Classifying oxides
Acidic oxide
An oxide, usually of a non-metal, that reacts with bases and alkalis to form a salt and water.
Basic oxide
An oxide of a metal that reacts with acids to form a salt and water.
- An oxide is a compound of an element with oxygen, and many oxides form when elements burn in air or oxygen.
- Oxides are classified by whether they react with acids or with bases.
- Basic oxides are oxides of metals, such as copper(II) oxide, CuO\text{CuO}CuO, and calcium oxide, CaO\text{CaO}CaO.
- Acidic oxides are oxides of non-metals, such as sulfur dioxide, SO2\text{SO}_2SO2, and carbon dioxide, CO2\text{CO}_2CO2.
- The type of oxide therefore depends on the metallic or non-metallic character of the element it contains.
- Metals sit on the left of the Periodic Table and non-metals on the right, so oxides of elements on the left are basic and oxides of elements on the right are acidic.
Basic oxides
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A basic oxide reacts with an acid to form a salt and water, so it behaves as a base.
metal oxide+acid→salt+water \text{metal oxide} + \text{acid} \rightarrow \text{salt} + \text{water} metal oxide+acid→salt+water -
Copper(II) oxide is a black, insoluble solid that disappears when warmed with dilute sulfuric acid, leaving a blue solution of copper(II) sulfate.
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Most basic oxides are insoluble in water, but calcium oxide reacts with water to form calcium hydroxide, which is an alkali.
CaO(s)+H2O(l)→Ca(OH)2(aq) \text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} CaO(s)+H2O(l)→Ca(OH)2(aq) -
The calcium hydroxide solution has a pH above 7, so it turns universal indicator blue or purple.
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Calcium oxide is spread on fields to raise the pH of acidic soil, because as a basic oxide it reacts with the acid in the soil.
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A basic oxide does not react with an alkali.
Basic oxides with hydrochloric acid
- Copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water.
- Calcium oxide reacts with hydrochloric acid to form calcium chloride and water.
- In both cases the metal ion carries a 2+2+2+ charge, so two HCl\text{HCl}HCl are needed for each oxide.
Acidic oxides
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An acidic oxide reacts with a base or an alkali to form a salt and water.
non-metal oxide+alkali→salt+water \text{non-metal oxide} + \text{alkali} \rightarrow \text{salt} + \text{water} non-metal oxide+alkali→salt+water -
Carbon dioxide reacts with sodium hydroxide solution to form sodium carbonate and water.
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Sulfur dioxide reacts with sodium hydroxide solution to form sodium sulfite and water.
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Many acidic oxides dissolve in water to form solutions with a pH below 7.
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Carbon dioxide gives a slightly acidic solution, while sulfur dioxide gives a more strongly acidic one.
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An acidic oxide does not react with an acid.
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The milky colour in the limewater test happens because carbon dioxide, an acidic oxide, reacts with calcium hydroxide, an alkali, to form insoluble calcium carbonate.
Acidic oxides with alkalis
- Carbon dioxide reacts with sodium hydroxide to give sodium carbonate and water.
- Sulfur dioxide reacts with sodium hydroxide to give sodium sulfite and water.
- Carbon dioxide reacts with limewater to give a white precipitate of calcium carbonate and water.
Testing the oxides of a metal and a non-metal
- Aim: to make an oxide of a metal and an oxide of a non-metal by burning, and to compare the pH of the solutions they form with water.
- Apparatus: two gas jars of oxygen with lids, two combustion spoons, magnesium ribbon, powdered sulfur, tongs, Bunsen burner, heat-resistant mat, distilled water, universal indicator solution, fume cupboard and eye protection.
- Method:
- Add about 20 cm320\ \text{cm}^320 cm3 of distilled water and five drops of universal indicator to each gas jar, and note that the water is green.
- Hold a 3 cm3\ \text{cm}3 cm strip of magnesium ribbon in tongs, light it in the Bunsen flame and lower it into the first jar while it burns.
- Replace the lid and shake the jar so the white powder of magnesium oxide mixes with the water.
- In a fume cupboard, heat a small amount of sulfur on a combustion spoon until it melts and burns with a blue flame, then lower it into the second jar and replace the lid.
- Remove the spoon once the flame goes out, replace the lid and shake the jar so the sulfur dioxide dissolves.
- Results: the water in the magnesium oxide jar turns blue at about pH 9 to 10, while the water in the sulfur dioxide jar turns red or orange at about pH 2 to 3.
- Conclusion: the metal forms a basic oxide and the non-metal forms an acidic oxide.
- Safety: wear eye protection, do not look directly at burning magnesium, and burn sulfur only in a fume cupboard because sulfur dioxide is toxic.
- Do not classify an oxide by whether it dissolves in water, because most basic oxides are insoluble.
- Do not say that carbon dioxide reacts with acids, because an acidic oxide reacts with bases and alkalis.
Classifying an oxide
- Decide first whether the element in the oxide is a metal or a non-metal.
- Link a metal to a basic oxide and a non-metal to an acidic oxide.
- Back up the classification with the reaction, stating that a basic oxide reacts with an acid and an acidic oxide reacts with a base, each forming a salt and water.
- Is the oxide of a metal usually acidic or basic?
- Classify sulfur dioxide and calcium oxide.
- What are the products when a basic oxide reacts with an acid?
- Name the salt formed when carbon dioxide reacts with sodium hydroxide solution.
- What colour would universal indicator show in water that has absorbed sulfur dioxide?
