Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statements about colloids is false?(a) When silver nitrate solution is added to potassium iodide solution, a negatively charged colloidal solution is formed (b) Freezing point of colloidal solution is lower than true solution at same concentration of a solute (c) Colloidal particles can pass through ordinary filter paper (d) When excess of electrolyte is added to colloidal solution, colloidal particle will be precipitated

Step-by-Step Solution

Answer

Statements (b) and (c) are false.

Step 1: Understanding Colloidal Characteristics

Colloids are heterogeneous mixtures where the particles are dispersed in a continuous medium. These particles vary typically in size from 1 nm to 1,000 nm. One of the key properties is that colloidal particles, while they do not settle under gravity, can be removed with selective membranes rather than ordinary filters.

Step 2: Reviewing Statement A

Statement (a) is true. When silver nitrate solution is added to potassium iodide solution, it forms a negatively charged colloidal solution composed of silver iodide in which the iodide ions impart a negative charge to the colloid.

Step 3: Reviewing Statement B

Statement (b) is false. The freezing point of a colloidal solution is higher than that of a true solution at the same concentration due to the colligative properties being less affected by larger colloidal particles compared to smaller ions or molecules in true solutions.

Step 4: Reviewing Statement C

Statement (c) is false. Colloidal particles are generally too large to pass through ordinary filter paper. Specialized methods are required to remove colloidal particles.

Step 5: Reviewing Statement D

Statement (d) is true. When excess electrolyte is added to a colloidal solution, it can cause the colloidal particles to precipitate out by neutralizing their charge, a process known as coagulation or flocculation.

Key Concepts

Colloidal Solution Characteristics

Colloids are unique mixtures that fall between true solutions and suspensions. The particles in a colloidal solution range in size between 1 nm and 1,000 nm, setting them apart from those in true solutions, where particles are molecular in scale. Unlike true solutions, colloids are not homogenous because their dispersed particles do not dissolve. However, these particles are small enough so they don't settle out when left undisturbed.

In everyday life, examples of colloids include milk, mayonnaise, and fog. All of these showcase typical colloidal characteristics: the particles remain in suspension and scatter light. This scattering, known as the Tyndall effect, helps differentiate colloids from true solutions. Moreover, while colloidal particles do not settle under the influence of gravity as in suspensions, they cannot be separated using regular filtration techniques without special equipment.

Despite being termed as solutions, colloids behave differently when it comes to colligative properties, such as the freezing and boiling points. Unlike true solutions, the colligative properties in colloids are modified due to the size of the dispersed particles. Hence, the freezing point of a colloidal solution is typically higher compared to a true solution of the same concentration.

Filter Paper and Colloids

One intriguing aspect of colloids is how their particles interact with filtering methods. Regular filter paper is not effective in removing colloidal particles from their medium due to their size and charge. This is because colloidal particles are just too big to pass through the pores of an ordinary filter paper.

Colloids need more sophisticated separation processes like ultrafiltration or dialysis. Ultrafiltration uses a special membrane that has smaller pores, precisely to allow the separation of the colloidal particles from the fluid they are in, without letting the dispersed particles escape. Dialysis, on the other hand, separates solutes by using the difference in diffusion rates through a semi-permeable membrane.

The inefficiency of regular filter paper means that in various industries and laboratories, more advanced filtering techniques are used to handle colloids. These advanced methods allow for both the extraction of colloidal particles for analysis and the removal of unwanted colloids from solutions.

Electrolyte Addition Effects on Colloids

Adding electrolytes to a colloidal solution can bring about drastic changes. One common reaction is the precipitation of colloidal particles, known as coagulation or flocculation. This process occurs when the charged particles of the colloid are neutralized by the oppositely charged ions in the added electrolyte. Once the particles lose their charge, they can no longer repel each other and start to aggregate, eventually forming larger particles that precipitate out of the mixture.

Coagulation is an important process applied in water treatment and wastewater management. By manipulating colloidal stability through electrolytes, it becomes possible to remove unwanted particles from water.

A delicate balance must be maintained since each colloid possesses a specific balance of charges that define its stability. Adding too much electrolyte can lead, not just to the precipitation of the colloidal particles, but also to unwanted aggregation and loss of desired properties in formulations. Thus, understanding and controlling the effects of electrolytes on colloidal systems is crucial in both scientific research and industrial applications.