Qualitative Analysis
At each stage of any test you are to record details of the following - CIE - A-Level Chemistry - Question 3 - 2016 - Paper 1
Question 3
Qualitative Analysis
At each stage of any test you are to record details of the following.
- colour changes seen
- the formation of any precipitate
- the solubilit... show full transcript
Worked Solution & Example Answer:Qualitative Analysis
At each stage of any test you are to record details of the following - CIE - A-Level Chemistry - Question 3 - 2016 - Paper 1
Step 1
Add aqueous sodium hydroxide.
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Answer
When aqueous sodium hydroxide is added to FA 5, a white precipitate indicates the presence of Al³⁺ ions if they are present. In FA 6, no precipitate confirms the absence of Al³⁺. Record the observations accordingly.
Step 2
Add aqueous ammonia.
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After adding aqueous ammonia, observe that FA 5 may show a precipitate. If soluble in excess, note that theions involved could include Al³⁺ or Zn²⁺. On the other hand, FA 6 might show little to no change, confirming the absence of these ions.
Step 3
Add a 1 cm depth of dilute hydrochloric acid, then transfer the mixture into a boiling tube and warm gently.
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When dilute hydrochloric acid is added, if gas is evolved in FA 5, it indicates the presence of carbonate ions. With FA 6, observing no reaction may indicate a lack of carbonate. Gently warming helps in confirming gas evolvement.
Step 4
Add two or three drops of acidified aqueous potassium manganate(VII).
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In FA 5, a color change might occur indicating redox activity, while FA 6 should be tested for the unchanged state which would be expected if no reducing agent is present.
Step 5
Add a 1 cm depth of aqueous barium chloride or barium nitrate, then add dilute hydrochloric acid.
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In this step, if a white precipitate forms, it may indicate the presence of sulfate ions in FA 5. Meanwhile, FA 6 can be analyzed similarly, with the absence of a precipitate suggesting no sulfate.
Step 6
Identify as many of the ions present in FA 5 and FA 6.
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For FA 5, the cation is identified as Al³⁺, and the anion is SO₄²⁻. For FA 6, with the absence of detectable types from the tests, you can conclude that 'unknown' ions are present.
Step 7
(i) Select reagents to test whether the anion in FA 7 contains the same element as the anion in FA 5.
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To test FA 7 for the sulfate anion, add BaCl₂ to check for a white precipitate indicating sulfate ions. If positive, this confirms the presence of SO₄²⁻ in FA 7.
Step 8
(ii) Select reagents to test whether the anion in FA 7 contains the same element as the anion in FA 6.
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Test FA 7 using HCl and add BaCl₂; a white precipitate confirms the SO₄²⁻ from FA 6. If there’s no precipitate, conjecture that the anion is different.
