A student generates a gas by adding dilute hydrochloric acid to marble chips (calcium carbonate). The gas is collected and tested. Which test correctly identifies the gas?
Show the answer and explanation
Correct answer
C. The gas turns limewater milky.
Principle or equation
Carbon dioxide is produced when an acid reacts with a carbonate. It is identified by turning limewater (calcium hydroxide solution) milky due to formation of insoluble calcium carbonate.
Why this answer is correct
The reaction is CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l). Carbon dioxide is colorless and odorless, does not support combustion, and turns limewater milky. A glowing splint relights in oxygen; a pop indicates hydrogen; bleaching is characteristic of chlorine.
Example
If CO2 is bubbled through limewater, a white precipitate of CaCO3 forms, making the solution milky.
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