Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Each of the following is true for white and red phosphorus except that they (a) can be oxidized by heating in air (b) can be converted into one another (c) are both soluble in \(\mathrm{CS}_{2}\) (d) consist of the same kind of atoms

Step-by-Step Solution

Answer

The statement that both are soluble in \( CS_2 \) is incorrect.

Step 1: Understand the Properties of White and Red Phosphorus

White phosphorus is highly reactive and can be oxidized by heating in air, while red phosphorus is more stable but can also be oxidized under certain conditions. Both forms are allotropes of phosphorus and can be converted into one another through heating or exposure to sunlight. Thus, they consist of the same kind of atoms, phosphorus.

Step 2: Evaluate Solubility in Carbon Disulfide

White phosphorus is known to be soluble in carbon disulfide (\(CS_2\)), whereas red phosphorus is insoluble in carbon disulfide. This is a key difference between the two forms.

Step 3: Identify the Incorrect Statement

Given the properties from the previous steps, the incorrect statement regarding both white and red phosphorus is their solubility in \( CS_2 \). Red phosphorus does not dissolve in carbon disulfide, contrary to what the statement suggests.

Key Concepts

White Phosphorus

White phosphorus is a fascinating and reactive allotrope of phosphorus. It is characterized by its molecular structure, which consists of P₄ tetrahedra. This structure results in a highly reactive form of phosphorus, often used in applications that require quick ignition, such as in military applications for smoke screens and incineration devices.

Key properties of white phosphorus include its ability to glow in the dark when exposed to air, a phenomenon known as chemiluminescence. This glowing effect is due to a slow reaction of phosphorus with oxygen, forming phosphorus pentoxide and releasing light.

White phosphorus is known for its dangerous and volatile nature. When exposed to air at temperatures above 30°C (86°F), it can spontaneously ignite, which is why it is stored under water or in inert gases to prevent contact with air. Its reactivity makes it a uniquely powerful yet hazardous substance.

Furthermore, white phosphorus is soluble in carbon disulfide ( CS₂ ), which distinguishes it significantly from its red counterpart. This solubility is utilized in chemical reactions and extractions where white phosphorus plays a role.

Red Phosphorus

Red phosphorus is a more stable and less reactive allotrope compared to white phosphorus. The stability is attributed to its amorphous network structure, setting it apart from the highly reactive P₄ molecules of white phosphorus.

The stability of red phosphorus makes it safer for handling and storage. Unlike white phosphorus, red phosphorus is not subject to spontaneous ignition in air at room temperature. This stability allows it to be used in everyday items like safety matches, fireworks, and in some flame retardants.

Conversion between white and red phosphorus is possible. When white phosphorus is heated in the absence of air or exposed to sunlight, it transforms into red phosphorus. This process involves the rearrangement of the P₄ tetrahedra into a more stable polymeric form.

While both white and red phosphorus are insoluble in water, red phosphorus is specifically characterized by its insolubility in carbon disulfide (CS₂). This property is crucial, especially when differentiating between the two allotropes in chemical experiments.

Solubility in Carbon Disulfide

The solubility of different substances in carbon disulfide ( CS₂ ) is an essential criterion for distinguishing between them, particularly in the study of phosphorus allotropes.

White phosphorus is soluble in carbon disulfide, making it useful in various chemical processes where it needs to be dissolved. This solubility attribute is one of the few methods to practically differentiate between white and red phosphorus without altering their chemical composition through a reaction.

In contrast, red phosphorus does not dissolve in carbon disulfide. This property provides an effective separation technique during experimental procedures. For instance, if a mixture of red and white phosphorus needs to be separated, using CS₂ could help dissolve out the white phosphorus, leaving the insoluble red phosphorus behind.

Understanding this solubility difference is key in chemical identification and separation processes, reinforcing the unique properties of each allotrope.