A sample of water from a stream, suspected to be contaminated with metal ions, was analysed - HSC - SSCE Chemistry - Question 10 - 2010 - Paper 1
Question 10
A sample of water from a stream, suspected to be contaminated with metal ions, was analysed.
The results of some tests on the water are recorded in the table.
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Worked Solution & Example Answer:A sample of water from a stream, suspected to be contaminated with metal ions, was analysed - HSC - SSCE Chemistry - Question 10 - 2010 - Paper 1
Step 1
Add dilute HCl: No change
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Answer
The test with dilute HCl shows no reaction, indicating that the water sample does not contain carbonate or sulfide ions that would usually produce carbon dioxide or hydrogen sulfide upon acidification.
Step 2
Add Na2SO4 solution: White precipitate formed
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Answer
The formation of a white precipitate upon adding Na2SO4 indicates the presence of barium ions (Ba2+) or lead ions (not listed as an option). Since BaSO4 is insoluble, a white precipitate suggests that Ba2+ is present.
Step 3
Flame test: Pale green colour
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Answer
A pale green flame indicates the presence of barium ions (Ba2+). Other tested ions such as Ca2+ emit a brick-red color, while Cu2+ gives a blue-green color, and Fe3+ emits a gold/yellow flame.
Step 4
Conclusion
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Answer
Based on the tests, the most likely contaminant in the water is Ba2+. Hence, the correct answer is (A) Ba2+.
