Oxides and chlorides with water
| English | Chinese | Pinyin |
|---|---|---|
| basic | 碱性 | jiǎn xìng |
| acidic | 酸性 | suān xìng |
| amphoteric | 两性 | liǎng xìng |
| hydrolyse | 水解 | shuǐ jiě |
| electronegativity | 电负性 | diàn fù xìng |
Acid or alkali from an oxide?
- Across Period 3 the oxides change from basic 碱性 to acidic 酸性.
- The middle one, $\text{Al}_2\text{O}_3$, is amphoteric 两性.
- The chlorides behave differently too.
Oxide and chloride hydrolysis route
Follow how Period 3 oxides and chlorides change pH in water.
Across Period 3, the oxides change from:
Metal oxides on the left are basic; non-metal oxides on the right are acidic; Al₂O₃ is amphoteric in between.
Sodium oxide dissolves in water to form an alkaline solution.
Metal oxides are basic; non-metal oxides acidic.
Oxides: basic → amphoteric → acidic
| Oxide | Nature |
|---|---|
| $\text{Na}_2\text{O}$, $\text{MgO}$ | basic (metal oxides) |
| $\text{Al}_2\text{O}_3$ | amphoteric |
| $\text{SiO}_2$ | weakly acidic |
| $\text{P}_4\text{O}_{10}$, $\text{SO}_3$ | acidic (non-metal oxides) |
- Metal oxides (left) are basic; non-metal oxides (right) are acidic.
Period 3 oxides change from basic to amphoteric to acidic
Match each Period 3 species to its behaviour in water.
Each item links the term to its correct meaning.
Amphoteric alumina
- $\text{Al}_2\text{O}_3$ (and $\text{Al(OH)}_3$) react with both acids and bases:
$$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$$$\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$$
Aluminium oxide is described as amphoteric because it:
Al₂O₃ reacts with acids (as a base) and with bases (as an acid) — it is amphoteric.
Chlorides with water
- NaCl and MgCl₂ are ionic — they just dissolve (near-neutral).
- SiCl₄ and PCl₅ are covalent — they hydrolyse 水解 (react with water) to give acidic solutions and HCl fumes:
$$\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$$
- The trend follows electronegativity 电负性: ionic (basic/neutral) on the left, covalent (acidic) on the right.
When SiCl₄ or PCl₅ is added to water, it:
These covalent chlorides hydrolyse with water, producing acidic solutions and fumes of HCl.
NaCl and MgCl₂ in water give a:
Ionic chlorides dissolve to give near-neutral solutions; covalent ones hydrolyse to acidic solutions.
You've got it
- oxides go basic → amphoteric → acidic across (metal oxides basic, non-metal oxides acidic)
- $\text{Al}_2\text{O}_3$ is amphoteric — reacts with both acids and bases
- NaCl/MgCl₂ dissolve (neutral); SiCl₄/PCl₅ hydrolyse to acidic solutions + HCl
- the trend follows the change from ionic (left) to covalent (right)