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7 (a) Qualitative tests are carried out on ionic substances to identify the ions present in the substances - Edexcel - GCSE Chemistry - Question 7 - 2019 - Paper 1
Question 7
7 (a) Qualitative tests are carried out on ionic substances to identify the ions present in the substances. (i) Explain why the test for a given ion must be unique ... show full transcript
Worked Solution & Example Answer:7 (a) Qualitative tests are carried out on ionic substances to identify the ions present in the substances - Edexcel - GCSE Chemistry - Question 7 - 2019 - Paper 1
Step 1
Explain why the test for a given ion must be unique to that ion.
Answer
The test for a given ion must be unique to that ion to ensure that the results obtained are specific and reliable. If a test is not unique, it may yield positive results for multiple ions, making it difficult to identify the correct ion in a mixture. This specificity is essential in qualitative analysis as it confirms the presence of individual ions without ambiguity.
Step 2
Step 3
Draw one straight line from the test for each ion to the observation that shows that ion to be present.
Answer
- Test for chloride ion: add dilute nitric acid followed by silver nitrate solution → white precipitate
- Test for iodide ion: add dilute nitric acid followed by silver nitrate solution → red precipitate
- Test for sulfate ion: add dilute hydrochloric acid followed by barium chloride solution → yellow precipitate
Step 4
Describe tests to show the presence of each of these metal ions.
Answer
To test for sodium ions, a flame test can be performed; the flame will be yellow.
For potassium ions, a flame test will show a lilac color.
Calcium ions can be detected using a flame test which gives an orange-red flame.
To test for aluminum ions, add sodium hydroxide solution; a white precipitate will form, which dissolves in excess NaOH.