Question

Which substance gives positive sol? (a) Gold (b) Starch (c) Ferric hydroxide (d) \(\mathrm{As}_{2} \mathrm{~S}_{3}\)

Short answer

The substance that gives a positive sol is Ferric Hydroxide.

Step-by-step solution

Identification of Charged Sol

Review the given options: Gold, Starch, Ferric Hydroxide, and \(\mathrm{As}_{2} \mathrm{S}_{3}\). Gold and Starch usually do not carry any charge. \(\mathrm{As}_{2} \mathrm{S}_{3}\) usually forms negatively charged sols in water.

Identify the Positive Sol

This leaves us with Ferric Hydroxide which forms a positively charged sol in water. This sol is commonly known as ferric hydroxide sol and its positive charge is due to the adsorption of ferric ions (Fe3+) on the colloidal particles.

Key concepts

Colloidal Systems

Colloidal systems are a fascinating area of chemistry, involving particles that are size-wise between true solutions and suspensions. These particles range from 1 to 1000 nanometers. Due to their small size, colloidal particles don't settle out like suspension particles and are small enough to be invisible to the naked eye, unlike those in a true solution.

In such systems, a dispersion medium (like water) plays host to dispersed particles (like minute solid particles or liquid droplets). These particles don't settle due to gravity, which gives colloidal solutions unique properties. For example, milk and smoke are both colloids, consisting of fat droplets and solid particles, respectively, dispersed in a medium. Understanding colloids is crucial for fields like material science, medicine, and environmental science, where they are widely applicable.

Ferric Hydroxide Sol

When talking specifically about ferric hydroxide sol, we're looking at a colloidal system where tiny particles of ferric hydroxide are dispersed throughout water. These minute particles endow the sol with a brownish hue. Generally, this colloidal solution is formed when ferric chloride reacts with hot water.

Ferric hydroxide sol is an example of a lyophobic sol, which means the colloidal particles have little to no affinity for the solvent and are quite readily precipitated. Due to its positively charged nature, this type of sol requires stabilizing agents to maintain the dispersion of particles. Ferric hydroxide sol has various applications, including its use in water purification processes.

Surface Chemistry

Surface chemistry is the study of chemical processes at interfaces between different phases, such as solid-liquid or gas-liquid interfaces. It's an essential area of chemistry that explains phenomena such as adsorption, catalysis, and the behavior of colloidal systems.

At the heart of surface chemistry is the concept of surface energy, which is higher than in the bulk of the material due to the presence of unsatisfied valence bonds. This increased energy level leads to reactivity at surfaces that can differ from the material's bulk properties. For instance, the surfaces play a key role in forming colloidal systems, where the interaction between the dispersed phase and the dispersion medium often occurs at the interface.

Charge Adsorption

Charge adsorption is vital in determining the stability and nature of colloidal systems. It occurs when ions from a solution are attracted to and held on the surface of a material. In the context of colloids, the surface of colloidal particles can adsorb ions, leading to a net charge on the particle.

For example, in the case of ferric hydroxide sol, the positively charged ferric ions (\( Fe^{3+} \)) adsorb onto the surface of the colloidal particles. This adsorption imparts an overall positive charge to the sol, which affects how the particles interact with each other and the medium. The charge on colloidal particles is a major factor influencing their stability, as well as their interaction with external electric fields and other charged entities.