Chemistry

This is a Multiple choice Questions as classified in NCERT Exemplar

Option (iii)

i.e., Decrease in the amount of dissolved oxygen in water is the answer since organic waste is oxidized by microorganisms in the presence of dissolved oxygen. Hence, oxygen decreases in water as a result it is harmful for aquatic life. 

This is a multiple choice answer as classified in NCERT Exemplar

(iv) Powdered sugar in hot water.

Explanation: (i) The solubility of a solute/sugar particle increases as its surface area increases. Powdered sugar dissolves more quickly due to its increased surface area.

(ii) Because it provides a cool sensation when touched, sugar dissolving is an endothermic process. Increasing the temperature or using hot water, according to LeChatelier's theory, facilitates sugar breakdown.

This is a Multiple choice Questions as classified in NCERT Exemplar

Option (i) i.e., rich in dissolved oxygen is the answer since the total amount of oxygen consumed by microorganisms in decomposing organic matter present in a certain volume of water is known as Biochemical Oxygen Demand (BOD) of water. If the BOD value is less than 5 ppm, then the sample of water is considered to be pure and is rich in dissolved oxygen. 

This is a Multiple choice Questions as classified in NCERT Exemplar

Option (i)

i.e., Its main components are produced by the action of sunlight on emissions of automobiles and factories is the answer since classical smog is a mixture of smoke, fog and sulphur dioxide and occurs in cold humid climates. While the gases released by automobiles and factories are not responsible for classical fog. 

This is a Multiple choice Questions as classified in NCERT Exemplar

Option (iii)

SO₂ is correct since photochemical fog is formed in the presence of sunlight in summers when NO and hydrocarbons are present in large amounts in the atmosphere. SO₂ is not responsible for photochemical fog. 

This is a Multiple choice Questions as classified in NCERT Exemplar

Option (i)

CO is correct since it does not absorb sunlight near the earth's surface and hence it is not radiated back to the earth. Greenhouse gases have this kind of property. 

This is a Assertion Type Questions as classified in NCERT Exemplar

Option (iii)

 i.e.,   Both A and R are not correct since the amount of oxygen required by bacteria to breakdown the organic matter present in a certain volume of a sample of water, is called BOD i.e., Biochemical Oxygen Demand. If the BOD of the water is < 5 ppm then it is clean but if it increases then the water pollution increases.

This is a Assertion Type Questions as classified in NCERT Exemplar

Option (i)

 i.e., Both A and R are correct, and R is the correct explanation of A is the answer since pesticides are the synthetic toxic chemicals with ecological repercussions and their repeated use gives rise to pests that are resistant to that group of pesticides and hence making the pesticides ineffective. Therefore, as insect resistance of DDT increased, other organic toxins such as Aldrin and Dieldrin got introduced in the market by the pesticide industry. Most of the organic toxins are water insoluble and are non-biodegradable. 

This is a Assertion Type Questions as classified in NCERT Exemplar

Option (iv)

i.e., A is not correct, but R is correct since ozone in the stratosphere is a product of ultraviolet radiations falling on oxygen molecules. The main cause of ozone layer depletion is the release of CFCs i.e., freons. With the depletion of the ozone layer, more ultraviolet radiation filters into the troposphere. These radiations lead to ageing of skin, cataract, sunburn, skin cancer etc.