Gas Effect on limewater P no change Q no change R turns milky S no change Four gases P, Q, R and S are each bubbled through limewater using the apparatus sh...
Question 1 Report
| Gas | Effect on limewater |
|---|---|
| P | no change |
| Q | no change |
| R | turns milky |
| S | no change |
Four gases P, Q, R and S are each bubbled through limewater using the apparatus shown, and other tests are carried out. The table records the results. Which gas is carbon dioxide?
Answer Details
Limewater is a solution of calcium hydroxide. Carbon dioxide is the only common gas that turns it milky, because it reacts to form an insoluble white precipitate of calcium carbonate:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
It is the fine suspension of solid calcium carbonate that makes the liquid look cloudy. In the table only gas R turns the limewater milky, so R is carbon dioxide.
Gases P, Q and S produce no change, which tells you only that none of them is carbon dioxide; it does not identify them, and other tests such as a glowing splint for oxygen or a lighted splint for hydrogen would be needed. A negative result narrows the possibilities but never names the gas.
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