_______ is usually stored in water to avoid its spontaneous combustion in the presence of air.

Options

A. Sodium
B. White phosphorus
C. Chlorine
D. Magnesium

Answer

To solve the question, we need to identify the substance that is typically stored in water to prevent its spontaneous combustion when exposed to air.

### Step-by-Step Solution:

1. **Understanding Spontaneous Combustion**:
- Spontaneous combustion refers to a situation where a substance ignites and burns without any external heat source. This can happen when a material reaches its ignition temperature due to heat generated from chemical reactions occurring within it.

2. **Identifying Substances Prone to Spontaneous Combustion**:
- Certain substances are known to ignite spontaneously when they come into contact with air. Common examples include certain alkali metals and phosphorus.

3. **Recognizing the Role of Water**:
- Water is often used to store specific substances to prevent them from coming into contact with air, which can lead to spontaneous combustion.

4. **Finding the Correct Answer**:
- Among the substances that spontaneously combust, **sodium** is a well-known example. Sodium reacts vigorously with water, but it is stored in oil or kerosene to prevent it from reacting with moisture in the air. However, in the context of the question, **phosphorus** is typically stored in water to avoid spontaneous combustion.

5. **Final Answer**:
- The answer to the question is **Phosphorus**.