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
Use the information in Figure 10 to name the cation and the anion in solid X.
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Answer
The cation in solid X can be identified as calcium ( ext{Ca}^{2+}) due to the red-orange flame test, which indicates the presence of calcium ions. The anion can be identified as chloride ( ext{Cl}^-) since the addition of dilute nitric acid followed by silver nitrate produces a white precipitate, indicating the presence of chloride ions.
Step 2
Give the reason why this test does not identify the cation in solid Y.
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Answer
The test does not identify the cation in solid Y because sodium hydroxide can produce a white precipitate with multiple cations. Therefore, the formation of a white precipitate does not uniquely indicate the presence of any specific cation.
Step 3
Give the reason why this test does not identify the anion in solid Y.
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The presence of a white precipitate could result from several anions, so this test does not uniquely identify any specific anion in solid Y.
Step 4
Give a reason why instrumental analysis may be better than other methods of analysis.
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Instrumental analysis may be better because it improves sensitivity and accuracy of results, allowing for more precise identification and quantification of ions than traditional methods.
Step 5
Write the ionic equation for this reaction.
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Answer
The ionic equation for the reaction of iron(II) sulfate with sodium hydroxide can be written as:
ightarrow ext{Fe(OH)}_2 (s)$$
Step 6
Explain this observation.
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The green iron(II) hydroxide precipitate gradually turns brown upon exposure to air because it is oxidized to iron(III) hydroxide ext{Fe(OH)}_3, which has a brown coloration. This oxidation occurs as iron(II) ions are converted to iron(III) ions on exposure to oxygen in the air.
