Question

The given graph shows changes in concentration of two substances X and Y in a river polluted by sewage. Which of the following options most appropriately identify X and Y?

A.X- carbon dioxide, Y- nitrogen compounds
B. X- nitrogen compounds, Y – carbon dioxide
C. X- nitrogen compounds, Y- oxygen
D. X- oxygen, Y- nitrogen compounds

Answer 1

Sewage mostly consists of compounds like, ammonium nitrate, nitrogen in large amounts, and phosphorus. These are all the organic matter of sewage, which is generated from the breakdown of microorganisms and other living species.

Complete answer:
We have been given a graph that shows to identify the substances labeled as X and Y. This is to be identified when sewage is added in that polluted river.
As we know sewage consists of mostly water, but some percentage of sewage also has organic matter as a result of degradation of organic materials. This organic matter includes mainly ammonium nitrate and other nitrogenous compounds. The nitrogenous compounds of the sewage are slowly consumed by the oxygen present in water. This oxygen proves a key to break down the nitrogenous compounds. These compounds are acted upon by bacteria that utilize the oxygen to convert the organic matter into gases (like carbon to carbon dioxide).
In the graph, the Y curve depicts the concentration in the sewage which is maximum when sewage is added and slowly degrades when the substance X acts upon it. So, it can be inferred that X must be the oxygen which is low when sewage is added, and this oxygen consumes and degrades the nitrogenous organic matter that degrades and lowers in concentration.
Hence, substance X is oxygen, while substance Y is the nitrogen compound.

So, option D is correct.

Note:
The downstream distance is distance in km down the river stream. The moment the sewage and the organic matter is added this concentration of nitrogen compounds decreases down the stream, while the oxygen consumed this matter in the upstream.