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Burn sodium and dissolve the white ash in water: the solution turns universal indicator purple. Burn sulfur and bubble the choking gas through water: the indicator turns red. Both are oxides, both formed by combining an element with oxygen, yet they behave as chemical opposites.
This split between basic oxides and acidic oxides is one of the clearest patterns in chemistry. It follows the periodic table, it explains acid rain and cement-making, and it’s a favourite exam topic.
The quick rule
- Metal oxides are basic. They react with acids to form a salt and water. If they dissolve in water, they form alkaline solutions.
- Non-metal oxides are acidic. They react with bases to form a salt and water. If they dissolve in water, they form acidic solutions.
A few oxides break the simple rule: amphoteric oxides react with both acids and bases, and neutral oxides react with neither.
Side-by-side comparison
| Basic oxides | Acidic oxides | |
|---|---|---|
| Typical elements | metals, especially Groups 1 and 2 | non-metals |
| Bonding | mainly ionic | mainly covalent |
| Examples | Na₂O, K₂O, MgO, CaO, CuO, FeO | CO₂, SO₂, SO₃, NO₂, P₄O₁₀, SiO₂ |
| With water (if they react) | form hydroxides → alkaline solution | form acids → acidic solution |
| React with | acids | bases and alkalis |
| Product of neutralisation | salt + water | salt + water |
| Typical state at room temperature | solid, high melting point | often gases or low-melting solids (SiO₂ is an exception) |
Basic oxides
Metal oxides contain the oxide ion, O²⁻, a strong base. It readily accepts protons.
With water, soluble metal oxides form hydroxides:
Na₂O + H₂O → 2NaOH CaO + H₂O → Ca(OH)₂
The reaction of calcium oxide (quicklime) with water is strongly exothermic; the mixture can get hot enough to boil. This is called “slaking” lime.
With acids, all basic oxides form a salt and water, whether they dissolve in water or not:
MgO + 2HCl → MgCl₂ + H₂O CuO + H₂SO₄ → CuSO₄ + H₂O
Insoluble basic oxides such as copper(II) oxide don’t change indicator colours in water, but they still neutralise acids. That’s why they’re bases but not alkalis; see bases vs alkalis.
Acidic oxides
Non-metal oxides are covalent. Many react with water to form acids:
| Oxide | Reaction with water | Acid formed |
|---|---|---|
| CO₂ | CO₂ + H₂O ⇌ H₂CO₃ | carbonic acid (weak) |
| SO₂ | SO₂ + H₂O ⇌ H₂SO₃ | sulfurous acid (weak) |
| SO₃ | SO₃ + H₂O → H₂SO₄ | sulfuric acid (strong) |
| NO₂ | 2NO₂ + H₂O → HNO₃ + HNO₂ | nitric and nitrous acids |
| P₄O₁₀ | P₄O₁₀ + 6H₂O → 4H₃PO₄ | phosphoric acid |
| Cl₂O₇ | Cl₂O₇ + H₂O → 2HClO₄ | perchloric acid (very strong) |
Oxides like these are sometimes called acid anhydrides: “acids without water”.
With bases, acidic oxides form a salt and water:
CO₂ + 2NaOH → Na₂CO₃ + H₂O SO₂ + Ca(OH)₂ → CaSO₃ + H₂O
The first reaction is why sodium hydroxide solution absorbs carbon dioxide from the air and slowly loses strength. The second is used to remove sulfur dioxide from power station exhaust (flue-gas desulfurisation).
Silicon dioxide is an unusual acidic oxide. It’s a giant covalent solid, insoluble in water, but it reacts with hot, concentrated alkali or with basic oxides at high temperature:
SiO₂ + 2NaOH → Na₂SiO₃ + H₂O SiO₂ + CaO → CaSiO₃
The second reaction is how impurities are removed as slag in the blast furnace.
Amphoteric oxides
Some metal oxides, from metals near the metal–non-metal boundary, react with both acids and bases:
- Aluminium oxide: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]
- Zinc oxide, lead(II) oxide, tin(II) oxide, beryllium oxide and chromium(III) oxide behave similarly.
The metal ions involved have a high charge density, so their bonds to oxygen have significant covalent character, halfway between ionic (basic) and covalent (acidic). More detail is in amphoteric substances.
