Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The recommended concentration of fluoride ion in drinking water is up to lppm as fluoride ion is required to make teeth enamel harder by converting \(\left[3 \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]\) to : (a) \(\left[\mathrm{CaF}_{2}\right]\) (b) \(\left[3\left(\mathrm{CaF}_{2}\right) \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]\) (c) \(\left[3 \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{CaF}_{2}\right]\) (d) [3\\{ \(\left.\left(\mathrm{Ca}(\mathrm{OH})_{2}\right\\} \cdot \mathrm{CaF}_{2}\right]\)

Step-by-Step Solution

Answer

Correct option: (c) \( 3\text{Ca}_3(\text{PO}_4)_2 \cdot \text{CaF}_2 \).

Step 1: Understand the chemistry involved

The conversion of tooth enamel requires fluoride ions to replace hydroxide ions in calcium hydroxide (\( \text{Ca(OH)}_2 \)). This process forms a harder layer on teeth by forming new compounds involving calcium and fluoride.

Step 2: Identify the reaction

To make teeth enamel harder, the fluoride ions react with existing compounds in the tooth enamel structure. Normally, the reaction would involve replacing \( \text{Ca(OH)}_2 \) with \( \text{CaF}_2 \), since fluoride ions replace hydroxide ions to form fluoride compounds.

Step 3: Choose the correct chemical formula

Among the given options, look for the formula where fluoride ions replace the hydroxide ions from \( \text{Ca(OH)}_2 \) to form a fluorapatite-like compound. This is likely to be the conversion to \( 3\left(\text{Ca}_3(\text{PO}_4)_2\right) \cdot \text{CaF}_2 \), instead of \( 3\left(\text{CaF}_2\right) \cdot \text{Ca(OH)}_2 \).

Step 4: Confirm the solution

Verify if the chosen option aligns with the intended chemical change where fluoride ions replace hydroxide ions to form a stable, protective layer. Thus, the correct chemical reaction involves forming \( 3\text{Ca}_3(\text{PO}_4)_2 \cdot \text{CaF}_2 \).

Key Concepts

Tooth Enamel Chemistry

Tooth enamel is the hard, outer surface layer of your teeth. It serves as the first line of defense against physical and chemical damage. Enamel mainly consists of hydroxyapatite, a crystalline calcium phosphate compound. When fluoride ions are introduced, they can replace the hydroxide ions in hydroxyapatite to form fluorapatite, which is more resilient to acids produced by plaque bacteria. This transformation strengthens the enamel and enhances resistance to decay.

Enamel is particularly vulnerable to acid erosion if fluoride is absent. The introduction of fluoride through drinking water or toothpaste helps maintain the hardness of enamel. It is an essential component in dental health as it reduces the risk of cavities and enhances overall enamel integrity.

Hence, understanding how fluoride influences these chemical processes is crucial in protecting your teeth from decay.

Calcium Fluoride Formation

Calcium fluoride ( CaF_2 ) plays a vital role in dental chemistry. It forms when fluoride ions ( F^- ) interact with calcium ions ( Ca^{2+} ) present in the tooth enamel. This process is significant because calcium fluoride forms a protective layer that provides long-lasting benefits to the teeth.

When fluoride in drinking water interacts with the enamel's calcium, it facilitates the formation of calcium fluoride, which acts as a reserve of fluoride. During times of acid attack, fluoride ions are slowly released, providing ongoing protection. This gradual release underscores the importance of consistent low-level fluoride exposure in preventing decay over time.

• Helps in creating a barrier against acidic substances.
• Allows slow release of fluoride ions, maintaining an ongoing protective effect.

The formation of calcium fluoride is a crucial mechanism by which fluoride strengthens and maintains dental health.

Fluorapatite

Fluorapatite is a modified form of the mineral apatite, distinguished by its incorporation of fluoride ions. It forms when tooth enamel undergoes fluoride treatment, replacing hydroxide ions with fluoride. The formation of this compound is a central reason why fluoride is so effective in preventing dental caries (tooth decay).

Fluorapatite has a greater resistance to acid than hydroxyapatite, the original mineral in enamel. This resistance is crucial because acids are primarily responsible for the demineralization of enamel, leading to cavities. When fluoride is integrated into the enamel structure, it forms fluorapatite, thereby increasing the enamel's threshold to resist demineralization.

• Increases enamel's acid-resistance compared to hydroxyapatite.
• Improves the overall stability and durability of enamel.

Thus, the role of fluoride in forming fluorapatite illustrates why fluoride treatments are so beneficial for long-term oral health.