Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Basic oxides
Many metal oxides are basic. Copper(II) oxide and calcium oxide react with acids to form salt and water. CuO + 2HCl -> CuCl2 + H2O. Calcium oxide also reacts with water to form calcium hydroxide, producing an alkaline solution if enough dissolves.
Acidic oxides
Many non-metal oxides are acidic. Carbon dioxide and sulfur dioxide react with bases. CO2 + 2NaOH -> Na2CO3 + H2O under suitable conditions. Dissolving such oxides in water may produce acidic solutions.
Metallic character link
Across a period, metallic character decreases and non-metallic character increases. Oxides therefore tend to change from basic through amphoteric to acidic. This is a broad trend used to interpret supplied information.
Classification evidence
Classify from reactions. An oxide that reacts only with acid is basic; one that reacts with base is acidic; one that reacts with both is amphoteric. Colour and physical state do not by themselves identify acid-base character.
Practical or data skill
Use supplied reaction data for several oxides and classify each as acidic, basic or amphoteric.
Examination tip
Support classification with the reagent it reacts with and the products salt plus water.
Review questions and suggested answers
Question 1
Classify CuO.
Suggested answer
Basic oxide.
Question 2
Classify CO2.
Suggested answer
Acidic oxide.
Question 3
What broad link exists across a period?
Suggested answer
Oxides tend to become less basic and more acidic as metallic character decreases.
