Problem 85 - Chemistry The Central Science

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 indefinitely. 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?

Step-by-Step Solution

Answer

(a) There is no colloid where both dispersed and dispersing substances are gases because gases mix uniformly to create a homogenous mixture, not a colloid with distinct dispersed and dispersing substances. This is due to gases having higher kinetic energy, causing their particles to move more freely and quickly. (b) To determine if a colored preparation is a solution or a colloid, perform the following tests: (1) Tyndall Effect - check for light scattering, (2) Filtration - examine the filtered solution for color and clarity, and (3) Check for sedimentation - observe if particles settle down after leaving the preparation undisturbed. These techniques will help identify the nature of the preparation.

Step 1: Part (a): Understanding colloids

A colloid is a mixture in which one substance (dispersed substance) is finely dispersed throughout another substance (dispersing substance) without forming any clear solution. In a colloid system, at least one of the phases must be in a condensed state of matter (solid or liquid). Both substances being gases would not form a colloid since they would mix uniformly, creating a homogeneous gaseous mixture instead of a colloid. The reason is that gases have higher kinetic energy and as a result, their particles move much more freely and quickly. This would cause their particles to mix easily and create a well-blended homogenous mixture as opposed to a colloid with distinct dispersed and dispersing substances.

Step 2: Part (b): Identifying a colloid or solution

To determine if a given brightly colored preparation is a solution or a colloid, the following techniques can be used: 1. Tyndall Effect: The Tyndall effect is the scattering of light by particles in a colloid or in a fine suspension. Shine a beam of light through the preparation and observe the path of the light. If the path of the light is visible as it passes through the preparation (light scattering), then it is a colloid. If the path of the light is not visible and the light just passes straight through, then it is a solution. 2. Filtration: Filter the preparation using filter paper or a very fine filter. If the filtered solution appears clear and the color remains the same or has very little change, then it is a solution. If the filtered solution appears cloudy or there is a significant change in color, then it is a colloid, which indicates that the particles have been separated by the filtration process. 3. Check for sedimentation: Leave the preparation undisturbed for some time. If the particles settle down at the bottom of the container, then it is a colloid. However, if no settling occurs and the preparation remains uniform even after a long time, it is most likely a solution. Using these techniques, one can identify whether the brightly colored preparation is a colloid or a solution.

Key Concepts

Tyndall Effect

Tyndall Effect is a phenomenon observed when light passes through a colloid. Unlike in true solutions, colloidal particles are large enough to scatter light, making the light path visible in the colloidal mixture. This phenomenon occurs due to the interaction between light and the particles of the dispersed phase, which are big enough to deflect light but too small to be seen with the naked eye.

For instance, when a beam of light enters a room filled with dust, the light can be seen as a beam. This is because the dust particles scatter the light. Similarly, in colloids, the particles will scatter light creating a visible effect known as the Tyndall effect. However, this effect isn't seen in true solutions, because the particles are too small to interfere with light. Therefore, the Tyndall effect serves as a simple but powerful way to distinguish a colloid from a true solution.

Homogeneous Mixtures

Homogeneous mixtures are blends of substances where the composition is uniform throughout the solution. Each part of a homogeneous mixture has the same properties. In terms of colloids and solutions, this concept is essential for understanding the difference between them. A solution is a homogeneous mixture as it contains particles such as ions or molecules that are evenly distributed at a molecular level.

In contrast, colloids have particles that are larger than those in a solution, ranging from 1 to 1000 nanometers, and do not settle out upon standing. This means they don't display the same uniformity at a molecular level, but they can appear uniform to the naked eye. Examples of homogeneous mixtures include salt water, air, and alloys.

Sedimentation in Colloids

Sedimentation in colloids is a process where particles in a colloid eventually settle out under the influence of gravity. The rate at which sedimentation occurs can be very slow due to the small size and the interactions between particles of the dispersed phase and the medium in which they are dispersed. This process is often used to differentiate colloids from true solutions.

In a true solution, the solute particles are dissolved at the molecular level and will not settle out at all. Sedimentation can be accelerated by centrifugation, employing a high-speed rotation to force particles to the bottom of the container. However, many colloids are very stable and do not show noticeable sedimentation; this stability can be attributed to the Brownian motion of particles and interactions that prevent particles from aggregating and settling.