Burn an element in oxygen and the oxide tells you whether it was a metal or a non-metal. Metal oxides are basic, non-metal oxides are acidic, and a few in the middle, such as aluminium oxide, behave as both.
Basic oxides are oxides of metals, such as calcium oxide and copper(II) oxide; they react with acids to form a salt and water. Acidic oxides are oxides of non-metals, such as carbon dioxide and sulfur dioxide; they react with bases to form a salt and water, and many dissolve in water to give acidic solutions. Extended:amphoteric oxides, such as aluminium oxide and zinc oxide, react with both acids and bases.
Fig. 1: three kinds of oxide
Section 01
Across the Periodic Table.
Step through the oxides of Period 3 in the simulator. Watch the pH of each oxide in water change from alkaline to acidic.
Metal oxides are basic: magnesium oxide + hydrochloric acid → magnesium chloride + water. Soluble ones, such as sodium oxide, dissolve to form alkalis.
Non-metal oxides are acidic: carbon dioxide + sodium hydroxide → sodium carbonate + water. Sulfur dioxide dissolves in rainwater to form acid rain.
Pattern: oxides become less basic and more acidic from left to right across a period, following the change from metal to non-metal.
Stage 1 · Learn
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An oxide reacts with hydrochloric acid to form a salt and water. It is most likely:
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Which oxide is acidic?
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Sodium oxide dissolves in water. The solution is:
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(Extended) An amphoteric oxide:
Section 02
Amphoteric oxides.
Extended. Some metal oxides near the metal and non-metal boundary react both ways.
Aluminium oxide with an acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O (acting as a base).
Aluminium oxide with a base: it also dissolves in hot concentrated sodium hydroxide to form a salt, sodium aluminate (acting as an acid).
Zinc oxide behaves the same way: it reacts with dilute acids and with sodium hydroxide solution.
Test for amphoteric: the oxide must react with (dissolve in) both an acid and an alkali.
Examiner trap
Not every metal oxide is simply basic: aluminium and zinc oxides are amphoteric. And "acidic oxide" means it reacts with bases; it does not contain hydrogen ions itself. Silicon dioxide is acidic even though it does not dissolve in water.
Stage 2 · Exam
Exam-style questions
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(Extended) Which pair are both amphoteric oxides?
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Element X burns to form an oxide that dissolves in water to give a solution of pH 3. X is most likely:
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Carbon dioxide reacts with aqueous calcium hydroxide (limewater). What type of reaction is this?
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(Extended) An oxide dissolves in dilute hydrochloric acid and also in aqueous sodium hydroxide. Which oxide could it be?
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Oxides, mastered.
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