Definition
Amphoteric oxide
An oxide that can react with both an acid and a base, forming a salt and water in each case.
- Some metal oxides behave as a basic oxide and as an acidic oxide.
- An amphoteric oxide reacts with acids, in the same way as a basic oxide, and with bases, in the same way as an acidic oxide.
- Both of these reactions produce a salt and water.
- The two most important amphoteric oxides are aluminium oxide, Al2O3\text{Al}_2\text{O}_3Al2O3, and zinc oxide, ZnO\text{ZnO}ZnO.
- Aluminium and zinc are metals, but they are less strongly metallic than metals such as sodium and calcium, so their oxides show both kinds of behaviour.
- Both oxides are white solids that are insoluble in water, so they do not change the colour of an indicator.
- An amphoteric oxide is identified by showing that it reacts with an acid and also with an alkali.
- Oxides can therefore be sorted into three groups by the reactions they show.
- Basic oxides react with acids only.
- Acidic oxides react with bases and alkalis only.
- Amphoteric oxides react with both acids and bases.
Zinc oxide
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With an acid, zinc oxide acts as a base and forms a zinc salt and water.
ZnO(s)+2HCl(aq)→ZnCl2(aq)+H2O(l) \text{ZnO(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{O(l)} ZnO(s)+2HCl(aq)→ZnCl2(aq)+H2O(l) -
With an alkali, zinc oxide acts as an acidic oxide and forms a salt called sodium zincate and water.
ZnO(s)+2NaOH(aq)→Na2ZnO2(aq)+H2O(l) \text{ZnO(s)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{ZnO}_2\text{(aq)} + \text{H}_2\text{O(l)} ZnO(s)+2NaOH(aq)→Na2ZnO2(aq)+H2O(l) -
In sodium zincate the zinc is part of the negative ion, ZnO22−\text{ZnO}_2^{2-}ZnO22−, rather than being a positive metal ion.
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In both reactions the white solid disappears and a colourless solution forms.
Aluminium oxide
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With an acid, aluminium oxide acts as a base and forms an aluminium salt and water.
Al2O3(s)+6HCl(aq)→2AlCl3(aq)+3H2O(l) \text{Al}_2\text{O}_3\text{(s)} + 6\text{HCl(aq)} \rightarrow 2\text{AlCl}_3\text{(aq)} + 3\text{H}_2\text{O(l)} Al2O3(s)+6HCl(aq)→2AlCl3(aq)+3H2O(l) -
With a warm alkali, aluminium oxide acts as an acidic oxide and forms a salt called sodium aluminate and water.
Al2O3(s)+2NaOH(aq)→2NaAlO2(aq)+H2O(l) \text{Al}_2\text{O}_3\text{(s)} + 2\text{NaOH(aq)} \rightarrow 2\text{NaAlO}_2\text{(aq)} + \text{H}_2\text{O(l)} Al2O3(s)+2NaOH(aq)→2NaAlO2(aq)+H2O(l) -
In sodium aluminate the aluminium is part of the negative ion, AlO2−\text{AlO}_2^{-}AlO2−.
Example
Balancing aluminium oxide with hydrochloric acid
- Aluminium forms Al3+\text{Al}^{3+}Al3+ ions and chloride ions are Cl−\text{Cl}^{-}Cl−, so the salt is AlCl3\text{AlCl}_3AlCl3.
- One Al2O3\text{Al}_2\text{O}_3Al2O3 contains two aluminium atoms, so it makes 2AlCl32\text{AlCl}_32AlCl3, which needs six chlorine atoms.
- Six chlorine atoms come from 6HCl6\text{HCl}6HCl, which also supplies six hydrogen atoms.
- Six hydrogen atoms combine with the three oxygen atoms from the oxide to make 3H2O3\text{H}_2\text{O}3H2O.
Practical
Showing that zinc oxide is amphoteric
- Aim: to show that zinc oxide reacts with both an acid and an alkali, and to compare it with copper(II) oxide, a basic oxide.
- Apparatus: four test tubes in a rack, spatula, zinc oxide, copper(II) oxide, dilute sulfuric acid, dilute sodium hydroxide solution, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, beaker of water at about 60 ∘C60\ ^\circ\text{C}60 ∘C as a water bath, and eye protection.
- Method:
- Put one small spatula measure of zinc oxide into each of two test tubes, and the same amount of copper(II) oxide into each of the other two.
- Add 5 cm35\ \text{cm}^35 cm3 of dilute sulfuric acid to one tube of each oxide.
- Add 5 cm35\ \text{cm}^35 cm3 of dilute sodium hydroxide solution to the other tube of each oxide.
- Stand all four tubes in the warm water bath for five minutes, shaking each one gently every minute.
- Record whether each solid dissolves and any colour change in the liquid.
- Results: zinc oxide disappears in both the acid and the alkali to give colourless solutions, while copper(II) oxide dissolves in the acid to give a blue solution but stays as a black solid in the alkali.
- Conclusion: zinc oxide is amphoteric because it reacts with an acid and an alkali, while copper(II) oxide is basic because it reacts with the acid only.
- Safety: wear eye protection, take extra care with sodium hydroxide because it damages the eyes even when dilute, and avoid breathing in the fine oxide powders.
Common Mistake
- Do not call aluminium oxide or zinc oxide basic just because they are metal oxides, because they also react with bases.
- Do not say that amphoteric oxides react with water, because they are insoluble and only react with acids and alkalis.
Exam technique
Showing that an oxide is amphoteric
- Give two reactions, one with an acid and one with a base or alkali.
- State that each reaction forms a salt and water.
- When an equation is asked for, name the zincate or aluminate as the salt formed with sodium hydroxide.
Self review
- What is an amphoteric oxide?
- Name two amphoteric oxides.
- What are the products when zinc oxide reacts with hydrochloric acid?
- Name the salt formed when zinc oxide reacts with sodium hydroxide solution.
- How could you show that copper(II) oxide is basic but zinc oxide is amphoteric?