Neutral oxides
A few non-metal oxides react with neither acids nor bases under normal conditions:
- Carbon monoxide, CO
- Nitrogen monoxide, NO
- Dinitrogen oxide (nitrous oxide), N₂O
- Water, H₂O, is sometimes included, though it can act as both an acid and a base.
These don’t form salts in simple neutralisation reactions, which is why they’re classed as neutral.
The Period 3 pattern
Crossing Period 3 from left to right shows the whole range:
| Oxide | Bonding | Nature | With water |
|---|---|---|---|
| Na₂O | ionic | strongly basic | NaOH, pH about 14 |
| MgO | ionic | basic | Mg(OH)₂, slightly soluble, pH about 10 |
| Al₂O₃ | ionic with covalent character | amphoteric | insoluble |
| SiO₂ | giant covalent | weakly acidic | insoluble |
| P₄O₁₀ | simple molecular | acidic | H₃PO₄, pH about 1–2 |
| SO₂ / SO₃ | simple molecular | acidic | H₂SO₃ / H₂SO₄, pH about 1–3 |
| Cl₂O₇ | simple molecular | strongly acidic | HClO₄, pH below 1 |
As you move across the period, the elements become more electronegative, the bonding changes from ionic to covalent, and the oxides change from basic to acidic. Explore these elements on the interactive periodic table and compare their electronegativity with the electronegativity trend.
Oxidation state matters too
For elements that form several oxides, the higher the oxidation state, the more acidic the oxide:
| Chromium oxide | Oxidation state | Nature |
|---|---|---|
| CrO | +2 | basic |
| Cr₂O₃ | +3 | amphoteric |
| CrO₃ | +6 | acidic (forms chromic acid) |
Manganese shows the same trend: MnO is basic, while Mn₂O₇ (manganese +7) is a strongly acidic, dangerously reactive oxide. A metal in a high oxidation state pulls electron density strongly from oxygen, behaving more like a non-metal.
Where these oxides matter
- Acid rain. Sulfur dioxide and nitrogen oxides from burning fuels dissolve in rainwater to form sulfuric and nitric acids.
- Ocean acidification. Carbon dioxide dissolving in seawater forms carbonic acid (see ocean pH).
- Agriculture. Farmers spread lime (calcium oxide, hydroxide or carbonate) to neutralise acidic soils.
- Cement and steel. Basic calcium oxide combines with acidic silica to make cement and to remove silica as slag.
- Flue-gas desulfurisation. Power stations spray limestone or lime slurry to capture sulfur dioxide.
- Glass-making. Acidic silica is melted with basic sodium and calcium oxides (from carbonates) to form glass.
Quick check
- Predict whether barium oxide is acidic, basic or amphoteric, and write its reaction with water.
- Write the equation for sulfur trioxide reacting with sodium hydroxide solution.
- Why is carbon monoxide not classed as an acidic oxide?
Answers: (1) Basic: BaO + H₂O → Ba(OH)₂. (2) SO₃ + 2NaOH → Na₂SO₄ + H₂O. (3) It doesn’t react with bases (or water) to form a salt under normal conditions, so it’s classed as neutral.
Common mistakes
- Thinking all oxides dissolve in water. Many basic oxides (CuO, Fe₂O₃) and SiO₂ don’t, but they still show basic or acidic behaviour with acids or alkalis.
- Calling every metal oxide basic. Watch for amphoteric oxides and high-oxidation-state metal oxides like CrO₃ and Mn₂O₇.
- Forgetting neutral oxides. CO, NO and N₂O don’t form salts.
- Calling CO₂ “carbonic acid”. CO₂ is the oxide; carbonic acid forms when it reacts with water.
Key takeaways
- Metal oxides are generally basic; non-metal oxides are generally acidic.
- Basic oxides contain O²⁻ and react with acids; acidic oxides react with bases and often form acids with water.
- Some oxides (Al₂O₃, ZnO) are amphoteric; a few (CO, NO, N₂O) are neutral.
- Across Period 3, oxides change from basic to amphoteric to acidic as bonding changes from ionic to covalent.
- Higher oxidation states make a metal’s oxide more acidic.
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