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CAIE A Level Chemistry 9701 · Group 17

Group 17: suggest question

Suggest4 marksStandard

In water treatment, chlorine is added to water. In the water, chlorine undergoes disproportionation to form chloride ions and chloric(I) acid, HOCl. HOCl is a weak acid that partially dissociates to form H+ and ClO− ions.

A water treatment plant adds chlorine to water that has been adjusted to pH 8.5. At this pH, most of the chlorine is present as ClO− ions rather than as HOCl.

Suggest why the plant might lower the pH of the water to about 6.5 before adding the chlorine, and explain how this improves the effectiveness of the treatment.

0 words · 4 marks

No account needed for your first marks. Your answer is used only to mark it.

How this question is marked

4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.

Suggest means: Apply what you know to an unfamiliar situation. There may be more than one acceptable answer. Reason from the principle you have learned to the new context, and say which principle you are using.

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