iGCSE Chemistry (0620) Core:12.5 Identification of ions and gases: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
The key observation is that damp red litmus paper turns white. Ammonia turns damp red litmus paper blue, not white. Carbon dioxide has no effect on litmus paper (though it turns limewater milky). Oxygen has no effect on litmus paper. Chlorine gas, however, is a powerful bleaching agent—it first turns damp litmus paper red (because chlorine dissolves in water to form an acidic solution), and then bleaches it, turning it white. The bleaching action is due to the oxidising property of chlorine. Therefore, the gas that turns damp red litmus paper white is chlorine, making option C correct.
▶️ Answer/Explanation
Detailed solution:
A yellow flame test indicates the presence of sodium ions ($\text{Na}^+$). Effervescence (fizzing) when dilute nitric acid is added means a carbonate is present, as carbon dioxide gas is produced. So the compound must contain both sodium and carbonate ions — that’s sodium carbonate ($\text{Na}_2\text{CO}_3$). Chlorides wouldn’t fizz with acid, and calcium gives a different flame colour.
▶️ Answer/Explanation
Detailed solution:
Copper(II) ions (Cu²⁺) react with aqueous sodium hydroxide to form a light blue precipitate of copper(II) hydroxide, which is insoluble in excess. Copper(II) sulfate is soluble, so with dilute sulfuric acid there is no visible change (no precipitate). In a flame test, copper(II) ions produce a characteristic blue-green flame. Sodium nitrate would not form a precipitate with sulfuric acid; barium nitrate gives a white precipitate with sulfuric acid and a light green flame; calcium nitrate gives a white precipitate with sodium hydroxide and an orange-red flame.