酸suān

Core Concept

An acid (酸) is a compound that ionizes in water to produce $\mathrm{H^+}$ as its only cation.

That final clause is the whole definition, and CSCA questions test it precisely. $\mathrm{NaHSO_4}$ produces $\mathrm{H^+}$ in solution, but it also produces $\mathrm{Na^+}$ — so it is a salt, not an acid. Producing hydrogen ions is necessary but not sufficient.

$\mathrm{HCl \rightarrow H^+ + Cl^-}$

$\mathrm{H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}}$

Classification

Strong vs Weak

	Strong acid 强酸	Weak acid 弱酸
Ionization	complete	partial, reversible
Equation arrow	$\rightarrow$	$\rightleftharpoons$
Examples	$\mathrm{HCl}$, $\mathrm{H_2SO_4}$, $\mathrm{HNO_3}$	$\mathrm{CH_3COOH}$, $\mathrm{H_2CO_3}$, $\mathrm{H_2S}$

Writing the wrong arrow is a marked error on CSCA free-response items.

By Number of Ionizable Hydrogens

• Monoprotic (一元酸): $\mathrm{HCl}$, $\mathrm{HNO_3}$
• Diprotic (二元酸): $\mathrm{H_2SO_4}$, $\mathrm{H_2CO_3}$
• Triprotic (三元酸): $\mathrm{H_3PO_4}$

A trap worth memorising: $\mathrm{CH_3COOH}$ (acetic acid) has four hydrogens but is monoprotic — only the one in the $\mathrm{-COOH}$ group ionizes. Count ionizable hydrogens, not total hydrogens.

Oxidizing vs Non-oxidizing

Concentrated $\mathrm{H_2SO_4}$ and $\mathrm{HNO_3}$ (any concentration) are oxidizing acids: they react with metals via the central atom, not $\mathrm{H^+}$, so they release $\mathrm{SO_2}$ or $\mathrm{NO}/\mathrm{NO_2}$ rather than $\mathrm{H_2}$.

Characteristic Reactions

1. With indicators: turns blue litmus red (紫色石蕊变红)
2. With active metals: $\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow}$
3. With basic oxides: $\mathrm{CuO + 2HCl \rightarrow CuCl_2 + H_2O}$
4. With bases (neutralization): $\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}$
5. With certain salts: $\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow}$

Reactions 2–5 are the "acid, base, oxide, salt" web that 酸/碱/盐/氧化物 questions — collectively the largest knowledge-point cluster in CSCA chemistry — are built on.

CSCA Practice Problems

💡 Note: These problems follow the CSCA exam syllabus and Chinese standardized test formats.

Example 1: Basic (Difficulty ★★☆☆☆)

Which of the following is an acid?

Options:

• A. $\mathrm{NaHSO_4}$
• B. $\mathrm{NaHCO_3}$
• C. $\mathrm{H_2CO_3}$
• D. $\mathrm{NaOH}$

Detailed Solution:

Apply the definition — the only cation produced must be $\mathrm{H^+}$:

• A: $\mathrm{NaHSO_4 \rightarrow Na^+ + H^+ + SO_4^{2-}}$ — produces $\mathrm{Na^+}$ too, so it is a salt
• B: also gives $\mathrm{Na^+}$ — a salt
• C: $\mathrm{H_2CO_3 \rightleftharpoons 2H^+ + CO_3^{2-}}$ — only $\mathrm{H^+}$ ✓
• D: produces $\mathrm{OH^-}$ — a base

Option A is the intended trap: it does release $\mathrm{H^+}$ and its solution is acidic, but that does not make it an acid.

Answer: C

---

Example 2: Intermediate (Difficulty ★★★☆☆)

Equal volumes of $0.1\ \mathrm{mol/L}$ $\mathrm{HCl}$ and $0.1\ \mathrm{mol/L}$ $\mathrm{CH_3COOH}$ are compared. Which statement is correct?

Options:

• A. Both have the same pH
• B. $\mathrm{HCl}$ has the lower pH
• C. $\mathrm{CH_3COOH}$ has the lower pH
• D. Neither conducts electricity

Detailed Solution:

1. Both are monoprotic at the same concentration, so both contain the same total amount of acid.

2. $\mathrm{HCl}$ is strong and ionizes completely: $c(\mathrm{H^+}) = 0.1\ \mathrm{mol/L}$.

3. $\mathrm{CH_3COOH}$ is weak and ionizes only partially: $c(\mathrm{H^+}) < 0.1\ \mathrm{mol/L}$.

4. Lower pH means higher $c(\mathrm{H^+})$, so $\mathrm{HCl}$ has the lower pH.

Answer: B

Worth noting: if instead they were neutralized by $\mathrm{NaOH}$, both would need the same amount — concentration of $\mathrm{H^+}$ differs, but total ionizable hydrogen does not.

Common Mistakes

❌ Mistake 1: Calling any $\mathrm{H^+}$-producing compound an acid

$\mathrm{NaHSO_4}$ is an acidic salt. The cation test is "only $\mathrm{H^+}$".

❌ Mistake 2: Counting all hydrogens as ionizable

$\mathrm{CH_3COOH}$ is monoprotic despite having four H atoms.

❌ Mistake 3: Using $\rightarrow$ for a weak acid

Weak acids need the reversible arrow $\rightleftharpoons$.

❌ Mistake 4: Expecting $\mathrm{H_2}$ from concentrated $\mathrm{H_2SO_4}$ or $\mathrm{HNO_3}$

Oxidizing acids give $\mathrm{SO_2}$, $\mathrm{NO}$, or $\mathrm{NO_2}$ instead.

Study Tips

1. ✅ Write the ionization equation whenever asked to classify — it settles acid vs salt instantly
2. ✅ Memorise the six common strong acids/bases; treat everything else as weak
3. ✅ Count only ionizable hydrogens
4. ✅ Distinguish "strong" from "concentrated" — they are unrelated properties
5. ✅ Learn the five characteristic reactions as a block; they recur constantly

---

💡 Exam Tip: 酸, 碱, 盐 and 氧化物 together form the single largest knowledge-point cluster in the CSCA chemistry bank. Most questions are classification items solved by writing one ionization equation.