Which of the following is not a property of hydrophilic sol? (a) High concentration of dispersed phase can be easily attained. (b) Coagulation is reversible. (c) Viscosity and surface tension are nearly as that of water. (d) The charge on the particles may depend on the \(\mathrm{pH}\) value; it may be positive, negative or even zero.

Colloidal systems are a fascinating state of matter where particles, ranging from 1 to 1000 nanometers in size, are dispersed throughout a solvent but do not settle out like typical suspensions. Instead, they exhibit a unique mixture of properties, including the Tyndall effect, which allows light to scatter as it passes through the medium.

In hydrophilic sols, which are types of colloidal systems, the dispersed phase has a high affinity for the aqueous environment. This compatibility is due to various functional groups on the particles' surfaces that can form hydrogen bonds with water. Moreover, the stability of these systems is mainly attributed to the particles' ability to repel each other, often resulting from their electric charge acquired in the presence of a polar solvent like water.

Understanding colloids is essential for fields ranging from materials science to medicine, as they can be tailored for specific industrial applications, drug delivery systems, and even in everyday products like paints and food.

Coagulation in colloids refers to the process where colloidal particles clump together, eventually precipitating out of the solution. It can be triggered by a variety of factors, including the addition of electrolytes, changes in temperature, or by mechanical means.

In hydrophilic colloids, the coagulation is particularly interesting because it is often reversible. When a dispersing agent, usually water, is added after coagulation, the individual particles can again become suspended and evenly distributed throughout the solvent.

It's important for students to comprehend that the reversible nature of coagulation in hydrophilic sols makes them distinct from hydrophobic sols, where the coagulation is typically irreversible. This reversibility lends hydrophilic colloids to widespread use in the food industry and biological processes where control over the colloidal state is important.

The surface charge of colloidal particles is a crucial factor that influences the stability of the colloidal system. In hydrophilic sols, this charge is not constant but varies with the pH of the solution.

Essentially, the particle charge is dependent on the acidic or basic nature of the medium. At low pH (highly acidic conditions), particles are more likely to acquire a positive charge, while at high pH (highly alkaline conditions), they tend to gain a negative charge. This is because of the ionizable groups on the particle’s surface that can either donate or accept protons depending on the pH.

Understanding this relationship is key to manipulating colloidal systems for desired outcomes. For example, knowing the optimal pH for maximum stability can prevent unwanted aggregation, which is essential in applications such as pharmaceuticals, water purification, and photographic films.