Which of the following is a false statement ?

Options

A. Lithium has greateer hardnes as compound to other alkali metals.
B. `LiHCO_(3)` and `Mg(HCO_(3))_(2)`, do not exist in the solid state.
C. Lithium and magnesium form nitrides on reacting with nitrogen but other alkali metals do not.
D. Alkali metal flurides are highly soluble in water.

Explanation

To determine which statement is false regarding alkali metals, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Analyze Statement A**: "Lithium has greater hardness as compared to the other alkali metals."
- **Explanation**: This statement is true. Lithium is indeed the hardest alkali metal. As we move down the group from lithium to cesium, the hardness decreases due to the decrease in ionization enthalpy and the weakening of metal-metal bonds. Therefore, this statement is correct.

2. **Analyze Statement B**: "Lithium hydrogen carbonate and magnesium hydrogen carbonate do not exist in the solid state."
- **Explanation**: This statement is also true. Lithium and magnesium bicarbonates are stable only in solution and do not form stable solid states. This is due to their low lattice enthalpy compared to their hydration enthalpy. Thus, this statement is correct.

3. **Analyze Statement C**: "Lithium and magnesium form nitrides on reacting with nitrogen, but other alkali metals do not."
- **Explanation**: This statement is true as well. Lithium and magnesium can react with nitrogen to form nitrides, while other alkali metals do not form nitrides under normal conditions. Therefore, this statement is also correct.

4. **Analyze Statement D**: "Alkali metal fluorides are highly soluble in water."
- **Explanation**: This statement is false. While many alkali metal fluorides are soluble in water, lithium fluoride (LiF) is an exception as it is covalent and has low solubility in water. Therefore, this statement is incorrect.

### Conclusion:
The false statement among the options provided is **Statement D**: "Alkali metal fluorides are highly soluble in water."

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