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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.

Related tags

Lime WaterCarbon DioxideCalcium HydroxideCalcium CarbonateLitmus TestPrecipitation Reaction

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