Question

(a) Explain the $\text{pH }$ change as the cause of tooth decay. How can tooth decay caused by $\text{pH }$ change be prevented?
(b) Explain how $\text{pH }$ the change in the lake water can endanger the lives of aquatic animals (like fish). What can be done to lessen the danger to the lives of aquatic animals in the lake?

Answer 1

Hint: $\text{pH }$ plays a vital role in determining the various natural phenomena.
a) Bacterial in the mouth oxidizes the food particles (carbohydrates, protein, etc.) and secrets acid. Saliva has a $\text{pH }$ value greater than 7 and neutralizes the acid secreted at the tooth. The drop in $\text{pH }$ creates a plaque coating on the tooth surface.
b) Aquatic animals can survive within a narrow range of $\text{pH }$. Increase or decrease in $\text{pH }$ decreases the chances of survival of animals. Therefore basic salt is added to the lake water to neutralize the acid.

Complete Solution :
a) Tooth decay gradually decays the enamel and dentine. Bacteria in the mouth softens or demineralize the enamel and secrets the acid. The acid is secreted by the action of bacteria through the oxidation of sugar and food particles. The decay of the tooth starts when the $\text{pH }$ of the mouth drops below $\text{ 5}\text{.5 }$ .
- On incorporation of food particles, the bacteria bind to the teeth and form a dental plaque. This plaque coating prevents the reach of saliva to the tooth layer. In absence of plaque, coating saliva neutralizes the acid. However, the plaque coating saliva could not neutralize the acid. Brushing the teeth removes the plaque.
- Tooth decay can be prevented by brushing and flossing the teeth or plaque. The antimicrobial rinse is also an effective way to reduce bacterial growth.
b) Oxides of nitrogen and sulphur dissolve in rainwater and make it acidic. This acid rain has a lower $\text{pH }$ value than normal rainwater. The acid rain lowers the $\text{pH }$ of lake water. The high acidity of lake water can kill aquatic animals. The fish or other aquatic animals cannot survive within the narrow range of $\text{pH }$ change.
- The high acidity of lake water can be reduced by the addition of basic salt. The acidity of a lake or river is neutralized by the addition of calcium carbonate. This prevents fish or aquatic animals from being killed.

Note: Note that, $\text{pH }$ has a wide significance in various natural phenomena. One should remember that the fish can survive in the narrow range $\text{ 6}\text{.5}-9.0\text{ }$.$\text{pH }$ of the water bodies also have a significant effect on the solubility and toxicity of various industrial chemicals and heavy metals in water.