Which of the following is an example of negatively charged sol?

$As_2S_3$

The correct answer is Option 2. $As_2S_3$.

A sol is a type of colloidal system where solid particles are dispersed in a liquid medium. These particles can range from a few nanometers to several micrometers in size. The properties of sols depend significantly on the charge of the dispersed particles, which can be positively, negatively, or neutral.

Types of Charged Sols

Negatively Charged Sols: Particles carry a net negative charge, which can stabilize the sol by repelling each other and preventing aggregation. Common examples include certain metal sulfides and some oxides.

Positively Charged Sols:  Particles have a net positive charge, often leading to different stability and interaction characteristics compared to negatively charged sols.

Neutral Sols:  Particles do not carry a significant charge, and their stability is primarily governed by van der Waals forces and other non-electrostatic interactions.

Analyzing the Options

1. \(Al_2O_3·xH_2O\) :

Aluminum oxide can form colloids, and in hydrated form, it typically behaves as a positively charged sol under acidic conditions (due to the release of Al³⁺ ions) and can be neutral or weakly positive depending on pH.

Charge: Generally not negatively charged.

2. \(As_2S_3\):

Arsenic trisulfide is a well-known example of a negatively charged sol, especially when dispersed in water.

The sol is formed when As₂S₃ particles dissociate into smaller particles, resulting in a net negative charge due to the presence of sulfide ions (S²⁻) in the solution. These anions help stabilize the sol by providing electrostatic repulsion among the dispersed particles, preventing aggregation.

Such sols can be used in various applications, including in the fields of chemistry and materials science.

3. TiO₂ (Titanium dioxide):

Titanium dioxide can form colloidal solutions and has variable charge depending on the pH of the solution.

In acidic conditions, TiO₂ tends to be positively charged, while at higher pH levels, it can acquire a negative charge. However, it is more commonly recognized as neutral or positively charged.

4. Blood:

Blood is a complex suspension of cells (like red and white blood cells) in plasma. It contains various proteins, ions, and other components that can impart both positive and negative charges. Blood does not represent a typical negatively charged sol and is generally considered a neutral suspension

Conclusion

From the analysis, \(As_2S_3\) (arsenic trisulfide) is the only example among the options that consistently represents a negatively charged sol. Its ability to stabilize in a colloidal form due to the presence of negatively charged sulfide ions allows it to function effectively in various chemical applications. Understanding the nature of these sols is crucial for applications in materials science, nanotechnology, and other fields.