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Chemistry Chapter 12 : Topic 1
Substances in Natural Water
1 / 20
Natural water sources such as rivers and lakes contain dissolved oxygen, which is considered important for aquatic life. Which statement best explains its importance?
Reasoning: Dissolved oxygen is essential for the respiration of fish and other aquatic organisms.
2 / 20
Which of the following substances found in natural water is considered beneficial in small amounts but harmful in excess?
Reasoning: Nitrates promote plant growth in small amounts but cause eutrophication when excessive.
3 / 20
Why are metal compounds in natural water considered dangerous?
Reasoning: Heavy metals like lead and mercury are toxic and can bioaccumulate in food chains.
4 / 20
Sewage in natural water is mainly harmful because it contains:
Reasoning: Sewage introduces pathogens and organic waste that pollute water.
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Which pollutant in water is mainly responsible for spreading water-borne diseases?
Reasoning: Microorganisms such as bacteria and viruses cause diseases like cholera.
6 / 20
Plastics in natural water are harmful because they:
Reasoning: Plastics do not decompose easily and form microplastics that affect aquatic life.
7 / 20
Excess fertilizers in water bodies mainly lead to:
Reasoning: Nitrates and phosphates cause excessive algal growth, reducing oxygen.
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Which combination of substances in water mainly comes from agricultural runoff?
Reasoning: Fertilizers contain nitrates and phosphates that wash into water bodies.
9 / 20
Phosphates in water mainly originate from:
Reasoning: Detergents and fertilizers are major sources of phosphates in water pollution.
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Why is dissolved oxygen considered a “useful substance” in water?
Reasoning: Aquatic organisms depend on dissolved oxygen for survival.
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Which of the following is NOT naturally beneficial in water ecosystems?
Reasoning: Sewage introduces harmful pathogens and pollutes water.
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A lake receives excessive fertilizers from nearby farms. What is the most likely effect?
Reasoning: Algal bloom consumes oxygen, leading to fish death.
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Which substance is most likely to cause long-term poisoning through food chains?
Reasoning: Heavy metals accumulate in organisms and magnify through food chains.
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Why are plastics considered persistent pollutants in water?
Reasoning: Plastics remain in the environment for a very long time.
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Which of the following best describes water from natural sources?
Reasoning: Natural water contains both beneficial and harmful dissolved substances.
16 / 20
What is the main danger of nitrates in drinking water?
Reasoning: Excess nitrates affect human health and aquatic balance.
17 / 20
Which process is most directly affected by water pollution from sewage?
Reasoning: Sewage reduces oxygen and introduces pathogens affecting aquatic life.
18 / 20
Which pair correctly represents harmful substances in natural water?
Reasoning: Both fertilizer chemicals and sewage / heavy metals are harmful pollutants.
19 / 20
Why is clean natural water still not always safe for drinking?
Reasoning: Pathogens and pollutants may be present even if water looks clean.
20 / 20
Which statement correctly distinguishes useful from harmful substances in natural water?
Reasoning: Water contains both essential (oxygen) and harmful (sewage, toxins) substances.
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