碱jiǎn

Core Concept

A base (碱) is a compound that ionizes in water to produce $\mathrm{OH^-}$ as its only anion.

It is the mirror image of the acid definition, and the same trap applies: producing $\mathrm{OH^-}$ is not enough — it must be the only anion. $\mathrm{Cu_2(OH)_2CO_3}$ releases $\mathrm{OH^-}$ but also $\mathrm{CO_3^{2-}}$, so it is a basic salt (碱式盐), not a base.

$\mathrm{NaOH \rightarrow Na^+ + OH^-}$

$\mathrm{Ba(OH)_2 \rightarrow Ba^{2+} + 2OH^-}$

Strong vs Weak

	Strong base 强碱	Weak base 弱碱
Ionization	complete	partial, reversible
Arrow	$\rightarrow$	$\rightleftharpoons$
Examples	$\mathrm{NaOH}$, $\mathrm{KOH}$, $\mathrm{Ba(OH)_2}$, $\mathrm{Ca(OH)_2}$	$\mathrm{NH_3 \cdot H_2O}$, $\mathrm{Cu(OH)_2}$, $\mathrm{Fe(OH)_3}$

A distinction CSCA loves: $\mathrm{Ca(OH)_2}$ is a strong base but only slightly soluble. "Strong" describes how completely the dissolved portion ionizes; "soluble" describes how much dissolves. They are independent properties, and confusing them is a marked error.

Solubility Rules Worth Memorising

Only $\mathrm{NaOH}$, $\mathrm{KOH}$, $\mathrm{Ba(OH)_2}$ and $\mathrm{NH_3 \cdot H_2O}$ are freely soluble. $\mathrm{Ca(OH)_2}$ is slightly soluble; all other common hydroxides are insoluble precipitates — which is exactly why they show up in 沉淀 questions.

Characteristic precipitate colours: $\mathrm{Cu(OH)_2}$ blue, $\mathrm{Fe(OH)_3}$ red-brown, $\mathrm{Fe(OH)_2}$ white turning grey-green then red-brown in air.

Characteristic Reactions

1. With indicators: turns red litmus blue; phenolphthalein turns pink
2. With acids (neutralization): $\mathrm{NaOH + HCl \rightarrow NaCl + H_2O}$
3. With acidic oxides: $\mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O}$
4. With certain salts: $\mathrm{2NaOH + CuSO_4 \rightarrow Cu(OH)_2\downarrow + Na_2SO_4}$

Reaction 3 is the basis of $\mathrm{CO_2}$ absorption questions; reaction 4 drives most precipitate-identification items.

CSCA Practice Problems

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

Example 1: Basic (Difficulty ★★☆☆☆)

Which statement about $\mathrm{Ca(OH)_2}$ is correct?

Options:

• A. It is a weak base because it is only slightly soluble
• B. It is a strong base even though it is only slightly soluble
• C. It is not a base because it is a solid
• D. It produces $\mathrm{H^+}$ in solution

Detailed Solution:

Solubility and strength are separate properties:

• Solubility — how much dissolves. $\mathrm{Ca(OH)_2}$ is only slightly soluble.
• Strength — how completely the dissolved portion ionizes. The part that does dissolve ionizes completely, so it is a strong base.

Option A is the intended trap, conflating the two.

Answer: B

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

$\mathrm{NaOH}$ solution is added dropwise to a solution containing both $\mathrm{FeCl_3}$ and $\mathrm{CuSO_4}$. What is observed?

Options:

• A. A white precipitate only
• B. A blue precipitate only
• C. A red-brown precipitate and a blue precipitate
• D. No precipitate forms

Detailed Solution:

1. Both $\mathrm{Fe(OH)_3}$ and $\mathrm{Cu(OH)_2}$ are insoluble, so both precipitate: $\mathrm{FeCl_3 + 3NaOH \rightarrow Fe(OH)_3\downarrow + 3NaCl}$ $\mathrm{CuSO_4 + 2NaOH \rightarrow Cu(OH)_2\downarrow + Na_2SO_4}$

2. Colours: $\mathrm{Fe(OH)_3}$ is red-brown, $\mathrm{Cu(OH)_2}$ is blue.

Answer: C

Exam habit worth building: whenever a hydroxide precipitates, state its colour — colour is frequently the actual thing being marked.

Common Mistakes

❌ Mistake 1: Equating "strong" with "soluble"

$\mathrm{Ca(OH)_2}$ is strong but sparingly soluble; $\mathrm{NH_3 \cdot H_2O}$ is very soluble but weak.

❌ Mistake 2: Calling any $\mathrm{OH^-}$-producing compound a base

Basic salts release $\mathrm{OH^-}$ too. The anion must be only $\mathrm{OH^-}$.

❌ Mistake 3: Writing $\rightarrow$ for ammonia solution

$\mathrm{NH_3 \cdot H_2O \rightleftharpoons NH_4^+ + OH^-}$ — weak, so reversible.

❌ Mistake 4: Omitting the $\downarrow$ symbol

Precipitates must be marked, and their colour stated when asked for observations.

Study Tips

1. ✅ Learn acid and base definitions as a matched pair — cation test vs anion test
2. ✅ Memorise the four soluble bases; everything else precipitates
3. ✅ Keep "strong/weak" and "soluble/insoluble" on separate axes
4. ✅ Learn precipitate colours — they are worth marks by themselves
5. ✅ Write full ionic equations to make the classification obvious

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💡 Exam Tip: 碱 appears 28 times in the sampled CSCA chemistry papers, right alongside 酸 and 盐. Most items are classification or precipitate-observation questions.