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Q.
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 $[3Ca_3(PO_4)_2.Ca(OH)_2]$ to :
The Fluoride ion converts $\left[3 Ca _3\left( PO _4\right)_2 \cdot Ca ( OH )_2\right]$ to $\left[3 Ca _3\left( PO _4\right)_2 \cdot CaF _2\right]$.
The reaction can be given as follow:
$
\begin{array}{l}
{\left[3 Ca _3\left( PO _4\right)_2 \cdot Ca ( OH )_2\right]+2 F ^{-} \rightarrow\left[3 Ca _3\left( PO _4\right)_2 \cdot CaF _2\right]} \\
\text { drinking water upto } 1 ppm \\
\text { harder teeth enamel } \\
\end{array}
$