Animated Solution for Chemistry - Surface Chemistry: The charges on the colloidal $\text{CdS}$ sol and ${\text{TiO}}_2$ sol are, respectively

Have you ever wondered why milk doesn't just separate into water and fat immediately, or why fog can hang in the air for hours without settling down? Welcome to the fascinating world of colloidal solutions! In this journey, we are going to decode a classic problem from Surface Chemistry that tests our understanding of why colloidal particles are so incredibly stable.

Imagine you have a beaker filled with a colloidal sol. The tiny particles dispersed in the liquid are constantly moving, bumping into each other. Why don't they just stick together, grow larger, and settle at the bottom due to gravity? The secret lies in electrical charges.

Colloidal particles are not electrically neutral. They carry a charge—either positive or negative. Because all the particles in a specific sol carry the same type of charge, they constantly repel each other. This electrostatic repulsion acts like an invisible shield, preventing the particles from coming close enough to aggregate. But where does this charge come from?

The most common reason is the preferential adsorption of ions from the dispersion medium. The surface of the colloidal particle acts like a magnet for certain ions, grabbing them and holding them tight, which gives the entire particle a net electrical charge.

Sulphides vs. Oxides

In chemistry, we have observed clear patterns regarding which types of compounds prefer which types of charges.

When we deal with metal sulphides (like ${\text{As}}_2{\text{S}}_3$, ${\text{Sb}}_2{\text{S}}_3$, or $\text{CdS}$), they have a strong tendency to adsorb negative ions, particularly sulphide ions (${\text{S}}^{2-}$), from the surrounding solution. As a result, metal sulphide sols are almost universally negatively charged.

On the flip side, when we look at metal oxides and hydroxides (like $\text{Fe}(\text{OH}{)}_3$, $\text{Al}(\text{OH}{)}_3$, or ${\text{TiO}}_2$), they behave differently. They prefer to adsorb positive ions, such as hydrogen ions (${\text{H}}^{+}$) or other metal cations. Consequently, metal oxide and hydroxide sols are generally positively charged.

Let's apply our logic to the first candidate in our problem: the Cadmium Sulphide ($\text{CdS}$) sol.

Cadmium sulphide is a classic metal sulphide. Following our established rule of thumb, the colloidal particles of $\text{CdS}$ will preferentially adsorb negative ions from the dispersion medium. This selective adsorption wraps the particles in a layer of negative charge. Therefore, we can confidently state that the $\text{CdS}$ sol is negatively charged.

Now, let's turn our attention to the second candidate: the Titanium Dioxide (${\text{TiO}}_2$) sol.

Titanium dioxide is a metal oxide. Based on the chemical behavior of metal oxides, these particles will attract and hold onto positive ions from their environment. This creates a positively charged layer around each particle. Hence, the ${\text{TiO}}_2$ sol is positively charged.

We have successfully decoded the nature of both sols. The $\text{CdS}$ sol carries a negative charge, and the ${\text{TiO}}_2$ sol carries a positive charge.

The question specifically asks for the charges respectively, meaning we must strictly follow the order in which the sols were presented.

1. $\text{CdS}$ sol $\to$ Negative 2. ${\text{TiO}}_2$ sol $\to$ Positive

Therefore, the correct sequence is negative and positive, which perfectly aligns with option (d).

Coagulation

Understanding the charge on a sol isn't just for answering multiple-choice questions; it has real-world applications, particularly in coagulation (or precipitation).

If you wanted to destroy the stability of these sols and force the particles to settle, you would need to neutralize their charges. According to the Hardy-Schulze rule, the most effective way to do this is by adding an electrolyte that provides an ion of the opposite charge, and the higher the valency of that ion, the better.

For our negatively charged $\text{CdS}$ sol, you would need a highly charged cation, like ${\text{Al}}^{3+}$ or ${\text{Ba}}^{2+}$. Conversely, for our positively charged ${\text{TiO}}_2$ sol, you would need a highly charged anion, like ${\text{PO}}_4^{3-}$ or ${\text{SO}}_4^{2-}$. Always keep these connections in mind, as they form the foundation of many advanced chemistry problems!