Question

(a) Why is there no colloid in which both the dispersed substance and the dispersing substance are gases? (b) Michael Faraday first prepared ruby-red colloids of gold particles in water that were stable for indefinite times. To the unaided eye these brightly colored colloids are not distinguishable from solutions. How could you determine whether a given colored preparation is a solution or colloid?

Short answer

(a) There is no colloid with both dispersed and dispersing substances being gases because the weak intermolecular forces in gases allow them to mix evenly with each other, forming a homogeneous mixture called a solution instead of a colloid. (b) To determine whether a given colored preparation is a solution or colloid, some methods include the Tyndall effect (light scattering), filtration, ultracentrifugation, and dialysis. These techniques exploit differences in light scattering, particle size, and permeability that separate solutions from colloids.

Step-by-step solution

Part (a) - Explanation for the absence of gas-gas colloids

A colloid is a mixture in which one substance is dispersed evenly throughout another substance. To understand why there is no colloid with both substances being gases, we must consider the molecular level of gases. Gases consist of individual molecules that are widely spaced apart and move randomly. The intermolecular forces between gas molecules are very weak, which allows the molecules to spread out and mix evenly with each other. As a result, when two gases mix, they form a homogeneous mixture called a solution instead of a colloid.

Part (b) - Methods to differentiate between a solution and a colloid

There are a few techniques that can be used to determine whether a given colored preparation is a solution or a colloid: 1. Tyndall effect: Shine a beam of light through the preparation, colloids scatter light, making the beam of light visible through the medium, while solutions do not scatter light and the beam remains invisible. 2. Filtration: Pass the preparation through a filter paper or membrane, colloids will be retained by the filter while solutions will pass through. 3. Ultracentrifugation: When subjected to high centrifugal forces, the dispersed particles in a colloid tend to sediment while the solute particles in a solution will not. 4. Dialysis: The preparation can be placed in a dialysis tube, with the dialysis membrane being selectively permeable to solute particles but not to colloid particles. A solution will pass through the membrane while a colloid will not. By using one or more of these methods, it would be possible to determine whether a given colored preparation is a solution or a colloid.

Key concepts

Tyndall Effect

The Tyndall effect is a phenomenon that helps distinguish colloids from true solutions based on their interaction with light.

When a beam of light passes through a true solution, it travels straight without being scattered because the particles are too small to affect the light path. In contrast, colloids have larger particles which scatter the light, making the beam visible as it goes through the colloid. This is similar to how dust particles in the air are visible in the path of sunlight entering through a window. The Tyndall effect is not only useful in telling colloids apart from solutions but also helps in studying the size of particles suspended in colloids.

Filtration

Filtration is a straightforward procedure used to separate particles from fluids (liquids or gases) by using a filter that allows only the fluid to pass through while catching the particles.

In the context of colloids versus solutions, filtration can be especially telling. A solution, having particles at the molecular level, will pass freely through a filter, whereas a colloid, with larger particles, may be retained by the filter. The size of the filter's pores is crucial; they must be small enough to block colloidal particles while allowing solvent molecules to pass through. This method provides an easy and practical way to identify whether a substance is a colloid or a solution.

Ultracentrifugation

Ultracentrifugation is a sophisticated technique that involves spinning a sample at very high speeds to separate substances based on their density.

For colloids and solutions, this method is efficient because the larger particles in colloids will move toward the bottom of the centrifuge tube under the high centrifugal forces applied, forming a sediment. This does not occur with solutions as the dissolved particles are too small and uniformly distributed to settle out. Ultracentrifugation is not only a method for differentiation but also a tool for purifying particles and studying their properties, such as size and mass.

Dialysis

Dialysis separates molecules in solution based on their size through a semipermeable membrane.

It's similar to filtration but is specifically designed for the selective passage of solutes. In dialysis, the colloidal particles are too large to pass through the pores of the dialysis membrane, whereas the smaller particles of a true solution can diffuse through. This process is commonly used in both laboratory procedures to purify colloids and in medicine, notably in kidney dialysis, where waste products are removed from the blood when the kidneys are not functioning properly.