Problem 16 - Chemistry in 30 days

Question

On adding few drops of dilute HCl to freshly precipitated ferric hydroxide, a red coloured colloidal solution is obtained. This phenomenon is known as (a) Peptization (b) Dialysis (c) Protective action (d) Dissolution

Step-by-Step Solution

Answer

(a) Peptization

Step 1: Understand the process

Peptization is the process where a precipitate is converted into a colloidal solution by adding a peptizing agent, usually an electrolyte. In the given exercise, dilute HCl acts as the peptizing agent.

Step 2: Consider the role of HCl

Dilute HCl, when added to ferric hydroxide, provides ions that stabilize the colloidal particles by imparting a charge, resulting in a red colloidal solution. This stabilization prevents the particles from aggregating, maintaining them in suspension.

Step 3: Identify the phenomenon

The process of forming a colloidal solution from a precipitate with the help of an electrolyte (in this case, dilute HCl) is known as peptization.

Key Concepts

Colloidal solution

A colloidal solution, also known simply as a colloid, is a mixture where one substance is dispersed evenly throughout another. It consists of tiny particles that are small enough to remain suspended and not settle out over time, but large enough to scatter light. This scattering is known as the Tyndall effect, which is why colloidal solutions often appear cloudy or opaque.

The particles in a colloid range from 1 nanometer to 1 micrometer in size, which is larger than those in a true solution but smaller than those in a suspension. To give you an idea, milk is a common example of a colloidal solution where fat particles are dispersed in water.

• Properties: Colloids generally show a uniform appearance and do not settle if left undisturbed.
• Stability: They are stable and maintain dispersion as the particles are often kept apart by repulsive forces.

Colloids play an essential role in many areas including food products, pharmaceuticals, and paints. They function efficiently due to their unique ability to stabilize mixtures where typical solutions would not suffice.

Ferric hydroxide

Ferric hydroxide is a chemical compound consisting of iron, oxygen, and hydrogen with the formula Fe(OH)₃. It often appears as a brown gelatinous precipitate when iron salts react with alkaline solutions.

In chemical reactions, ferric hydroxide is produced during hydrolysis reactions where iron(III) ions combine with hydroxide ions. This process often results in a fluffy precipitate that can be difficult to filter due to its colloidal nature.

• Formation: Ferric hydroxide can form in nature, contributing to rust when iron is exposed to moisture and air.
• Uses: It is used in water purification processes to remove contaminants and in laboratory experiments as a precipitating agent.

In the context of the given exercise, ferric hydroxide acts as the precipitate that transforms into a colloidal solution upon interaction with an electrolyte. This transformation is crucial to achieving a stable colloidal dispersion.

Electrolyte

An electrolyte is a substance that, when dissolved in water, dissociates into ions and conducts electricity. Electrolytes are essential for various biological and chemical processes because of their ability to carry charge through solutions.

In the realm of chemistry, electrolytes can be acids, bases, or salts, each providing ions necessary for various reactions and processes. Common electrolytes include sodium chloride (table salt) and hydrochloric acid.

• Functions: Electrolytes maintain electrical neutrality, conduct electric current, and help stabilize colloidal solutions.
• Role in Colloidal Solutions: In peptization, electrolytes supply charges that disperse and stabilize colloidal particles, preventing them from clumping. This is why dilute HCl, acting as an electrolyte, is key in transforming ferric hydroxide precipitate into a colloidal solution.

Understanding electrolytes is crucial not only for grasping chemical reactions but also for exploring practical applications like battery functions and bodily fluid regulation.