Question

# Assertion: Soluble fluoride is often added to drinking water to bring its concentration up to 1 ppm.Reason: ${ F }^{ - }$ ion concentration above 2 ppm causes brown mottling of teeth.a) Both assertion and reason are correct and reason is the correct explanation for assertion.b) Both assertion and reason are correct but reason is not the correct explanation for assertion.c) Assertion is correct but reason is incorrect.d) Both assertion and reason are incorrect.

Hint: Fluoride ion concentration in the drinking water is crucial since it helps in hardening of the enamel and hence makes the teeth strong but at the same time at high concentration can cause harmful effects such as brown mottling of teeth.

Complete step by step solution:
First, let us look into why soluble fluoride in drinking water is important.
When water-soluble fluorides are consumed, they are absorbed by the gastrointestinal tract. If the drinking water contains fluorides around 10 mg/litre, its concentration in the blood is also the same. The absorbed fluorides are incorporated into teeth and bones and this process is reversible. Fluoride is an important element for both animals and humans. But there are many harmful effects of long-term fluoride ingestion through drinking water. It primarily affects skeletal tissues. However, low concentration of fluoride in drinking water is responsible for protection against dental caries. Therefore soluble fluoride is added to drinking water such that its concentration becomes 1 ppm. The fluoride ions convert the enamel present on the surface of the teeth which is also called hydroxyapatite $[3(C{ a }_{ 3 }(P{ O }_{ 4 }{ ) }_{ 2 }.Ca(OH{ ) }_{ 2 }]$ into fluorapatite $[3(C{ a }_{ 3 }(P{ O }_{ 4 }{ ) }_{ 2 }.Ca{ F }_{ 2 }]$ which is very hard in nature and hence makes the teeth strong.

Now let us look at the adverse effects of soluble fluorides.

If the fluoride content is high (above 2 ppm), it can cause dental fluorosis which is also called mottling of teeth. It is caused if drinking water containing high concentration of fluoride is consumed during the time of teeth formation or calcination due to which the enamel appears to be chalky and eventually undergoes brown discoloration.

Therefore the option (b) both assertion and reason are correct but reason is not the correct explanation for assertion is correct.

Note: It is not clear how the fluoride concentration present in the food affects the skeleton tissues. Generally, mottling of teeth does not take place in temperate areas where the fluoride ion concentration is below 1.5-2 mg per litre of drinking water.