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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

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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.

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Answer

The test for a specific ion must be unique to that ion to avoid false positives. If multiple ions react similarly to the test, it would be impossible to determine the presence of one specific ion in a mixture. A unique test ensures that the observed result can be confidently attributed only to the target ion, providing clarity and reliability in identifying ionic substances.

Step 2

State the name of the gas produced in the test if carbonate ions are present.

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The gas produced when carbonate ions (CO₃²⁻) react with dilute hydrochloric acid is carbon dioxide (CO₂). This reaction typically results in effervescence as the gas escapes.

Step 3

Tests for three ions are described. Draw one straight line from the test for each ion to the observation that shows that ion to be present.

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Test for IonObservation
Chloride ionwhite precipitate
Iodide ionyellow precipitate
Sulfate ionwhite precipitate

Step 4

Describe tests to show the presence of each of these metal ions.

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Answer

To test for sodium ions, a flame test can be performed; a bright yellow flame indicates the presence of sodium. For potassium ions, similarly, a flame test produces a lilac flame. To identify calcium ions, adding a few drops of sodium hydroxide (NaOH) will result in a white precipitate of calcium hydroxide (Ca(OH)₂). Lastly, for aluminum ions, a similar approach with NaOH may yield a white precipitate initially, but dissolves in excess NaOH to form a colorless solution, confirming the presence of aluminum.

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