When carbon dioxide gas is passed through lime water (calcium hydroxide solution), it produces a white precipitate which is of:
calcium carbonate
Carbon dioxide reacts with calcium hydroxide in lime water: Ca(OH)2 + CO2 → CaCO3 + H2O. The insoluble CaCO3 formed is what makes the lime water turn milky, and this is the standard laboratory test for carbon dioxide.
Passing excess gas afterwards dissolves the precipitate as soluble calcium hydrogen carbonate, so that compound is not the white solid itself; calcium oxide is the product of heating limestone, and calcium sulphate needs sulphuric acid or a sulphate source, which is not present here.
Hence, the answer is calcium carbonate.
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