A student added dilute acid to sodium carbonate and passed the gas into lime water. Milkiness appeared first and then disappeared on passing excess gas. What is the correct reason?
Answer and explanation
Correct answer: First calcium carbonate then soluble calcium hydrogen carbonate forms
The governing reactions are carbonate–acid reaction and the limewater test for carbon dioxide. Acid plus sodium carbonate releases CO2. Initially, CO2 reacts with limewater: CO2 + Ca(OH)2 → CaCO3↓ + H2O, and insoluble CaCO3 causes milkiness. With excess CO2, CaCO3 + CO2 + H2O → Ca(HCO3)2, which is soluble, so the milkiness disappears. Hence option A is correct.
Frequently asked questions
What is the correct answer to this question?
First calcium carbonate then soluble calcium hydrogen carbonate forms
Why is this the correct answer?
The governing reactions are carbonate–acid reaction and the limewater test for carbon dioxide. Acid plus sodium carbonate releases CO2. Initially, CO2 reacts with limewater: CO2 + Ca(OH)2 → CaCO3↓ + H2O, and insoluble CaCO3 causes milkiness. With excess CO2, CaCO3 + CO2 + H2O → Ca(HCO3)2, which is soluble, so the milkiness disappears. Hence option A 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.
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