Metal oxides form when metals react with oxygen; they are classed as bases because they neutralise acids to produce a salt and water. Non-metal oxides, by contrast, are acidic and dissolve in water to form acidic solutions. This acid-base contrast between metal and non-metal oxides is a fundamental pattern in KS3 chemistry.
How do metal oxides form?
A metal oxide forms when a metal reacts with oxygen gas. The general word equation is:
metal + oxygen → metal oxide
Examples:
- 4Na + O₂ → 2Na₂O (sodium oxide)
- 2Mg + O₂ → 2MgO (magnesium oxide)
- 4Fe + 3O₂ → 2Fe₂O₃ (iron(III) oxide — commonly known as rust)
- 2Cu + O₂ → 2CuO (copper(II) oxide)
The speed of this reaction varies dramatically with position in the reactivity series. Potassium and sodium react vigorously with oxygen at room temperature, their surfaces tarnishing almost immediately in air. Copper reacts only slowly, forming a thin tarnish layer over years. Gold and platinum do not react with oxygen at all under normal conditions.
Heating generally accelerates the reaction: magnesium burns brilliantly white in air (or pure oxygen) to form magnesium oxide; iron powder ignites in pure oxygen to form iron(III) oxide.
Why are metal oxides described as bases?
A base is a substance that can react with an acid and neutralise it. Metal oxides are classified as bases because they accept protons (H⁺ ions) from acids in neutralisation reactions, producing a salt and water but no gas.
The general pattern is:
metal oxide + acid → salt + water
This is a form of neutralisation, and it is the acid-base behaviour that defines a metal oxide as a base — not whether it dissolves in water.
Most metal oxides do not dissolve in water (they are insoluble bases). However, the oxides of the most reactive metals (Group 1 and Group 2) do dissolve in water to produce alkaline solutions, making them both bases and alkalis:
- Na₂O + H₂O → 2NaOH (sodium hydroxide, a strong alkali)
- CaO + H₂O → Ca(OH)₂ (calcium hydroxide, a weaker alkali)
The key distinction: all alkalis are bases, but not all bases are alkalis. An alkali must dissolve in water; a base only needs to neutralise an acid.
How do metal oxides react with acids?
The general equation metal oxide + acid → salt + water produces specific products depending on which acid is used:
| Acid used | Salt produced | Example reaction |
|---|---|---|
| Hydrochloric acid (HCl) | A chloride | CuO + 2HCl → CuCl₂ + H₂O (copper chloride) |
| Sulfuric acid (H₂SO₄) | A sulfate | MgO + H₂SO₄ → MgSO₄ + H₂O (magnesium sulfate) |
| Nitric acid (HNO₃) | A nitrate | ZnO + 2HNO₃ → Zn(NO₃)₂ + H₂O (zinc nitrate) |
To name the salt: take the metal from the oxide and the anion from the acid (chloride from HCl, sulfate from H₂SO₄, nitrate from HNO₃).
Worked example: Write the word equation and identify the salt when copper(II) oxide reacts with sulfuric acid.
- Word equation: copper(II) oxide + sulfuric acid → copper(II) sulfate + water
- Formula equation: CuO + H₂SO₄ → CuSO₄ + H₂O
Copper sulfate is the salt; water is the other product. No gas is produced — this is different from the reaction of a metal carbonate with acid (which releases CO₂) or the reaction of a reactive metal with acid (which releases hydrogen).
What are non-metal oxides and why are they different?
Non-metal oxides are compounds formed when non-metals react with oxygen. Unlike metal oxides, non-metal oxides are characteristically acidic:
- Carbon dioxide (CO₂): dissolves in water to form carbonic acid (H₂CO₃), a weak acid. This is why rainwater is naturally slightly acidic (pH ~5.6) and why fizzy drinks are acidic.
- Sulfur dioxide (SO₂): dissolves in water to form sulfurous acid (H₂SO₃), and oxidises in the atmosphere to form sulfuric acid — the main contributor to acid rain.
- Nitrogen dioxide (NO₂): dissolves in water to form nitric acid — another contributor to acid rain.
