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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 solid is lithium carbonate based on the red flame is not justified because several other compounds can produce a red flame during a flame test. For instance, the presence of strontium or lithium salts may also result in a red flame. A more definitive test would be needed, such as a reaction with dilute acid, to confirm the presence of carbonate ions.
Step 2
Step 3
2. Dilute hydrochloric acid was added to a solution of the substance followed by a few drops of barium chloride solution. A white precipitate formed.
Answer
The formation of a white precipitate indicates the presence of sulfate ions (SO₄²⁻) when barium chloride (BaCl₂) is added. This is because barium sulfate (BaSO₄) is insoluble in water and precipitates out.
Step 4
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 the iron(II) solution, a green precipitate of iron(II) hydroxide (Fe(OH)₂) is formed. In contrast, when sodium hydroxide is added to the iron(III) solution, a reddish-brown precipitate of iron(III) hydroxide (Fe(OH)₃) appears. This difference in color helps distinguish between the two iron cations.