Problem 85 - Chemistry The Central Science In Si Units

Question

(a) Do colloids made only of gases exist? Why or why not? (b) In the 1850 s, Michael Faraday prepared ruby-red colloids of gold nanoparticles in water that are still stable today. These brightly colored colloids look like solutions. What experiment(s) could you do to determine whether a given colored preparation is a solution or colloid?

Step-by-Step Solution

Answer

(a) No, colloids made solely of gases do not exist. (b) Use the Tyndall effect to determine if the mixture is a colloid (light scattering) or a solution (no scattering).

Step 1: Understanding Colloids and Gases

Colloids consist of two phases - a dispersed phase and a continuous phase. For a substance to be a colloid, the dispersed particles must not settle under gravity and must remain evenly distributed throughout the continuous phase. Gas in gas systems cannot form colloids because there is no distinction between dispersed and continuous phases; gases mix completely and result in a homogeneous solution.

Step 2: Answering Part (a)

Based on the understanding that gases mix homogeneously, colloids made solely of gases do not exist because a colloid requires a distinct dispersed phase and a continuous phase, which cannot be provided by gases alone.

Step 3: Experiment to Determine Type of Mixture

To distinguish between a solution and a colloid, you can perform the Tyndall effect experiment. Shine a beam of light through the mixture. Colloids will scatter the light, making the beam visible as it passes through. In contrast, solutions will not scatter light, and the beam will remain invisible.

Step 4: Results of the Tyndall Effect

If the beam of light is visible passing through the preparation, it indicates the presence of a colloid (i.e., the dispersed particles are scattering the light). If the light beam is invisible, the preparation is likely a true solution (particles are too small to scatter light).

Key Concepts

Gas-phase systems

Understanding gas-phase systems involves recognizing how gases behave when combined. Gases naturally mix thoroughly with each other, creating a homogenous solution. This property stems from the complete intermingling of gas molecules due to their high kinetic energy and non-restrictive space.
In contrast to liquids or solids where particles can be suspended and form distinct phases, gases don't have this property. For a mixture to be classified as a colloid, it needs clearly distinguishable dispersed and continuous phases.
Since gases mix completely, thus forming only a single, uniform phase, they cannot exhibit the separate phases essential for forming colloids. This distinction is crucial for defining why gas-phase-only colloids don't exist.

Tyndall effect

The Tyndall effect is a simple yet effective experiment used to distinguish between colloids and solutions. It is based on the scattering of light by particles in a colloid. Here's how it works:

• First, shine a beam of light through your mixture.
• If the mixture is a colloid, the dispersed particles scatter the light, and you will see the light beam passing through the mixture.
• In a true solution, the particles are too small to scatter light, so the beam remains invisible as it travels through.

This scattering occurs because the particles in a colloid are large enough (usually between 1 nm and 1 μm) to interfere with and scatter light waves.
By observing the visibility of the light beam, you can conclude whether a substance is a colloid or a solution.

Dispersed and continuous phases

In colloids, understanding dispersed and continuous phases is fundamental. These phases define the structure and behavior of colloidal systems.

• The **dispersed phase** consists of small particles or droplets distributed throughout another substance. These particles are typically between 1 nm and 1 μm in size and do not settle due to gravity.
• The **continuous phase** is the medium in which these particles are distributed. It holds and evenly envelops the dispersed particles.

In practical terms, think of a colloid like fog. The tiny water droplets represent the dispersed phase, while the air acts as the continuous phase.
Both phases are necessary for creating and maintaining a stable colloid. Each phase must be distinct, meaning there should be a clear boundary or difference between the two.
Understanding these phases helps in identifying whether a mixture, at a macroscopic level, functions as a colloid.