Colloids

The dispersal of a precipitated material into colloidal solution by the action of an electrolyte in solution is called peptisation and the electrolyte is called a peptising agent.

Concept: Chemistry (Surface Chemistry) - Coagulation and Hardy-Schulze Rule.

Step 1: Identify the nature of the sol. The question specifies a "negatively charged sol". To cause coagulation (precipitation) of a negative sol, neutralizing positively charged ions (cations) are required.

Step 2: Eliminate the incompatible options. Options (A) and (B) are complex anions (negatively charged). They will not be effective in coagulating a negatively charged sol because like charges repel. We strictly consider the cations: (C) and (D) .

Step 3: State the Hardy-Schulze rule. The Hardy-Schulze rule states that the coagulating power of an active flocculating ion is directly proportional to its valency (magnitude of charge). The higher the positive charge on the cation, the greater its power to neutralize and precipitate the negative sol.

Step 4: Compare the charges of the available cations. The Barium ion ( ) has a charge of +2. The Aluminum ion ( ) has a charge of +3.

Step 5: Determine the ion with the greatest coagulating power. Because the charge of is greater than the charge of , will exhibit a significantly higher coagulating power according to the Hardy-Schulze rule. $ $

Step 1: Identify the colloidal system formed.
When is added to excess , silver iodide ( ) sol is formed.
Step 2: Apply the rule of preferential adsorption.
In a colloidal solution, the ion present in excess is preferentially adsorbed on the surface of colloidal particles.
Step 3: Determine the adsorbed ion.
Since is present in excess, iodide ions are adsorbed on AgI particles, giving them a negative charge.
Step 4: Conclusion.
Thus, the species adsorbed on AgI colloidal particles is .

Step 1: Understanding coagulation.
Coagulation is the process of clumping together of particles in a sol, leading to the formation of a gel or precipitate. Certain actions like heating, addition of electrolytes, or electrophoresis can lead to coagulation. However, adding water to a sol often dilutes it, which does not promote coagulation.
Step 2: Analyzing the options.
(A) Heating of an egg in boiling water: Incorrect. Heating an egg leads to coagulation of proteins.
(B) Addition of an electrolyte to sol: Incorrect. Addition of electrolytes like NaCl to a sol can lead to coagulation by neutralizing the charges on sol particles.
(C) Electrophoresis: Incorrect. Electrophoresis causes particles to move toward electrodes, which may lead to coagulation.
(D) Addition of water to gold sol: Correct — Adding water dilutes the sol and prevents coagulation, making it the exception.
Step 3: Conclusion.
The correct answer is (D) Addition of water to gold sol, as it does not lead to coagulation.

Step 1: Meaning of precipitating power.
Precipitating power of an ion depends on its charge and its ability to neutralize oppositely charged ions in solution.

Step 2: Effect of ionic charge.
Ions with higher charge have greater electrostatic attraction and hence greater ability to cause precipitation.

Step 3: Comparison of given ions.
Among the given ions, has the highest positive charge, making it most effective in precipitating oppositely charged ions.

Step 4: Conclusion.
Thus, the ionic species with highest precipitating power is (D) }.