The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively

Q: What is mutual coagulation?

When two oppositely charged sols are mixed in roughly equal proportions, they neutralize each other's charge and get coagulated. This is called mutual coagulation.

Q: Why does electrophoresis cause coagulation?

During electrophoresis, colloidal particles move towards the oppositely charged electrode, lose their charge upon contact, and aggregate to form a precipitate.

Q: What is the Hardy-Schulze rule?

It states that the coagulating power of an electrolyte depends on the valency of the active ion (the ion carrying a charge opposite to that of the colloidal particles). Higher the valency, greater is the coagulating power.

Q: Why does sodium sulphate coagulate both sols?

Sodium sulphate dissociates into $\text{Na}^+$ and $\text{SO}_4^{2-}$ ions. The positive $\text{Na}^+$ ions coagulate the negative gold sol, while the negative $\text{SO}_4^{2-}$ ions coagulate the positive iron (III) hydroxide sol.

Q: Why is $\text{Mg}^{2+}$ more effective than $\text{Cl}^-$ in coagulation?

According to the Hardy-Schulze rule, the coagulating power increases with the charge of the ion. Since $\text{Mg}^{2+}$ has a higher charge (+2) compared to $\text{Cl}^-$ (-1), it is more effective in causing coagulation.

Question

The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged, respectively. Which of the following statement is not correct?

Options:

(A) Coagulation in both sols can be brought about by electrophoresis

(B) Mixing the sols has no effect

(C) Sodium sulphate solution causes coagulation in both sols

(D) Magnesium chloride solution coagulates the gold sol more readily than the iron (III) hydroxide sol

Sigma Percentile · Accepted Answer

### Analyzing the Setup
We are given two colloidal solutions: iron (III) hydroxide and colloidal gold. The problem explicitly states their charges. Iron (III) hydroxide is a positively charged sol, while colloidal gold is a negatively charged sol. Our goal is to evaluate four statements regarding their coagulation properties and identify the one that is **incorrect**.

### The Power of Electrophoresis
Let's look at the first statement. Electrophoresis is a phenomenon where colloidal particles move under the influence of an applied electric field. Since colloidal particles carry a charge, they will naturally migrate towards the oppositely charged electrode. Once they reach the electrode, they lose their charge, become neutral, and aggregate to form a precipitate. This means electrophoresis can indeed bring about coagulation in both positively and negatively charged sols. Therefore, the first statement is perfectly correct.

### The Clash of Charges: Mutual Coagulation
Now, what happens if we simply mix these two sols together? We have a positively charged iron (III) hydroxide sol and a negatively charged gold sol. When they are mixed, the positively charged particles and the negatively charged particles attract each other. They neutralize each other's charge. Without the repulsive forces keeping them apart, the particles come together and coagulate. This beautiful phenomenon is known as **mutual coagulation**.

The second statement claims that mixing the sols has no effect. Based on our understanding of mutual coagulation, this statement is blatantly false! Mixing them will definitely cause coagulation.

### The Role of Electrolytes
Let's verify the remaining statements just to be absolutely sure. The third statement involves adding sodium sulphate ($	ext{Na}_2	ext{SO}_4$). This electrolyte provides $	ext{Na}^+$ and $	ext{SO}_4^{2-}$ ions. The positive $	ext{Na}^+$ ions will neutralize and coagulate the negative gold sol. Simultaneously, the negative $	ext{SO}_4^{2-}$ ions will neutralize and coagulate the positive iron (III) hydroxide sol. Thus, sodium sulphate can coagulate both sols. This statement is correct.

### The Hardy-Schulze Rule in Action
Finally, let's examine the fourth statement involving magnesium chloride ($	ext{MgCl}_2$). This gives us $	ext{Mg}^{2+}$ and $	ext{Cl}^-$ ions. The $	ext{Mg}^{2+}$ ions will target the negative gold sol, while the $	ext{Cl}^-$ ions will target the positive iron sol.

Here, we must invoke the **Hardy-Schulze rule**, which states that the coagulating power of an ion is directly proportional to its valency. The $	ext{Mg}^{2+}$ ion has a valency of 2, whereas the $	ext{Cl}^-$ ion has a valency of 1. Because of its higher charge, $	ext{Mg}^{2+}$ will be much more effective at coagulating the gold sol than $	ext{Cl}^-$ will be at coagulating the iron sol. Therefore, magnesium chloride coagulates the gold sol more readily. This statement is also correct.

### Final Conclusion
Having analyzed all the options, it is crystal clear that the only incorrect statement is the second one. Mixing oppositely charged sols definitely has an effect—it causes mutual coagulation!

Animated Solution for Chemistry - Surface Chemistry: The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged, respectively. Which of the following statement is not correct?

Select Answer:

Visualized Solution

$Analyzing the Given Information$

$Evaluating Option (a): Electrophoresis$

$Evaluating Option (b): Mutual Coagulation$

$Evaluating Option (c): Effect of Sodium Sulphate$

$Evaluating Option (d): Hardy-Schulze Rule$

$Final Conclusion$

The Sigma Insight: Colloids, Micelles and and Emulsions

Analyzing the Setup We are given two colloidal solutions: iron (III) hydroxide and colloidal gold

The Power of Electrophoresis Let's look at the first statement

The Clash of Charges

The Role of Electrolytes Let's verify the remaining statements just to be absolutely sure

The Hardy-Schulze Rule in Action Finally, let's examine the fourth statement involving magnesium chloride ( $MgCl_{2}$ )

Final Conclusion Having analyzed all the options, it is crystal clear that the only incorrect statement is the second one

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