Question

At DO below 4 mg/l, water at normal temperature is considered as
A. Heavily polluted
B. Moderately polluted
C. Little polluted
D. Nearly pure

Answer 1

Dissolved oxygen refers to the level of oxygen dissolved in water. Dissolution of oxygen depends on various factors such as temperature, pressure, and salinity. In colder temperature and high pressure levels of dissolved oxygen is higher in water. There are certain temperature ranges suitable for survival of living organisms. Aquatic life depends on presence to DO in water.

Complete step by step answer: Dissolved oxygen is a very important factor in assessing water quality. Water quality influences aquatic life. Oxygen molecules or non compound oxygen not bound to any other element. Free oxygen molecules are present within water same as the way salt and sugar does.
There is a specific range to dissolved oxygen that may affect the living of aquatic organisms such that if the level of DO is below 5mg/l that may be considered to be hazardous to the aquatic living organisms. Pollution levels of water bodies are measured on the basis of the presence of oxygen in it. It may be highly polluted, moderately polluted, less polluted or nearly pure. Temperature, pressure and salinity affect the solubility of oxygen in water. If the temperature is colder than the level of DO will be high, same as in pressure, greater the pressure, more will be the DO but in case of salinity DO will be more when salinity is lower than normal.
Dissolved oxygen is necessary for organisms living in water bodies. These organisms include fish, invertebrates, bacteria, and plants. They use it for their respiration. Bottom feeders need oxygen in a minimal amount, while shallow water fish need it in higher amounts. Microbes also need DO for decomposing organic material at the bottom of a body of water.
As we have discussed above, that level of dissolved oxygen should not be below than 5mg/l because it can be harmful for aquatic life so at normal temperature, any water body showing less than 5mg/l DO then it will be highly polluted.
So, the correct answer is option A that is ‘heavily polluted’.

Note: 100 percent of air saturation is observed in a stable body of water. It means that water is holding as many as dissolved oxygen gas molecules as it can in equilibrium. Dissolved oxygen is lower at deep sea levels. Increasing temperature decreases solubility of oxygen in water. At room temperature it may decrease from 10 to 8mg/l. Saltwater can hold about 20 percent less dissolved oxygen than fresh water at the same temperature and pressure.