- Sulfur trioxide (SO₃): dissolves in water to form sulfuric acid directly.
Some non-metal oxides, such as carbon monoxide (CO), are neutral — they do not react with water to form an acid or a base.
| Oxide type | Example | Behaviour in water |
|---|---|---|
| Metal oxide (base) | MgO | Does not dissolve, but neutralises acids |
| Metal oxide (alkali) | Na₂O | Dissolves to form NaOH (alkaline) |
| Non-metal oxide (acidic) | CO₂, SO₂ | Dissolves to form an acid |
| Non-metal oxide (neutral) | CO | Does not react with water |
Which metal oxides dissolve in water to form alkalis?
Only the most reactive metals — those in Group 1 (alkali metals: lithium, sodium, potassium) and some in Group 2 (calcium, barium) — form oxides that dissolve readily in water:
- Sodium oxide: Na₂O + H₂O → 2NaOH
- Calcium oxide (quicklime): CaO + H₂O → Ca(OH)₂ — this reaction is strongly exothermic and the product (slaked lime) is used in agriculture to neutralise acidic soils.
- Lithium oxide: Li₂O + H₂O → 2LiOH
Metal oxides from less reactive metals (copper, iron, aluminium) are insoluble in water and will not form alkalis, though they still neutralise acids.
How do metal oxides fit into the reactivity series?
The ease with which a metal forms an oxide reflects its position in the reactivity series:
- Reactive metals (potassium, sodium, calcium, magnesium): react readily with oxygen at room temperature or on mild heating; their oxides are stable.
- Medium reactivity (aluminium, zinc, iron, copper): react with oxygen on strong heating; the rate of reaction decreases down the series.
- Unreactive metals (silver, gold, platinum): do not react with oxygen at all under normal conditions; they do not form oxides and so remain shiny and untarnished.
The reactivity series also predicts displacement reactions between metals and metal oxides: a more reactive metal can "displace" a less reactive metal from its oxide (this is the principle behind the thermite reaction: aluminium displaces iron from iron(III) oxide with the release of a large amount of heat).
Frequently asked questions
What is a metal oxide in chemistry?
A metal oxide is a compound formed when a metal reacts with oxygen. It consists of metal ions bonded to oxide ions (O²⁻). Metal oxides are classified as bases: they react with acids in neutralisation reactions to produce a salt and water. Some metal oxides — mainly those of Group 1 and 2 elements — also dissolve in water to form alkaline solutions, making them alkalis as well as bases.
What is the general equation for metal oxide plus acid?
The general equation is: metal oxide + acid → salt + water. The salt produced depends on which acid you use: hydrochloric acid gives a chloride salt, sulfuric acid gives a sulfate salt, and nitric acid gives a nitrate salt. No gas is produced in this type of reaction, which distinguishes it from the reaction of a metal with acid (produces hydrogen) or a metal carbonate with acid (produces carbon dioxide).
How are metal oxides and non-metal oxides different?
Metal oxides are basic — they neutralise acids, and some dissolve in water to form alkalis. Non-metal oxides are acidic — they dissolve in water to form acids (e.g. CO₂ forms carbonic acid; SO₂ forms sulfurous acid). This acid-base contrast relates to the bonding: metal oxides contain ionic bonds between metal cations and O²⁻ ions (making them bases), while non-metal oxides contain covalent bonds and release H⁺ ions when they react with water (making them acids).
Why does calcium oxide react so vigorously with water?
Calcium oxide (CaO, also called quicklime) reacts exothermically with water in a reaction sometimes called "slaking": CaO + H₂O → Ca(OH)₂. A large amount of heat is released, enough to make the mixture steam and even crack containers if too much water is added at once. The product, calcium hydroxide (slaked lime), is a mild alkali used to neutralise acidic soils (raising pH) and in making mortar and cement. The vigour of the reaction reflects the strong ionic bonds in CaO and the large lattice energy released when Ca(OH)₂ forms.
For Socratic KS3 chemistry with Professor Curie — predicting whether a new oxide will be acidic or basic by asking first whether its central element is a metal or a non-metal, then checking against the experimental evidence — visit aitutors.me.