盐yán

Core Concept

A salt (盐) is a compound made of a metal cation (or $\mathrm{NH_4^+}$) combined with an acid-radical anion (酸根离子).

$\mathrm{NaCl \rightarrow Na^+ + Cl^-}$

$\mathrm{(NH_4)_2SO_4 \rightarrow 2NH_4^+ + SO_4^{2-}}$

Salts are the category that catches everything failing the acid and base tests, which is why 盐 appears 27 times in the sampled CSCA papers — usually as the correct answer to a classification question whose distractors look like acids or bases.

Classification

Type	Chinese	Definition	Example
Normal salt	正盐	no ionizable $\mathrm{H}$, no $\mathrm{OH}$	$\mathrm{NaCl}$, $\mathrm{Na_2SO_4}$
Acid salt	酸式盐	contains ionizable $\mathrm{H}$	$\mathrm{NaHSO_4}$, $\mathrm{NaHCO_3}$
Basic salt	碱式盐	contains $\mathrm{OH}$	$\mathrm{Cu_2(OH)_2CO_3}$

The key insight: an acid salt is still a salt. $\mathrm{NaHSO_4}$ makes its solution acidic, but it is not an acid — its cations are not exclusively $\mathrm{H^+}$.

Why a Salt Solution Is Not Always Neutral

Salt hydrolysis (盐类水解) makes 盐溶液的酸碱性及原因 a recurring CSCA knowledge point. The rule is compact:

谁弱谁水解，谁强显谁性 — the weak partner hydrolyzes; the strong partner sets the pH.

Formed from	Solution	Example
strong acid + strong base	neutral	$\mathrm{NaCl}$
strong acid + weak base	acidic	$\mathrm{NH_4Cl}$
weak acid + strong base	basic	$\mathrm{CH_3COONa}$, $\mathrm{Na_2CO_3}$
weak acid + weak base	depends on relative strengths	$\mathrm{CH_3COONH_4}$

So $\mathrm{Na_2CO_3}$ — washing soda — gives a basic solution even though it is a salt, because $\mathrm{CO_3^{2-}}$ comes from the weak acid $\mathrm{H_2CO_3}$ and pulls $\mathrm{H^+}$ from water.

Common Preparation Routes

1. acid + base → salt + water
2. acid + metal → salt + $\mathrm{H_2}$
3. acid + basic oxide → salt + water
4. base + acidic oxide → salt + water
5. salt + salt → two new salts (requires a precipitate)

Route 5 needs a driving force — a precipitate, gas, or water — or no reaction occurs.

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 a salt?

Options:

• A. $\mathrm{HNO_3}$
• B. $\mathrm{NaHSO_4}$
• C. $\mathrm{KOH}$
• D. $\mathrm{CaO}$

Detailed Solution:

• A: only $\mathrm{H^+}$ as cation → acid
• B: $\mathrm{Na^+ + H^+ + SO_4^{2-}}$ — a metal cation with an acid radical → salt (an acid salt) ✓
• C: only $\mathrm{OH^-}$ as anion → base
• D: metal + oxygen, no acid radical → basic oxide

Option B is deliberately chosen because it looks acidic. The test is the cation set, not the pH.

Answer: B

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

Which salt solution is basic?

Options:

• A. $\mathrm{NaCl}$
• B. $\mathrm{NH_4Cl}$
• C. $\mathrm{Na_2CO_3}$
• D. $\mathrm{Na_2SO_4}$

Detailed Solution:

Apply 谁弱谁水解，谁强显谁性 to each:

• A: $\mathrm{HCl}$ (strong) + $\mathrm{NaOH}$ (strong) → neutral
• B: $\mathrm{HCl}$ (strong) + $\mathrm{NH_3 \cdot H_2O}$ (weak) → the weak base hydrolyzes, solution acidic
• C: $\mathrm{H_2CO_3}$ (weak) + $\mathrm{NaOH}$ (strong) → the weak acid radical hydrolyzes, solution basic ✓
• D: $\mathrm{H_2SO_4}$ (strong) + $\mathrm{NaOH}$ (strong) → neutral

The hydrolysis: $\mathrm{CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-}$

Answer: C

Common Mistakes

❌ Mistake 1: Calling an acid salt an acid

$\mathrm{NaHSO_4}$ and $\mathrm{NaHCO_3}$ are salts. Their solutions may be acidic or basic, but their classification comes from the cation test.

❌ Mistake 2: Assuming every salt solution is neutral

Only strong-acid + strong-base salts are neutral.

❌ Mistake 3: Thinking "salt" means table salt

$\mathrm{NaCl}$ is one salt among thousands.

❌ Mistake 4: Writing a salt + salt reaction with no driving force

Without a precipitate, gas, or water forming, the ions simply coexist and nothing happens.

Study Tips

1. ✅ Classify by ions, never by pH — write the ionization equation first
2. ✅ Recite 谁弱谁水解，谁强显谁性 for every hydrolysis question
3. ✅ Remember acid salts are salts, however acidic their solutions
4. ✅ Learn the five preparation routes as a set
5. ✅ Check for a driving force before writing any salt–salt reaction

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💡 Exam Tip: 盐 (27 occurrences) sits with 酸, 碱 and 氧化物 as the largest CSCA chemistry cluster, and 盐溶液的酸碱性及原因 adds more. One classification rule plus one hydrolysis rule covers nearly all of it.