Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

Smoke is a colloidal dispersion of a (a) solid in a gas (b) liquid in a gas (c) gas in a solid (d) gas in a gas

Step-by-Step Solution

Answer

Smoke is a colloidal dispersion of a solid in a gas.

Step 1: Understanding Colloid Types

Colloids consist of two phases: a dispersed phase and a continuous phase. Identifying what is dispersed and what makes up the medium helps in classifying the colloid.

Step 2: Identifying Smoke Composition

Smoke originates from burning substances. It primarily contains solid particles, such as ash and soot, suspended in a gas, specifically air.

Step 3: Matching with Options

Given the multiple-choice options, identify which one describes a solid dispersed in a gas. Option (a) suggests a solid in a gas.

Step 4: Determining the Correct Answer

Smoke matches the colloidal dispersion of solid particles within a gaseous medium, confirming that option (a) is correct.

Key Concepts

Solid in a Gas

When we talk about a 'solid in a gas' colloidal dispersion, we're referring to a mixture where tiny solid particles are spread throughout a gas. Smoke is a great example of this kind of blend. Imagine when you see smoke rising from a fire; it consists of tiny particles of ash and soot floating in the air. This means that the solid part (the ash and soot) is evenly distributed within a gaseous phase (the air).
Such mixtures have unique properties. They do not settle quickly like suspensions and are not as clear-cut as solutions. The tiny particles remain evenly spread for a long time. This gives the smoke its visible and smoky appearance, characteristic of a solid in a gas colloid.

Colloid Classification

Colloids are fascinating mixtures comprised of two phases: the dispersed phase and the continuous phase. To classify a colloid, it's crucial to know which substance is in which phase.

• Dispersed Phase: This is the substance that is present in a small quantity, which spreads throughout the medium.
• Continuous Phase: This is the substance that makes up the larger amount and hosts the dispersed particles.

With these definitions in mind, colloids can be categorized based on the states of the dispersed and continuous phases. Common classifications include:

• Solid in Gas: e.g., Smoke (solid particles like ash in air).
• Liquid in Gas: e.g., Fog (tiny water droplets in air).
• Gas in Solid: e.g., Pumice (gas bubbles in a solid matrix).
• Gas in Gas: Not typical as gases diffuse into one another rather than form colloids.

Understanding colloid classification helps us identify mixtures like smoke accurately, determining which phase acts as dispersed and which as the medium.

Dispersed Phase

In a colloid, the dispersed phase is the component present in smaller amounts and is spread throughout the continuous phase. Smoke serves as an excellent example, where solid particles such as ash and soot act as the dispersed phase. These particles are tiny and remain suspended in the air for extended periods.
Due to their small size, particles in the dispersed phase do not fall under the influence of gravity in the same way larger particles would in suspensions. This explains why smoke can float and spread through the air for miles. The unique properties of colloids, including their stability and behavior, largely depend on the interaction between the dispersed phase and the continuous phase.
By understanding the role of the dispersed phase, we can get insights into how colloids behave, interact, and can be utilized in different applications ranging from industrial processes to atmospheric science.