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JEE MAIN Chemistry QUESTION #5566
Question 1
The recommended concentration of fluoride ion in drinking water is up to 1 ppm. At this level, fluoride hardens tooth enamel by converting $[3Ca_3(PO_4)_2 \cdot Ca(OH)_2]$ (hydroxyapatite) to:
  • $[3Ca_3(PO_4)_2 \cdot CaF_2]$✔️
  • $[3\{Ca(OH)_2\} \cdot CaF_2]$
  • $[CaF_2]$
  • $[3(CaF_2) \cdot Ca(OH)_2]$
Correct Answer Explanation
Fluoride ions replace the hydroxide ions in hydroxyapatite: $3Ca_3(PO_4)_2 \cdot Ca(OH)_2 + 2F^- \to 3Ca_3(PO_4)_2 \cdot CaF_2 + 2OH^-$. The product, fluoroapatite $[3Ca_3(PO_4)_2 \cdot CaF_2]$, is harder and more resistant to acid attack than hydroxyapatite, thus protecting teeth from decay.