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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 accurate identification. If a test is not exclusive, it may yield false positives from other ions present, leading to misinterpretation of results. Thus, a unique test helps in confirming the presence of a specific ion without confusion or overlap with other possible substances.
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
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Test for chloride ion: add dilute nitric acid followed by silver nitrate solution → white precipitate.
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Test for iodide ion: add dilute nitric acid followed by silver nitrate solution → yellow precipitate.
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Test for sulfate ion: add dilute hydrochloric acid followed by barium chloride solution → red 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 where a yellow flame indicates the presence of sodium. For potassium ions, a flame test will yield a lilac flame. For calcium ions, a flame test gives an orange-red flame. Lastly, aluminum can be tested by adding sodium hydroxide; if a white precipitate forms which dissolves in excess sodium hydroxide, it confirms the presence of aluminum ions.