Carbon dioxide is passed into a solution. Milkiness appears first and then disappears. Which substance causes the initial milkiness?
Answer and explanation
Correct answer: Calcium carbonate
This is the lime-water test for carbon dioxide. Initially, carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate, which appears as a white precipitate and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. If excess carbon dioxide is passed, the precipitate reacts further to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the milkiness disappears. Thus option A explains the initial change.
Frequently asked questions
What is the correct answer to this question?
Calcium carbonate
Why is this the correct answer?
This is the lime-water test for carbon dioxide. Initially, carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate, which appears as a white precipitate and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. If excess carbon dioxide is passed, the precipitate reacts further to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the milkiness disappears. Thus option A explains the initial change.
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.
