Which of the following is kept in water

Options

A. White phosphorus
B. sodium
C. potassium
D. calcium

Explanation

To determine which of the following substances is kept in water, we need to analyze the reactivity of the given options.

### Step-by-Step Solution:

1. **Identify the Substances**: The question mentions sodium, potassium, calcium, and white phosphorus. We need to evaluate the reactivity of each of these substances with water and air.

2. **Reactivity of Sodium, Potassium, and Calcium**:
- Sodium (Na), potassium (K), and calcium (Ca) are alkali and alkaline earth metals.
- These metals are highly reactive with water. For example, sodium reacts vigorously with water to produce sodium hydroxide and hydrogen gas, which can ignite due to the heat of the reaction.
- Similarly, potassium reacts even more violently with water, and calcium also reacts, though less vigorously than sodium and potassium.

3. **Reactivity of White Phosphorus**:
- White phosphorus (P4) is a non-metal and is known for its high reactivity, particularly with oxygen in the air.
- It can spontaneously ignite when exposed to air, which makes it very hazardous if not stored properly.

4. **Conclusion**:
- Since sodium, potassium, and calcium react with water, it is not practical to keep them in water. Instead, they are usually stored in oil to prevent contact with moisture and air.
- White phosphorus, on the other hand, is kept in water to prevent it from catching fire when exposed to air. Water acts as a barrier to oxygen, thus preventing combustion.

5. **Final Answer**:
- The substance that is kept in water is **white phosphorus**.