Basic Anhydrides

In-depth explanation of Basic Anhydrides

Basic anhydrides are metal oxides that react with water to form hydroxides, which are bases. Sodium oxide gives sodium hydroxide and calcium oxide gives calcium hydroxide. Metal oxides are generally ionic, so they are basic and dissolve in acids to form salts and water. Their reaction with water is often vigorous for the oxides of the most reactive metals. The contrast with acid anhydrides shows how bonding determines the acid-base character of oxides.

Going deeper: Basic Anhydrides

Key ideas: Metal oxides of groups 1 and 2 are basic oxides that react with water to give hydroxides: Na2O + H2O gives 2NaOH, CaO + H2O gives Ca(OH)2 (slaked lime, strongly exothermic), BaO and K2O similarly. They react with acids to give salt and water: MgO + 2HCl gives MgCl2 + H2O. Oxidation number is preserved.

Common mistakes and connections: Mistakes: expecting transition metal oxides to dissolve (most are insoluble, such as Fe2O3 and CuO, though they still react with acids) and forgetting Al2O3 and ZnO are amphoteric. Basicity increases down a group, and ionic oxides containing O2- are very strong bases that react completely with water.

Visualization of Basic Anhydrides

Across period 3 electronegativity rises from Na to Cl, and oxides change from basic (Na2O, MgO) through amphoteric Al2O3 to acidic (SO3, Cl2O7). The interactive lesson shows this as a live animation or chart.

Worked example and use case

Calcium oxide, also called quicklime, reacts vigorously with water to form slaked lime, calcium hydroxide.

Where Basic Anhydrides fits in Acids, Bases & Electrochemistry

Basic Anhydrides is topic 103 of 105 in Acids, Bases & Electrochemistry.

Related topics

• SuperacidsAdvanced
• Hard and Soft Acids and BasesAdvanced
• Acid AnhydridesIntermediate
• Water HardnessIntermediate
• AlkalinityAdvanced

Browse all 105 topics in Acids, Bases & Electrochemistry