The substance used as a smoke screen in warfare is .

Options

A. ` SiCl_4`
B. ` Sn Cl_4`
C. `PbCl_4`
D. `GECl_4`

Answer

To solve the question regarding the substance used as a smoke screen in warfare, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Purpose of Smoke Screens**: Smoke screens are utilized in warfare to conceal troop movements and positions from the enemy. They create a visual barrier that makes it difficult for the enemy to see.

**Hint**: Consider why smoke screens are necessary in military operations.

2. **Identifying Common Substances Used**: Various chemicals can create smoke screens, but we need to identify which one is specifically noted for its use in warfare.

**Hint**: Think about chemicals that produce dense smoke or fumes when they react with water.

3. **Analyzing the Given Options**: The question provides multiple choices. Among them, we will focus on silicon tetrachloride (SiCl4) as a potential candidate.

**Hint**: Look for the chemical that is known for producing a significant amount of smoke or fumes when it reacts with water.

4. **Chemical Reaction of SiCl4**: When SiCl4 is exposed to water, it undergoes a hydrolysis reaction:
\[ \text{SiCl}_4 + 2 \text{H}_2\text{O} \rightarrow \text{Si(OH)}_4 + 4 \text{HCl} \]
This reaction produces silicon hydroxide and hydrochloric acid.

**Hint**: Consider what products are formed and how they contribute to the smoke effect.

5. **Formation of Dense White Fumes**: The hydrochloric acid (HCl) produced can react with ammonia (NH3) in the air to form ammonium chloride (NH4Cl), which appears as dense white fumes:
\[ \text{HCl} + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} \]

**Hint**: Think about the visibility and density of the fumes produced from this reaction.

6. **Conclusion**: Based on the reactions and properties discussed, SiCl4 is confirmed to be the substance used as a smoke screen in warfare due to its ability to produce dense white fumes when it reacts with water and subsequently with ammonia.

**Final Answer**: The substance used as a smoke screen in warfare is **SiCl4**.