iGCSE Chemistry (0620) Core:10.1 Water: Exam Style Questions Paper 1- New Syllabus
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Detailed solution:
Dissolved oxygen is actually beneficial—aquatic life needs it to survive. Nitrates and phosphates, however, are harmful. They come from fertilisers and detergents, and they cause excessive plant growth. When these plants die and decompose, the process uses up oxygen, leading to deoxygenation and damage to aquatic ecosystems. So only 2 and 3 are harmful.
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Detailed solution:
Anhydrous cobalt(II) chloride is blue and turns pink when it comes into contact with water. Anhydrous copper(II) sulfate is white and turns blue when hydrated by water. Both are classic chemical tests used to detect the presence of water. Litmus and methyl orange are acid-base indicators; they change colour based on pH, not specifically due to water. So substances 1 and 2 are correct.
▶️ Answer/Explanation
Detailed solution:
In the treatment of domestic water supplies, activated carbon (also called activated charcoal) is used to remove tastes and odours from the water. Carbon works through a process called adsorption, where impurities stick to the large surface area of the carbon particles. Chlorine is used for disinfection (killing microbes), not for removing tastes and odours. Calcium and copper are not used for this purpose in water treatment. Therefore, the element used to remove tastes and odours is carbon, making option B correct.