Identifying Positively Charged Sols

Colloidal solutions, or sols, consist of tiny particles dispersed in a continuous medium. These dispersed particles carry an electric charge. The charge on the colloidal particles is important for the stability of the sol because particles with the same charge repel each other, preventing aggregation and settling.

The charge on a sol arises due to various reasons, including:

• Adsorption of ions from the dispersion medium.
• Dissociation of surface molecules.
• Frictional electrification.

Sols are broadly classified based on the charge carried by their dispersed phase particles: positively charged sols and negatively charged sols.

Analysis of Given Options for Sol Charge

Let's examine the options provided and determine the typical charge of each sol:

• Starch Sol: Starch is a carbohydrate polymer. Starch sols are generally formed by dispersing starch in water. Starch sols are typically negatively charged. This is often due to the adsorption of hydroxyl ions or dissociation of acidic groups if present after hydrolysis.
• Gum Sol: Gums are natural polysaccharides. Like starch, gum sols (e.g., Gum arabic) are usually negatively charged. This charge arises from the presence of acidic groups (like carboxylic acids) or the adsorption of anions.
• Gold Sol: Gold sols are examples of metallic sols. Metallic sols, like gold, silver, and platinum sols, are generally negatively charged. This negative charge is often attributed to the adsorption of anions from the preparation method (e.g., using reducing agents that leave behind anions).
• Blood: Blood is a complex fluid, and it can be considered a colloidal system where components like proteins (e.g., albumin, globulins, haemoglobin) are dispersed in plasma. Proteins have complex structures and can carry positive or negative charges depending on the pH of the medium. However, in normal physiological conditions (around pH 7.4), the haemoglobin molecules within the red blood cells, which contribute significantly to the colloidal nature, carry a net positive charge. Other plasma proteins also contribute to the overall charge balance. Therefore, blood is considered a positively charged sol.

Conclusion on Positively Charged Sols

Based on the common characteristics of these sols, blood is the example among the options that is considered a positively charged sol under normal conditions.

Revision Table: Types of Sols and Charges

Understanding the charge of a sol is crucial for its stability and for techniques like electrophoresis. Here's a quick recap of common types of sols and their charges:

Additional Information: Formation and Properties of Sols

Sols are colloidal dispersions where a solid is dispersed in a liquid medium. Their stability is greatly influenced by the charge on the dispersed particles. The presence of ions in the dispersion medium also plays a significant role.

• Origin of Charge: The charge can originate from selective adsorption of ions. For example, if $\text{AgNO}_3$ solution is added to KI solution, AgI precipitate forms. If KI is in excess, iodide ions ($\text{I}^-$) are adsorbed on the AgI surface, resulting in a negatively charged sol. If $\text{AgNO}_3$ is in excess, silver ions ($\text{Ag}^+$) are adsorbed, resulting in a positively charged sol.
• Electrophoresis: This is a phenomenon where colloidal particles move under the influence of an electric field. Positively charged particles move towards the cathode, while negatively charged particles move towards the anode. This technique can be used to determine the charge on the sol particles.
• Coagulation: Adding electrolytes to a sol can cause the colloidal particles to lose their charge and aggregate, leading to precipitation. This process is called coagulation or flocculation. The effectiveness of an ion in causing coagulation depends on its valency (Hardy-Schulze rule). For negatively charged sols, cations are effective coagulating agents, and their effectiveness increases with increasing positive charge ($\text{Na}^+ < \text{Ba}^{2+} < \text{Al}^{3+}$). For positively charged sols, anions are effective, and their effectiveness increases with increasing negative charge ($\text{Cl}^- < \text{SO}_4^{2-} < \text{PO}_4^{3-}$).