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The recommended concentration of fluoride ion in drinking water is up to 1 ppm as fluoride ion is required to make teeth enamel harder by converting [3Ca3(PO4)2.Ca(OH)2] to:

1. [CaF2]
2. [3(CaF2).Ca(OH)2]
3. [3(Ca3(PO4)2.CaF2]
4. [3(Ca(OH)2.CaF2]

Correct answer: [3(Ca3(PO4)2.CaF2]

Solution

Fluoride ions react with hydroxyapatite in tooth enamel, which is represented by the formula [3Ca3(PO4)2.Ca(OH)2], to form fluorapatite, denoted as [3(Ca3(PO4)2.CaF2]. This conversion enhances the enamel's resistance to decay, making it harder and more durable.

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