iGCSE Chemistry 0620 Extended - 12.5 Identification of ions and gases- Exam Style Questions Paper 2- New Syllabus
▶️ Answer/Explanation
Copper(II) ions (\(\mathrm{Cu^{2+}}\)) give a blue-green flame in a flame test, and with aqueous sodium hydroxide form a light blue precipitate of copper(II) hydroxide that is insoluble in excess NaOH.
(A) Yellow flame = Na⁺, but Na⁺ gives no precipitate with NaOH.
(C) Light green flame is not characteristic of any common cation; green ppt. soluble in excess indicates Cr³⁺, but chromium gives no distinct flame colour.
(D) Red flame = Li⁺ or Ca²⁺ (orange-red), but red-brown ppt. indicates Fe³⁺, a mismatch.
✅ Answer: (B)
(A) Yellow flame = Na⁺, but Na⁺ gives no precipitate with NaOH.
(C) Light green flame is not characteristic of any common cation; green ppt. soluble in excess indicates Cr³⁺, but chromium gives no distinct flame colour.
(D) Red flame = Li⁺ or Ca²⁺ (orange-red), but red-brown ppt. indicates Fe³⁺, a mismatch.
✅ Answer: (B)
▶️ Answer/Explanation
Correct Answer: D
Explanation: Flame test colours for common metal ions:
– Sodium (Na⁺) gives a yellow flame.
– Potassium (K⁺) gives a lilac flame.
– Copper(II) (Cu²⁺) gives a blue-green flame.
– Calcium (Ca²⁺) gives an orange-red flame.
✅ X = sodium (yellow), Y = potassium (lilac), Z = copper(II) (blue-green).
▶️ Answer/Explanation
A gas that turns damp red litmus paper blue is ammonia (NH3), which is an alkaline gas. Ammonia is produced when a base (such as a basic oxide) reacts with an ammonium salt. For example, calcium oxide (a basic oxide) reacts with ammonium chloride to produce ammonia, calcium chloride, and water. The other combinations would not produce ammonia gas.
✅ Answer: (B)
✅ Answer: (B)