Saline hydrides are known to react with water violently producing fire. The fire cannot be extinguished by `CO_2` because

Options

A. `CO_2` is lighter than the gas evolved
B. `CO_2` is heavier than the gas evolved
C. `CO_2` gets reduce by the evolved gas
D. `CO_2` gets oxidised by the evolved gas

Answer

To solve the question regarding why the fire produced by the reaction of saline hydrides with water cannot be extinguished by carbon dioxide (CO2), we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Saline Hydrides**:
- Saline hydrides are compounds like sodium hydride (NaH) and lithium hydride (LiH). These compounds react vigorously with water.

2. **Reaction with Water**:
- When saline hydrides react with water, they produce a strong base (like NaOH) and hydrogen gas (H2).
- The general reaction can be represented as:
\[ \text{NaH} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2 \uparrow \]
- This reaction is highly exothermic, meaning it releases a significant amount of heat.

3. **Fire Hazard**:
- The hydrogen gas produced is flammable and can easily catch fire due to the heat generated from the exothermic reaction.

4. **Using CO2 as an Extinguisher**:
- Carbon dioxide (CO2) is commonly used as a fire extinguisher because it displaces oxygen, which is necessary for combustion.
- However, in this scenario, CO2 cannot effectively extinguish the fire produced by the hydrogen gas.

5. **Why CO2 Fails**:
- When CO2 is introduced to the fire, it can react with the sodium hydride (NaH) to form sodium carbonate (Na2CO3) and release hydrogen gas. This reaction can further fuel the fire instead of extinguishing it:
\[ \text{2NaH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2 \uparrow \]
- Therefore, CO2 is reduced in this reaction, which means it cannot serve as an effective fire extinguisher.

6. **Conclusion**:
- The fire cannot be extinguished by CO2 because CO2 is reduced during the reaction with the saline hydride, leading to the production of more flammable hydrogen gas.

### Final Answer:
The fire cannot be extinguished by CO2 because CO2 is reduced in the reaction with the saline hydride, producing more hydrogen gas.