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Two reactants in a flask produce bubbles of gas that turn limewater milky. The gas is
Explanation
The gas is Carbon Dioxide (CO2). When CO2 is passed through limewater, which is an aqueous solution of calcium hydroxide (Ca(OH)2), it reacts to form calcium carbonate (CaCO3). Calcium carbonate is a white, insoluble solid that precipitates out of the solution, giving it a characteristic "milky" or cloudy appearance. The chemical reaction is represented as:
Ca(OH)2 + CO2 → CaCO3 + H2O
While sulfur dioxide (SO2) can also turn limewater milky by forming calcium sulfite, CO2 is the standard gas associated with this test in basic chemistry. Carbon monoxide (CO) and nitrogen dioxide (NO2) do not react with limewater to produce a milky precipitate.