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Qualitative tests are used to identify ions - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2016 - Paper 1
Question 6
Qualitative tests are used to identify ions. (a) A student carries out a flame test on an unknown solid. A red flame is seen. The student concludes that the solid i... show full transcript
Worked Solution & Example Answer:Qualitative tests are used to identify ions - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2016 - Paper 1
Step 1
Explain why this conclusion is not justified.
Answer
The conclusion that the unknown solid is lithium carbonate based solely on a red flame is not justified for several reasons.
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Flame Test Limitations: Different substances can produce similar flame colors. While lithium compounds do produce a red flame, other substances might also produce similar colors. Therefore, a single test is not definitive for identifying a compound.
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Presence of Other Ions: The unknown solid may contain other ions or compounds that produce a red flame, leading to a misidentification.
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Lack of Additional Tests: Identification should be corroborated with other tests, such as acid reactions or precipitation tests, to confirm the presence of carbonate ions before concluding that the solid is lithium carbonate.
Step 2
1. A flame test on the solid substance produced a yellow colour.
Answer
A yellow flame indicates the presence of sodium ions (Na⁺), as sodium is known to produce a bright yellow flame when subjected to a flame test. This suggests that the solid likely contains sodium ions along with any other ions present.
Step 3
2. Dilute hydrochloric acid was added to a solution of the substance followed by a few drops of barium chloride solution.
Answer
When dilute hydrochloric acid is added, it forms a soluble chlorides solution, and the addition of barium chloride results in a white precipitate of barium sulfate (BaSO₄) if sulfate ions (SO₄²⁻) are present. Therefore, this indicates that the substance contains both sulfate ions and possibly other ions.
Step 4
Describe the test to show the gas is ammonia.
Answer
To test for ammonia gas, the following procedure can be applied:
- Warm the sodium hydroxide solution with the ammonium solution.
- Test the gas evolved by bringing a damp litmus paper or red cabbage indicator near the gas.
- If the paper turns blue, it confirms the presence of ammonia, as ammonia is alkaline and will change the color of the indicator.
Step 5
Describe what is seen when sodium hydroxide solution is added to each of these solutions.
Answer
When sodium hydroxide solution is added to an iron(II) solution:
- A green precipitate of iron(II) hydroxide (Fe(OH)₂) will form, indicating the presence of iron(II) ions.
When sodium hydroxide solution is added to an iron(III) solution: 2. A brown precipitate of iron(III) hydroxide (Fe(OH)₃) will form, indicating the presence of iron(III) ions.
Thus, the different colors of the precipitates allow for the distinguishing between iron(II) and iron(III) ions.