Question

Brown mottling of teeth occurs when ${F^ - }$ ion concentration in drinking water becomes:
A.Less than $1$ ppm
B.Greater than $1$ ppm
C.Greater than $5$ ppm
D.It does not depend on ${F^ - }$ ion concentration.

Answer 1

Hint: Presence of fluoride ions in drinking is necessary because it helps to strengthen the enamel of teeth. But if concentration is higher than a particular value, it also damages the teeth.

Complete step by step answer:
The water soluble fluoride ions are absorbed by the gastrointestinal tract. The fluorides ions are present in teeth and bones and it strengthens them. Hence fluoride ions should be present in certain concentrations. But the concentration level rises above the required value, it is harmful. Even though low concentration strengthens the teeth, high concentration of fluoride ions destroys them. The allowed concentration of fluoride ions in drinking water is $1$ ppm. If the concentration of fluoride ions are higher than this value, brown mottling of teeth occurs.
Hence the correct option is B.

Additional information-
This disorder is commonly called dental fluorosis. It mainly occurs when drinking water containing high fluoride concentration is consumed at the time of formation of teeth. It is a painless disease. Symptoms of this disorder are,
-Brown spot on tooth enamel
-Pitting of tooth enamel.
-Permanent damage of teeth.

Note:
Usually mottling of teeth is very less in temperature areas. Because in those areas, fluoride ion concentration is very low in drinking water.
Excess fluoride can enter the body not only through drinking water but also through toothpastes and food. If fluoride ion concentration is greater than $5$ ppm, it leads to decay of tooth