Question

Excess fluoride (over $10$ $ppm$) in drinking water can cause
A.Harmful effect of bones and teeth
B.Methemoglobinemia
C.Kidney damage
D.Laxative effect

Answer 1

Water used for drinking must be pure otherwise it can cause serious diseases . Therefore international standards have been laid down for the water to be used for drinking . Fluoride is added externally in water in a limited amount for purifying it.

Complete step by step answer:
Chemicals that are allowed to be present and the tolerable limit up to which they are permitted according to the international standards are discussed below:
-Fluoride : Fluoride is added externally to the water to make it able to drink . The tolerable limit for fluoride in water is $1$ $ppm$ or $1$ $mg$ $\mathop {dm}\nolimits^{ - 3}$. A limited amount of fluoride protects teeth against decay , but concentration of fluoride above $2$ $ppm$ causes brown mottling of teeth . High concentration of fluoride ( over$10$ $ppm$) in water is harmful to our bones and teeth .
-Lead : Lead is used for transportation of water in our houses . The international standard limit for lead is $50$ $ppm$. Excess amounts of lead in drinking water causes lead poisoning which damages our kidneys , liver , brain and reproductive system.
-Sulphates : The international standard limit for sulphate in drinking water is less than $50$ $ppm$. Higher concentration of sulphates in drinking water causes a laxative effect .
-Nitrates : The standard limit for nitrates in drinking water is $50$ $ppm$. Excess amounts of nitrates in water cause methemoglobinemia also known as blue baby syndrome which may be linked to stomach cancer .
Hence option (A) is correct.

Note:
The fluoride ions make the enamel on the teeth much harder by converting hydroxyapatite ${\left[ {3C{a_3}{{\left( {P{O_4}} \right)}_2}.Ca{{\left( {OH} \right)}_2}} \right]_{}}$, the enamel on the surface of the teeth into much harder fluorapatite ${\left[ {3C{a_3}{{\left( {P{O_4}} \right)}_2}.Ca{F_2}} \right]_{}}$.