Qualitative tests are used to identify ions in compounds - Edexcel - GCSE Chemistry - Question 8 - 2018 - Paper 1
Question 8
Qualitative tests are used to identify ions in compounds.
(a) Solid X contains two ions.
The tests for these ions and their results are shown in Figure 10.
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Worked Solution & Example Answer:Qualitative tests are used to identify ions in compounds - Edexcel - GCSE Chemistry - Question 8 - 2018 - Paper 1
Step 1
name of cation
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Answer
The cation can be deduced from the red-orange flame test, which indicates the presence of sodium ions (Na⁺).
Step 2
name of anion
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Answer
The anion can be determined from the formation of a white precipitate upon adding dilute nitric acid followed by silver nitrate. This indicates the presence of chloride ions (Cl⁻).
Step 3
(i) Give the reason why this test does not identify the cation in solid Y.
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The formation of a white precipitate does not specify the cation present, as multiple cations can produce a similar result with sodium hydroxide.
Step 4
(ii) Give the reason why this test does not identify the anion in solid Y.
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Sodium hydroxide is not used to test for anions; it only precipitates certain cations.
Step 5
Give a reason why instrumental analysis may be better than other methods of analysis.
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Instrumental analysis provides improved sensitivity, accuracy, and reliability of results compared to qualitative methods.
Step 6
(i) Write the ionic equation for this reaction.
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The ionic equation is:
Fe3++3OH−→Fe(OH)3(s)
Step 7
(ii) Explain this observation.
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Answer
The green iron(II) hydroxide precipitate turns brown due to its oxidation by oxygen in the air, forming iron(III) hydroxide, which is brown.
