Consider the following statements: I. The leaves of nettle, a herbaceous plant, cause painful stings when touched accidentally because of the secretion of formic acid. II. When gypsum is heated at 100°C, it forms calcium carbonate hemihydrate, called Plaster of Paris. III. When moistened with water, anhydrous copper sulphate forms a green-coloured copper sulphate dihydrate. Which of the statements given above is/are correct?
I only
Statement I is correct: nettle leaves bear stinging hairs that inject formic acid into the skin on accidental contact, producing a painful sting. Statement II is incorrect: heating gypsum (CaSO4·2H2O) at about 100°C removes three-quarters of its water of crystallisation to give calcium sulphate hemihydrate (CaSO4·½H2O), i.e. Plaster of Paris, not calcium carbonate hemihydrate. Statement III is incorrect: moistening white anhydrous copper sulphate with water gives blue copper sulphate pentahydrate (CuSO4·5H2O), not a green dihydrate; this colour change is used as a standard test for the presence of water. Hence, only statement I is correct.
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