Animated Solution for Chemistry - Surface Chemistry: Which one of the following statements is false for hydrophilic sols ?

When we dive into the fascinating world of surface chemistry, colloids stand out as a unique state of matter. Among them, hydrophilic sols (a specific type of lyophilic sol where water is the dispersion medium) are particularly interesting. The term "hydrophilic" literally translates to "water-loving." This simple translation holds the key to understanding all their physical and chemical properties.

In a hydrophilic sol, the particles of the dispersed phase have a profound affinity for the dispersion medium, which is water. Imagine dropping some starch or gelatin into warm water. The particles don't just sit there; they actively interact with the water molecules, wrapping themselves in a thick, protective blanket of hydration.

Let's break down the statements given in the question one by one to see how this "water-loving" nature dictates their behavior.

1. Stability and Electrolytes: Because the colloidal particles are surrounded by a robust hydration sphere, they are inherently highly stable. They don't rely heavily on an electrical double layer to prevent them from clumping together. Therefore, unlike their lyophobic (water-hating) counterparts, they do not require the addition of electrolytes to maintain their stability. The first statement is absolutely true.

2. Reversibility: What happens if you evaporate all the water from a starch solution? You are left with a solid residue. But the magic of hydrophilic sols is that they are highly forgiving. If you simply add water back to that residue and give it a good shake, the sol is reconstituted immediately. This means they are reversible in nature. The second statement is also true.

3. Coagulation: Coagulation is the process of bringing colloidal particles together so they form larger aggregates and settle down. For hydrophilic sols, the thick layer of water molecules acts as a physical barrier, preventing the particles from coming close enough to aggregate. To coagulate them, you would first need to strip away this water layer (dehydration) and then neutralize their charge, which requires massive amounts of electrolytes. Hence, they cannot be easily coagulated. The fourth statement is true.

Viscosity

Now we arrive at the third statement regarding viscosity. Viscosity is essentially a measure of a fluid's resistance to flow. Think about pure water; it flows very easily. Now think about a concentrated gelatin solution or honey. They are thick, sticky, and resist flowing.

When hydrophilic particles are dispersed in water, they undergo extensive solvation. They trap a large number of water molecules around them, effectively increasing the size of the dispersed units and creating significant internal friction within the fluid. Because of this high concentration of the dispersed phase and the extensive hydration, the viscosity of hydrophilic sols is much higher than that of the pure dispersion medium ($H_{2}O$).

Therefore, the statement claiming that their viscosity is of the order of that of $H_{2}O$ is blatantly false. This makes it the correct answer to our question. Always remember: water-loving colloids make things thick and viscous!