A student reacts calcium oxide with dilute hydrochloric acid and separately reacts calcium oxide with water. Describe what happens in each reaction and explain how these reactions show that calcium oxide is a basic oxide.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Calcium oxide reacts with dilute hydrochloric acid:
CaO + 2HCl → CaCl₂ + H₂O
Calcium oxide reacts with water:
CaO + H₂O → Ca(OH)₂
The first reaction produces a salt and water – the classic neutralisation of an acid by a basic oxide. The second reaction gives calcium hydroxide, an alkaline solution. These reactions show that CaO is a basic oxide because it reacts with an acid to give a salt and water, and it forms an alkaline hydroxide when it reacts with water.
CaO + 2HCl → CaCl₂ + H₂O
Calcium oxide reacts with water:
CaO + H₂O → Ca(OH)₂
The first reaction produces a salt and water – the classic neutralisation of an acid by a basic oxide. The second reaction gives calcium hydroxide, an alkaline solution. These reactions show that CaO is a basic oxide because it reacts with an acid to give a salt and water, and it forms an alkaline hydroxide when it reacts with water.
Examiner tips
- Use the correct equations with stoichiometry, include the products CaCl₂ and Ca(OH)₂, and mention water. Explain that the acid–oxide reaction gives a salt and water (neutralisation) and that the oxide–water reaction gives an alkali. Keep the explanation concise and link both reactions to the definition of a basic oxide.
Common mistakes
- Writing the wrong products (e.g., CaO + HCl → CaClO + H₂O) or omitting the water in the acid reaction. Failing to state that Ca(OH)₂ is an alkaline solution or not linking the reactions to the definition of a basic oxide.
Mark scheme (4 marks)
- Calcium oxide reacts with dilute hydrochloric acid to form a salt (calcium chloride) and water
- Calcium oxide reacts with water to form calcium hydroxide (an alkali / alkaline solution)
- A basic oxide is one that reacts with acids to form a salt and water only (neutralisation)
- Calcium oxide is a basic oxide because it reacts with acid (to neutralise it) but does not react with bases/alkalis
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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