An acid is added to a white solid and a gas evolves that turns lime water milky. Excess of the same gas removes the milkiness. What is the final soluble substance?
Answer and explanation
Correct answer: Calcium hydrogen carbonate
The governing concept is the reaction of carbon dioxide with limewater. The gas turns limewater milky because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When CO₂ is passed in excess, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the milkiness disappears. Therefore option C is correct; calcium carbonate is only the intermediate product.
Frequently asked questions
What is the correct answer to this question?
Calcium hydrogen carbonate
Why is this the correct answer?
The governing concept is the reaction of carbon dioxide with limewater. The gas turns limewater milky because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When CO₂ is passed in excess, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the milkiness disappears. Therefore option C is correct; calcium carbonate is only the intermediate product.
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.
