Two reactants in a flask produce bubbles of gas that turn limewater milky. The gas is
When carbon dioxide gas is bubbled through limewater, it reacts with calcium hydroxide to form a precipitate of calcium carbonate (CaCO₃), which is insoluble in water and makes the solution appear milky or cloudy.
The chemical reaction is:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
If excess carbon dioxide is bubbled through the milky solution, the calcium carbonate precipitate reacts further to form soluble calcium bicarbonate, Ca(HCO₃)₂, and the milky appearance disappears.
CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)
Other gases listed:
– SO₂ (Sulfur dioxide) also reacts with limewater, but forms calcium sulfite (CaSO₃), which is also insoluble and causes turbidity. However, the reaction with CO₂ is the most common and characteristic test for CO₂.
– NO₂ (Nitrogen dioxide) is an acidic gas and would react with Ca(OH)₂, but it typically doesn’t produce a milky precipitate with limewater as characteristically as CO₂.
– CO (Carbon monoxide) is a neutral gas and does not react with limewater.
Given the standard chemical tests, turning limewater milky is the definitive test for CO₂.