Question

Choose the best answer: A colloidal dispersion of one liquid in another is called \((\mathbf{a})\) a gel, \((\mathbf{b})\) an emulsion, \((\mathbf{c})\) a foam (d) an aerosol.

Short answer

The colloidal dispersion of one liquid in another is best described as an emulsion, where one liquid is dispersed throughout the other in the form of small droplets without actually mixing. So the correct answer is \((\mathbf{b})\) an emulsion.

Step-by-step solution

Definition of Gel

A gel is a semi-solid or jelly-like substance consisting of a solid framework in a continuous liquid phase.

Definition of Emulsion

An emulsion is a mixture of two immiscible liquids (liquids that do not mix), such as oil and water, in which one liquid is dispersed throughout the other in the form of small droplets.

Definition of Foam

A foam is a substance that is formed by trapping pockets of gas in a liquid or solid. Examples include whipped cream.

Definition of Aerosol

An aerosol is a suspension of fine solid particles or liquid droplets in a gas, such as air.

Identifying the Colloidal Dispersion of One Liquid in Another

Based on the definitions above, the colloidal dispersion of one liquid in another is best described as an emulsion. In an emulsion, the liquid particles are dispersed throughout the other liquid without actually mixing with it. This closely resembles the situation described in the exercise. So the correct answer is \((\mathbf{b})\) an emulsion.

Key concepts

Colloidal Dispersion

When someone talks about colloidal dispersions, they are referring to a mixture where tiny particles are spread throughout another substance. This dispersion happens at a microscopic level. The particles do not dissolve, but instead remain suspended in the medium. These dispersed particles are what gives colloids their unique properties.
One of the routes colloidal dispersions can form is through an emulsion. An example is oil in water. Within the liquid, tiny oil droplets spread out, forming a stable dispersion that does not separate easily. Despite being mixed, the droplets do not dissolve, allowing the mixture to retain its unique characteristics.

• Colloids are larger than simple solutions but smaller than suspensions.
• They often display interesting optical effects, like the Tyndall effect, where light scatters through the dispersed particles.
• Common colloidal dispersions include milk (emulsion) and fog (aerosol).

Immiscible Liquids

When liquids are described as immiscible, it means they do not mix together to form a homogeneous mixture. A classic example is oil and water, which remain separate even when mixed. This phenomenon occurs because the molecules of the liquids are more attracted to themselves than to each other.
In emulsions, immiscible liquids are combined using an emulsifying agent to become a stable mixture, where one liquid is caught in small droplets inside the other. This creates a lasting emulsion despite their natural tendency to separate. For instance, mayonnaise is made from oil and water, bound together using egg yolk as an emulsifier.

• Immiscible liquids maintain distinct layers when not agitated or emulsified.
• They require an emulsifier to stay stable in an emulsion.
• Examples of emulsifiers include egg yolks and lecithin.

Chemical Definitions

To understand emulsions and other complex mixtures, recognizing basic chemical definitions is crucial. Key terms like solution, suspension, and colloid form the foundation for understanding how substances interact.
A solution is a homogeneous mixture where substances dissolve completely, e.g., salt in water. Suspensions, on the other hand, contain larger particles suspended in a medium but not dissolved, such as sand in water. Colloids fall between these two, with microscopic particles that are small enough to stay suspended and large enough to scatter light.

• Solution: Completely dissolved, uniform mixture.
• Suspension: Larger particles that settle upon standing.
• Colloid: Fine particles stay suspended, cause light scattering.

Understanding these definitions helps to identify and describe complex mixtures like emulsions. This knowledge aids in comprehending why colloids display unique behaviors and properties.