Chemistry 9th Chap 12 : Topic 2 - Maryam Publications

Chemistry Chapter 12 : Topic 2

Beneficial Substances in Water

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Natural water contains dissolved oxygen, which is essential for aquatic ecosystems. Which statement best explains its role?

A) It increases water temperature and speeds up evaporation.

B) It removes dissolved minerals from water.

C) It reacts with salts to form pure water.

D) It supports respiration of fish and other aquatic organisms.

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Why is dissolved oxygen considered a naturally beneficial substance in water?

A) It prevents all types of pollution.

B) It kills harmful bacteria in water.

C) It makes water chemically pure.

D) It is required for survival of aquatic organisms.

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Some metal compounds in natural water are beneficial because they:

A) Always make water safe for drinking

B) Remove harmful gases from water

C) Provide essential trace minerals needed by living organisms

D) Increase the boiling point of water

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Which of the following best describes the role of trace metal ions in natural water?

A) They have no biological importance.

B) They are always toxic and must be removed completely.

C) They convert water into a pure substance.

D) They act as essential nutrients in very small amounts.

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Why can iron compounds in water sometimes be beneficial?

A) They help in photosynthesis of aquatic plants.

B) They sterilize water naturally.

C) They provide essential iron required for blood formation in animals.

D) They increase oxygen production in water.

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Which statement correctly differentiates beneficial substances from harmful ones in natural water?

A) Beneficial substances are needed in controlled amounts for life processes.

B) Only gases in water are beneficial.

C) All dissolved substances are harmful.

D) Beneficial substances are those that increase pollution.

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Calcium compounds in natural water are important because they:

A) Increase toxicity of water

B) Help in strengthening bones and shells of organisms

C) Remove oxygen from water

D) Cause water to become acidic

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Magnesium ions present in water are beneficial because they:

A) Convert water into a gas

B) Reduce dissolved oxygen

C) Destroy aquatic life

D) Are essential for chlorophyll formation in plants

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Why are naturally occurring mineral ions in water not always harmful?

A) They are always diluted beyond usefulness.

B) They are required in trace amounts for biological functions.

C) They are chemically inactive.

D) They evaporate quickly.

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Which of the following is an example of a beneficial dissolved substance in water?

A) Plastic particles

B) Mercury compounds

C) Dissolved oxygen

D) Sewage waste

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What is the significance of balanced mineral content in natural water?

A) It increases water hardness only for industrial use.

B) It supports biological processes in living organisms.

C) It eliminates all microbes.

D) It makes water completely pure.

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Why is excessive removal of all minerals from drinking water not ideal?

A) It reduces essential nutrients needed by the body.

B) It increases pollution.

C) It causes water to evaporate.

D) It makes water radioactive.

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Which of the following correctly explains the role of dissolved oxygen in water ecosystems?

A) It increases salinity of water.

B) It converts water into hydrogen gas.

C) It is used in respiration by aquatic organisms.

D) It causes corrosion of metals only.

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Natural water containing trace minerals is considered beneficial because it:

A) Becomes chemically unstable

B) Becomes toxic automatically

C) Loses all dissolved gases

D) Provides essential nutrients for organisms

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Which statement best describes the dual nature of substances in natural water?

A) All substances are harmful.

B) Some substances are beneficial in small amounts while others are harmful in excess.

C) All substances are beneficial.

D) Water contains only gases.

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Why is magnesium in water considered biologically important?

A) It removes oxygen from water.

B) It increases water hardness only.

C) It kills bacteria in water.

D) It is a part of chlorophyll in plants.

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Which of the following is TRUE about beneficial substances in water?

A) They are required in controlled trace amounts.

B) They are useful only in large concentrations.

C) They are never naturally occurring.

D) They are always organic compounds.

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What happens if dissolved oxygen level in water decreases significantly?

A) Aquatic organisms may suffocate

B) Aquatic life thrives more

C) Water becomes distilled

D) No change occurs

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Why is natural water considered a mixture of beneficial and harmful substances?

A) Because it has no dissolved gases.

B) Because it includes essential nutrients and potential pollutants.

C) Because it contains only salts.

D) Because it is always polluted.

20 / 20

Which best explains why some metal compounds in water are beneficial?

A) They eliminate all bacteria.

B) They increase evaporation rate.

C) They act as essential micronutrients for living organisms.

D) They always purify water.

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