iGCSE Chemistry (0620) Core:7.2 Oxides: Exam Style Questions Paper 1- New Syllabus
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Detailed solution:
Oxides are classified based on the character of the element that forms them. Metal oxides are typically basic—they react with acids to form a salt and water. Non-metal oxides are typically acidic—they react with bases to form a salt and water. Lithium and sodium are both Group I metals, so lithium oxide ($\mathrm{Li_2O}$) and sodium oxide ($\mathrm{Na_2O}$) are both basic oxides. They dissolve in water to form alkaline solutions of lithium hydroxide and sodium hydroxide. Phosphorus is a non-metal, so phosphorus oxide is acidic. Sulfur is also a non-metal, so sulfur dioxide ($\mathrm{SO_2}$) is acidic—it dissolves in water to form sulfurous acid and is a major cause of acid rain. Therefore, the basic oxides are 1 (lithium oxide) and 3 (sodium oxide).
▶️ Answer/Explanation
Detailed solution:
Basic oxides are formed exclusively by metals, and the more metallic the element, the more basic its oxide. Group I alkali metals are the most metallic elements on the whole table, so their oxides (like $Na_2O$ or $K_2O$) are strongly basic. Period 3 is a mix—it starts with metals like sodium and magnesium that form basic oxides, but then moves into non-metals like sulfur which form acidic oxides. Group VIII noble gases are almost completely unreactive and don’t form oxides at all under normal conditions. So only Group I guarantees that its elements form purely basic oxides.
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Detailed solution:
Acidic oxides are usually oxides of non-metals. Carbon dioxide ($\text{CO}_2$) is a non-metal oxide and dissolves in water to form carbonic acid, so it’s acidic. Barium oxide, copper(II) oxide, and magnesium oxide are all metal oxides, and metal oxides are typically basic, reacting with acids rather than bases.