Limewater turns cloudy with CO2 from exhaled air

Steps:

• Limewater (calcium hydroxide solution) is initially clear and turns milky when CO2 is present due to calcium carbonate formation.
• Test-tube 1 contains exhaled air, rich in CO2 from respiration, so CO2 reacts with limewater over 10 minutes.
• Test-tube 2 contains inhaled air, low in CO2, so no significant reaction occurs.
• After 10 minutes, tube 1 becomes cloudy while tube 2 stays clear.

Why B is correct:

• Exhaled air in tube 1 produces CO2, which reacts with Ca(OH)₂ to form insoluble CaCO₃ precipitate, clouding limewater; inhaled air in tube 2 lacks sufficient CO2.

Why the others are wrong:

• A: Ignores CO2 in tube 1, which should cloud it.
• C: Reverses the tubes; tube 1 clouds, not tube 2.
• D: Assumes CO2 in both, but tube 2 has minimal CO2.

Final answer: B