Some reactive elements must be stored under water or other liquids to prevent dangerous reactions with air. Which one of the following elements is commonly preserved under water for safety?

Introduction / Context:
Certain chemical elements are so reactive with oxygen in the air that they must be stored under liquids to prevent spontaneous combustion or rapid oxidation. This is a practical safety issue in laboratories and is frequently tested in basic chemistry questions. Knowing which elements require special storage conditions helps you understand their reactivity and the precautions needed when handling them.

Given Data / Assumptions:

• The elements listed are sodium, sulphur, red phosphorus and white phosphorus.
• The question asks which element is preserved under water.
• We assume common laboratory practice for storing these substances.
• We distinguish between storage under water and storage under oil or in ordinary containers.

Concept / Approach:
White phosphorus is highly reactive and can ignite spontaneously in air at relatively low temperatures. To prevent contact with oxygen, it is stored under water, which acts as a barrier between the solid and the air. Sodium is also reactive with air and especially with water, but it is typically stored under kerosene or mineral oil, not under water, because water would react violently with sodium. Sulphur and red phosphorus are much less reactive and can be stored in ordinary containers under normal conditions.

Step-by-Step Solution:
Step 1: Consider sodium. Although it is reactive, especially with water, safe practice is to store it under kerosene, not under water.Step 2: Consider sulphur. It is relatively stable in air and is stored in closed containers without special liquid protection.Step 3: Consider red phosphorus. It is less reactive than white phosphorus and can be stored safely in closed bottles without needing water cover.Step 4: Consider white phosphorus. It is extremely reactive with oxygen and likely to ignite spontaneously when exposed to air.Step 5: To prevent this, white phosphorus is stored under water so that it does not contact air until deliberately removed.Step 6: Therefore, the element commonly preserved under water is white phosphorus.

Verification / Alternative check:
Laboratory manuals often specifically instruct that white phosphorus must always remain covered with water and should be handled with forceps under water for cutting or transferring. In contrast, sodium is shown stored in jars filled with kerosene. General chemistry texts also explain the difference in reactivity between white and red phosphorus, emphasising the need for underwater storage of the white allotrope. These consistent safety recommendations confirm that white phosphorus is the element preserved under water.

Why Other Options Are Wrong:
Option a, sodium, is not stored under water because the reaction between sodium and water produces hydrogen gas and sodium hydroxide and can be explosive. Option b, sulphur, does not require such precautions and is stable in air under normal conditions. Option c, red phosphorus, is more stable and safer than white phosphorus and does not need to be stored under water. Therefore, none of these options match the specific storage requirement described in the question.

Common Pitfalls:
Students sometimes remember that sodium is stored under a liquid and incorrectly assume that the liquid is water rather than kerosene. Another pitfall is to mix up red and white phosphorus and forget which allotrope is highly reactive. To avoid these errors, associate white phosphorus with high reactivity, low ignition temperature and underwater storage, while associating sodium with storage under kerosene and red phosphorus with relatively safe storage in bottles.

Final Answer:
The element that is commonly preserved under water is white phosphorus.