Problem 5 - Chemistry in 30 days

Question

Which of the following sols is negatively charged? (a) Arsenious sulphide (b) Aluminum hydroxide (c) Ferric hydroxide (d) Silver iodide in silver nitrate solution

Step-by-Step Solution

Answer

Arsenious sulphide (a) is negatively charged.

Step 1: Identify the Colloidal System

Recognize the substances given in the options as colloidal systems. A sol is a colloidal solution where solid particles are dispersed in a liquid.

Step 2: Understand the Charge of Colloids

Colloidal particles can acquire a charge through adsorption of ions. It's crucial to determine whether the colloidal substance has a tendency to adsorb positive or negative ions.

Step 3: Analyze the Charge in Each Option

(a) Arsenious sulphide sol is negatively charged. (b) Aluminum hydroxide sol is positively charged. (c) Ferric hydroxide sol is positively charged. (d) Silver iodide in silver nitrate solution typically carries a negative charge when excess iodide ions are present. However, in excess silver nitrate, it's likely positively charged.

Step 4: Identify the Correct Answer

From analyzing the charges, arsenious sulphide is the sol that is negatively charged under typical conditions due to adsorption of sulphide ions which are negatively charged.

Key Concepts

Charge of Colloids

Colloidal particles possess the intriguing ability to acquire a charge, which plays a significant role in their stability and interactions. The charge on colloids typically stems from the adsorption process - where ions from the surrounding medium adhere to the particle surface. This adsorption of ions can render the colloid with either a positive or negative charge.
These charges contribute to the stability of the sol by preventing particles from aggregating, thanks to electrostatic repulsion. When like-charged particles attempt to come close to one another, this repulsion keeps them apart, maintaining a balanced and stable colloidal solution.
Considering the nature of ions surrounding the colloid is essential for determining its charge. Understanding this concept is vital in applications ranging from industrial processes to biological systems.

Arsenious Sulphide

Arsenious sulphide (\( As_2S_3 \)) is a particularly fascinating colloidal sol because it typically carries a negative charge. This is attributed to its ability to adsorb negatively charged sulphide ions (\( S^{2-} \)) from the surrounding solution.
In a watery medium, the sulphide ions preferentially attach to the surface of the arsenious sulphide particles, imparting them with a negative charge. This ensures that the particles remain dispersed in the solution, thereby preventing aggregation and settling. Such negative charges also allow arsenious sulphide sols to find uses in various chemical applications such as photography and as pigments due to their stable dispersion characteristics.

Silver Iodide

Silver iodide (\( AgI \)) is an interesting colloid because its charge can differ based on the solution composition. Silver iodide can adsorb different ions, altering its surface charge. In the presence of excess iodide ions, the charge on silver iodide sol is generally negative due to adsorption of the iodide ions (\( I^- \)).
Conversely, when the concentration of silver ions is higher, such as in an excess of silver nitrate (\( AgNO_3 \)), the sol may become positively charged due to the adsorption of silver ions (\( Ag^+ \)).
This dual capability underlines the importance of solution conditions in determining colloidal charge, making silver iodide sols useful in various fields, including photographic development and cloud seeding experiments.

Aluminum Hydroxide

Aluminum hydroxide (\( Al(OH)_3 \)) functions prominently as a positively charged colloid. This positive charge arises mainly because of its tendency to attract and adsorb hydroxide ions (\( OH^- \)) onto its surface, thereby exposing a surplus of aluminum ions (\( Al^{3+} \)) which inherently carry a positive charge.
The behavior of aluminum hydroxide as a colloid finds practical utility in applications such as water purification and as an antacid in medicine. The strong positive charge helps in coalescing impurities, effectively facilitating their removal from water.

Ferric Hydroxide

Ferric hydroxide (\( Fe(OH)_3 \)) is another example of a positively charged colloidal sol. The positive charge of ferric hydroxide originates from the adsorption of hydroxide ions which result in predominant ferric ions (\( Fe^{3+} \)) at the particle's surface.
This surface-positive nature is utilized in processes like coagulation, where ferric hydroxide can help neutralize negatively charged particles, causing them to clump together for easier removal. This property is crucial in industries that require water treatment processes, highlighting the practical significance of ferric hydroxide in environmental and industrial applications.