Question

A sample of water has no fluoride (F) ions in it. What is the maximum limit (in $mg~L^{-1}$) of soluble fluoride salt to be added to this water to convert it into safe drinking water?

• A. 1--2
• B. 5--10
• C. 10--20
• D. $x>20$

Hint:

Too little fluoride causes dental problems, while excessive fluoride causes fluorosis. Safe drinking water generally contains about: \[ 1-2~mgL^{-1} \] fluoride ions.

Answer 1

The Correct Option is A

Concept: Fluoride ions are required in small amounts for healthy teeth and bones. However, excessive fluoride causes fluorosis.

Step 1: Recall the recommended concentration.
According to drinking water standards, fluoride concentration should be approximately \[ 1~mgL^{-1} \] and should not exceed about \[ 2~mgL^{-1} \] for safe consumption.

Step 2: Interpret the question.
Since the water initially contains no fluoride ions, fluoride salts may be added until the safe upper limit is reached. Thus the acceptable range is \[ 1-2~mgL^{-1} \]

Step 3: State the answer.
\[ \boxed{1-2~mgL^{-1}} \]