Problem 190 - A Complete Resource Book in Chemistry for JEE Main

Question

The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged, respectively. which of the following statements is incorrect? (a) magnesium chloride solution coagulates, the gold sol more readily than the iron (III) hydroxide sol (b) sodium sulphate solution causes coagulation in both sols (c) mixing of the sols has no effect (d) coagulation in both sols can be brought about by electrophoresis

Step-by-Step Solution

Answer

Option (c) is incorrect.

Step 1: Analyze Colloidal Charges

We need to understand how charged particles interact. Colloidal iron (III) hydroxide is positively charged, while colloidal gold is negatively charged. This means they will attract ions with opposite charges to neutralize and coagulate.

Step 2: Evaluate Option (a)

Magnesium chloride contains Mg²⁺ ions, which are effective in causing coagulation of negatively charged colloidal particles like gold sol. Due to opposites attracting, gold sol will coagulate more readily with a positive ion like Mg²⁺.

Step 3: Evaluate Option (b)

Sodium sulphate solution contains SO₄²⁻ ions. These negative ions will cause coagulation of positively charged sol like iron (III) hydroxide. Na⁺ ions from sodium sulphate can also interact with negatively charged gold sol promoting coagulation in both.

Step 4: Evaluate Option (c)

When positively charged iron (III) hydroxide and negatively charged gold sols mix, their opposite charges can neutralize each other, leading to coagulation. Thus, mixing has an effect rather than no effect as the statement suggests.

Step 5: Evaluate Option (d)

Electrophoresis involves movement of charged particles in an electric field to electrode of opposite charge. Both sols, depending on charge, will migrate towards their respective opposites and could potentially coagulate, making the statement correct.

Step 6: Conclusion

Given the evaluations, option (c) is the incorrect statement because mixing oppositely charged sols does indeed have an effect, usually resulting in coagulation.

Key Concepts

Colloidal Particles

Colloidal particles are tiny particles dispersed evenly throughout another substance. They are larger than molecules or ions but small enough to remain suspended without settling quickly due to gravity. Imagine them as microscopic particles dancing smoothly in a liquid or gas.

Colloids are neither solutions nor suspensions. They occupy a unique middle ground where particles range from about 1 nanometer to 1000 nanometers in size. Because of their size, colloidal particles exhibit interesting behavior that differs from activities seen in typical solutions or larger particle suspensions.

Among some important properties of colloids are their ability to scatter light, known as the Tyndall effect, and their capacity to remain stable without precipitating. In colloidal systems, particles usually carry an electrical charge; this property plays a crucial role in maintaining their stability by preventing particles from clumping together.

Electrophoresis

Electrophoresis is a method used to separate charged particles under the influence of an electric field. Picture it as a gentle tug-of-war between charged particles and oppositely charged electrodes.

• Movement Principle: The particles are pulled towards an electrode with the opposite charge. For example, positively charged particles migrate towards the negative electrode.
• Field Application: An electric field is applied to the colloidal mixture, and different particles move at different speeds based on size, charge, and medium resistance.
• Coagulation Role: As particles migrate, their movement can lead to coagulation when particles aggregate or neutralize charges on each other.

Electrophoresis isn't limited to colloids. It's a method with significant applications in biology, particularly in separating molecules like DNA based on size.

Charge Interaction in Colloids

Colloids stabilize when particles bear charges. This keeps them from sticking together and settling. Let's explore the role of these charges in coagulation.

• Opposite Attraction: Particles with opposite charges attract. This interaction is vital in processes like coagulation, where oppositely charged colloidal particles can neutralize each other, leading to agglomeration.
• Same Charge Repulsion: Particles with the same charge repel each other, which helps stabilize the colloid by keeping particles apart.
• Ionic Influence: Ions in the surrounding medium can enhance or disrupt this balance. By adding electrolytes, like magnesium chloride or sodium sulfate, the charges on the colloidal particles can neutralize, causing the particles to come together, which results in coagulation.

In a practical sense, this dance of charges—attraction, repulsion, and ionic influence—dictates how well a colloidal system performs and how susceptible it is to processing methods that aim to coagulate the dispersed particles.