The solubility order for alkali metal fluoride in water is :

Options

A. `LiF lt RbF lt KF lt NaF`
B. `RbF lt KF lt NaF lt LiF`
C. `LiF lt NaF lt KF lt RbF`
D. `LiF gt NaF gt KF gt RbF`

Answer

To determine the solubility order of alkali metal fluorides in water, we can analyze the factors that affect solubility, particularly focusing on the size of the ions involved.

### Step-by-Step Solution:

1. **Identify the Alkali Metal Fluorides**: The alkali metal fluorides we are considering are Lithium Fluoride (LiF), Sodium Fluoride (NaF), Potassium Fluoride (KF), and Rubidium Fluoride (RbF).

2. **Understand the Role of Ion Size**: The solubility of ionic compounds in water is influenced by the size of the ions. In general, larger ions tend to have higher solubility in water because the lattice energy decreases with increasing ionic size, making it easier for the ionic compound to dissolve.

3. **Compare the Sizes of Alkali Metal Ions**: The alkali metal ions increase in size as we move down the group in the periodic table:
- Li⁺ (smallest)
- Na⁺
- K⁺
- Rb⁺ (largest)

4. **Consider the Fixed Anion Size**: The size of the fluoride ion (F⁻) remains constant across these compounds. Therefore, the comparison of solubility will depend solely on the size of the cations (Li⁺, Na⁺, K⁺, Rb⁺).

5. **Establish the Solubility Trend**: Since the solubility is directly proportional to the difference in size between the cation and the anion, we can conclude that:
- RbF will have the highest solubility because Rb⁺ is the largest cation.
- KF will follow, as K⁺ is smaller than Rb⁺ but larger than Na⁺.
- NaF will be next, as Na⁺ is smaller than K⁺.
- LiF will have the lowest solubility because Li⁺ is the smallest cation.

6. **Final Solubility Order**: Based on the above analysis, the order of solubility of alkali metal fluorides in water is:
- RbF > KF > NaF > LiF

### Conclusion:
The solubility order for alkali metal fluoride in water is:
**RbF > KF > NaF > LiF**