An unknown solid reacts with acid to give a gas that turns limewater milky. Which ion may be present in the solid?
Answer and explanation
Correct answer: Carbonate ion
The key qualitative test is that carbon dioxide turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. An acid reacts with a carbonate ion to release carbon dioxide, for example CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Therefore the unknown solid may contain carbonate ions. Chloride and sulphate do not normally produce this gas in the stated test, and hydroxide neutralises acid without releasing CO₂, so option C is correct.
Frequently asked questions
What is the correct answer to this question?
Carbonate ion
Why is this the correct answer?
The key qualitative test is that carbon dioxide turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. An acid reacts with a carbonate ion to release carbon dioxide, for example CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Therefore the unknown solid may contain carbonate ions. Chloride and sulphate do not normally produce this gas in the stated test, and hydroxide neutralises acid without releasing CO₂, so option C is correct.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
