Question

Which of the following environments is richest in free oxygen?
A. Warm fresh water
B. Atmosphere
C. Cold fresh water
D. Salt water

Answer 1

Hint: Oxygen is the most important, abundant and major component of the Earth’s atmosphere. Life is unpredictable without oxygen. And when life originated, it is believed that oxygen gas was the first to be produced around \[2.7\] billion years ago by the process of photosynthesis by tiny blue green algae or other microbes.

Complete explanation:
Option A Warm fresh water: In warm water due to heating, the molecules of water move faster and allow oxygen to escape from it and thereby less dissolved oxygen remain present in the water.
Option A is not correct.
Option B Atmosphere:Tiny organisms like blue green algae, some microbes and plants conduct photosynthesis using free sunshine, water and carbon dioxide to produce carbohydrates and oxygen as well causing the abundance of oxygen in the atmosphere.
So, option B is correct.
Option C Cold fresh water: Cold fresh water also contains less space for oxygen molecules to bind. Solid ice can be thought of as an example as it is solid so less space is available but it can hold more than warm and saltier water.
Option C is not correct.
Option D Salt water: Salt water contains sodium and chloride ions dissolved in it and thereby lesser space is available for oxygen molecules to bind. It is mainly seawater which has approximately \[35\] grams of salt in each litre of water. There is only \[0.203\] atm of partial pressure of oxygen at sea level which cannot be a good source of free oxygen.
Option D is not correct.

Therefore, Option B is the correct answer.

Note:
Before the evolution of cyanobacteria there was no free oxygen on earth because oxygen is highly reactive and unstable in free form. After the evolution of cyanobacteria, the environment was converted into an oxidising environment after which life came into existence. Now the atmosphere is considered as the best source of free oxygen.