An unknown solution turns red litmus blue and forms a white precipitate when carbon dioxide is passed through it. Which substance is most likely present in the solution?
Answer and explanation
Correct answer: Lime water
Red litmus turning blue indicates that the solution is basic. Lime water is an aqueous solution of calcium hydroxide, \(\mathrm{Ca(OH)_2}\). When carbon dioxide is passed through it, a white precipitate of calcium carbonate forms: \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O}\). Therefore, option A is correct. Sodium chloride solution is generally neutral, whereas vinegar and dilute sulphuric acid are acidic, so they do not turn red litmus blue. Exam tip: The milky appearance of lime water on passing \(\mathrm{CO_2}\) is a key test for calcium carbonate formation.
Frequently asked questions
What is the correct answer to this question?
Lime water
Why is this the correct answer?
Red litmus turning blue indicates that the solution is basic. Lime water is an aqueous solution of calcium hydroxide, \(\mathrm{Ca(OH)_2}\). When carbon dioxide is passed through it, a white precipitate of calcium carbonate forms: \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O}\). Therefore, option A is correct. Sodium chloride solution is generally neutral, whereas vinegar and dilute sulphuric acid are acidic, so they do not turn red litmus blue. Exam tip: The milky appearance of lime water on passing \(\mathrm{CO_2}\) is a key test for calcium carbonate formation.
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.
