CBSE Questions for Class 8 Chemistry Pollution Of Air And Water Quiz 2 - MCQExams.com

Air is a mixture because:

(i) The properties of the constituents of air (oxygen, nitrogen, carbon dioxide etc.) are not changed. For example, pure oxygen is a supporter of combustion. Oxygen present in the air also supports combustion. Pure carbon dioxide turns lime water milky. Carbon dioxide present in the air also turns lime water milky.

(ii) The proportions of the constituents of air vary from place to place. Even in the same place, the proportion varies from time to time. In a compound, the proportions of different constituents always remain fixed.

(iii) On distillation of liquefied air, nitrogen distills off first, leaving behind oxygen in the liquid form. This is the characteristic of a mixture, not of a compound.

(iv) No thermal change occurs when the constituents of air (oxygen, nitrogen etc.) are mixed together.

(v) Liquid air does not have a fixed boiling point. This is the characteristic of a mixture, not of a compound.

Photochemical smog is produced when pollutants from the combustion of fossil fuels react with sunlight. The energy in the sunlight converts the pollutants into other toxic chemicals. For photochemical smog to form, there must be other pollutants in the air, specifically nitrous oxides and other volatile organic compounds (VOCs).

Hence, the correct option is $$B$$

• If the ozone layer becomes depleted, then more UV rays will reach the Earth. Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants, like more skin cancers, sunburns and premature aging of the skin.
• Continued warming from the release of greenhouse gases (GHGs) into the atmosphere is expected to have a substantial impact on the environment, human health and the economy, like global warming, will decrease snow, sea ice and glacier coverage, resulting in rising sea levels and increased coastal flooding. Rising temperatures will also thaw permafrost in the Arctic.
• Effects of Greenhouse gases would eventually lead to the melting of ice worldwide, especially at the Earth's poles due to Global warming. This includes mountain glaciers, ice sheets covering West Antarctica and Greenland, and Arctic sea ice.

• The progressive heating up of the surface of the Earth is called the greenhouse effect or global warming.

• $$CO_2$$ and $$H_2O$$ block a part of the heat emitted by the Earth and reflect it back. Due to this, the Earth's surface gets heated up.

• 50% of $$CO_2$$ content of air can increase the surface temperature of the Earth by $$3^oC$$.

• In addition to $$CO_2$$, the other greenhouse gases are $$CFC$$ (17%), $$O_3$$ (18%), $$NO$$ (4%) and water vapour (2%).