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Vanessa Boey Khai Ling
Vanessa Boey Khai Ling

primary 6 chevron_right chevron_right Singapore

PSLE Science - Please Explain why answer is A :)

Date Posted: 1 month ago
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1 month ago
From Test 1 , you can deduce that there is displacement of iodide ions and they are oxidised to I2. The gas mixture is likely to contain either chlorine or bromine.

Test 2 suggests that the gas mixture contains a strong oxidising agent to oxidise Fe(II) ions in aqueous FeCl2(green) to Fe(III) ions in FeCl3 (yellow, usually brownish yellow). Both chlorine and bromine are able to oxidise


Bromine gas forms hydrobromic acid (HBr) and hypobromous acid (HOBr) when dissolved in water. Similarly , chlorine gas forms hydrochloric acid (HCl) and hypochlorous acid (HOCl) when dissolved in water.

The hypobromous and hypochlorous acids partially dissociate to form H+ ions and the respective hypobromite and hypochlorite ions, which are oxdising agents.

Thus, when chlorine or bromine gas dissolves in the water of the damp litmus papers, the acidic solutions formed turns blue litmus red. The oxidising agents are responsible for turning the red litmus white ('bleaching')

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Jefri Sanusi Teguh
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