CIE iGCSE Co-Ordinated Science C12.5 Identification of ions and gases Exam Style Questions Paper 1
▶️ Answer/Explanation
Ammonia is an alkaline gas, and alkalis turn damp red litmus paper blue. This is a characteristic test for ammonia gas.
Understanding the other gases:
- Carbon dioxide is acidic and turns damp blue litmus paper red (or turns limewater milky).
- Chlorine is a bleaching gas and bleaches damp litmus paper (turns it white).
- Hydrogen is neutral and does not affect litmus paper (it is tested with a lighted splint, producing a ‘pop’ sound).
Therefore, ammonia is the gas that turns damp red litmus paper blue.
✅ Answer: (A)
▶️ Answer/Explanation
With aqueous ammonia, \(\mathrm{Cu^{2+}}\) gives a light blue precipitate that dissolves in excess to form a dark blue solution.
\(\mathrm{Zn^{2+}}\) gives a white precipitate that dissolves in excess to form a colourless solution.
\(\mathrm{Ca^{2+}}\) and \(\mathrm{Fe^{2+}}\) do not dissolve in excess ammonia.
✅ Answer: (D)
\(\mathrm{Zn^{2+}}\) gives a white precipitate that dissolves in excess to form a colourless solution.
\(\mathrm{Ca^{2+}}\) and \(\mathrm{Fe^{2+}}\) do not dissolve in excess ammonia.
✅ Answer: (D)
▶️ Answer/Explanation
When acidified aqueous silver nitrate is added to a solution containing chloride ions, a white precipitate of silver chloride (AgCl) forms. This is a specific test for chloride ions. Copper ions would give a coloured precipitate, hydroxide ions would not give a precipitate with silver nitrate, and sodium ions would not react.
✅ Answer: (A)
✅ Answer: (A)
▶️ Answer/Explanation
Sodium ions produce a characteristic yellow flame in the flame test. Lithium gives red, potassium gives lilac, and copper(II) gives blue-green. These colours are used to identify metal ions in compounds.
✅ Answer: (D)
✅ Answer: (D)
▶️ Answer/Explanation
Limewater (calcium hydroxide solution) is the standard test for carbon dioxide — when \(\text{CO}_2\) is bubbled through it, a white precipitate of calcium carbonate forms, turning the solution milky/cloudy.
The reaction is: \(\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\).
✅ Answer: (D)
The reaction is: \(\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\).
✅ Answer: (D)