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.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Both Assertion and Reason are incorrect

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Solution

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
Soluble fluoride is added at average concentration of about 1 part per million or below to prevent tooth decay. The tooth enamel is made up of hydroxyapatite which consists of calcium, phosphorus and hydroxide. The bacteria produces acid, which tends to dissolve this hydroxyapatite. So, fluoride contains calcium and phosphorus which make up fluoroapatite and hence avoids the tooth decay.

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