Question

Thermal pollution causes a decrease in dissolved oxygen. Is the above statement true?
A. True
B. False

Answer 1

Hint: The solubility in case of liquid solution with gases as solute is inversely proportional to the temperature.

Complete answer
We know that solubility of a solute can be affected by various factors for example, nature of solute or that of solvent, temperature, pressure etc. here, we will have a look at the effect of temperature on the solubility when we have solutions of gases in liquid solvent. For such solutions, we have Henry’s law that can be expressed as:
\[p = {K_{\rm{H}}}x\]
Here, \[p\] is the partial pressure of the gas above the solution, \[{K_{\rm{H}}}\] is the Henry’s law constant for the gas and \[x\] is the mole fraction of the gas in solution. Henry’s law constant is characteristic for a gas at a given temperature so can also be related to the nature of the gas.
For dependence of Henry’s law constant at temperature, we can say that they have a directly proportional relationship. So, for an increase in temperature, we will observe an increase in the Henry’s law constant for a gas and consequently a decrease in the solubility.
Now, let’s consider the given situation of thermal pollution and its effect on the dissolved oxygen. It is well known to us that water bodies have dissolved oxygen in them that helps the aquatic life to survive in there but thermal pollution of water will cause a sudden change in the temperature of water that might be due to addition of hot or cold water. As we just discussed, change in the temperature would affect the solubility of oxygen. Mostly, we will observe thermal pollution as an increase in the temperature which will cause a decrease in the dissolved oxygen.

Hence, the given statement is true and the correct option is A.

Note:
Thermal pollution can also be due to addition of cold water even though it is not observed as much as due to hot water.