The fluoride ions make the enamel of teeth much harder by converting, which of the following substance/compound

• A

  $\left[3 \mathrm{Mg}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{Ca}(\mathrm{OH})_2\right]$
• Correct answer
  B

  $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{Ca}(\mathrm{OH})_2\right]$
• C

  $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{CaF}_2\right]$
• D

  $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{MgCl}_2\right]$

Solution

Teeth enamel contain $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{Ca}(\mathrm{OH})_2\right]$ which on reacting with fluoride ion is converted into $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \cdot \mathrm{CaF}_2\right]$ much harder.

Hence, the correct answer is option: 'B'.