a proton, forms its conjugate base, and every base, on gaining a proton, forms its conjugate acid.

Relationship between pH and pOH: water undergoes self-ionisation:

$H_2O \rightleftharpoons H^+ + OH^-$ , with $K_w = [H^+][OH^-] = 10^{-14}$ at 25°C.

Taking $-\log$ of both sides:

$-\log K_w = -\log[H^+] - \log[OH^-]$

$pK_w = pH + pOH$

Since $K_w = 10^{-14}$ at 25°C, $pK_w = 14$ , so:

$pH + pOH = 14$

✓Final answer
$H^+$

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Q.i. What are acids and bases according to Arrhenius theory ?

Concept understanding — Ionization of Acids and Bases

Q.i. What are acids and bases according to Arrhenius theory ?

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