Question

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

Short answer

The correct answer is (b) an emulsion, as it best fits the description of a colloidal dispersion of one liquid in another.

Step-by-step solution

Understand the terms

In order to find the correct answer, it is important to first understand the meaning of each term provided in the options: (a) A gel: A gel is a semi-solid substance that consists of a network of solid particles or liquid droplets dispersed in a liquid. Gels are typically not colloidal dispersions of one liquid in another. (b) An emulsion: An emulsion is a colloidal dispersion in which small droplets of one liquid are dispersed in another liquid in an unstable state. The two liquids do not mix completely due to their immiscibility. (c) A foam: A foam is a colloidal dispersion in which a gas is dispersed in a liquid or solid medium. This option involves a gas and not a liquid in another liquid. (d) An aerosol: An aerosol is a suspension of liquid or solid particles in a gas. In an aerosol, the liquid or solid particles are dispersed in a gas medium, and not in another liquid.

Identify the correct term for the given situation

Based on the definitions discussed in Step 1, it becomes clear that an emulsion best fits the description of a colloidal dispersion of one liquid in another. Solution: The correct answer is (b) an emulsion.

Key concepts

colloidal dispersion

Colloidal dispersions are fascinating mixtures where one substance is evenly dispersed in another at a microscopic level. Unlike true solutions, the dispersed particles in a colloid are usually larger, ranging from 1 nm to 1 µm. But these particles are still small enough not to settle under the influence of gravity.
A key characteristic of colloidal dispersions that sets them apart from other mixtures is the stability imparted by forces like electrostatic charges or hydrophobic interactions. These forces prevent the dispersed particles from clumping together.
Colloids can appear in different forms depending on the state of the dispersed phase and the dispersion medium:

• An emulsion, like mayonnaise, is a colloid where both the dispersed substance and the medium are liquids.
• A foam, such as whipped cream, is a colloid where gas bubbles are dispersed in a liquid.

Each type of colloidal dispersion has unique properties, making them vital in various industries from food to pharmaceuticals.

immiscibility

Immiscibility refers to the inability of two liquids to mix and form a homogenous solution. When two liquids are immiscible, they tend to separate into layers when combined. This characteristic is because the molecules in each liquid prefer to stay with their own kind rather than mix with different molecules.
An everyday example of immiscibility can be seen in oil and water. When you try to mix them, they quickly separate because the non-polar oil molecules avoid the polar water molecules.
Immiscibility plays a crucial role in forms of colloidal dispersions such as emulsions. Although the two liquids in an emulsion are technically immiscible, tiny droplets of one liquid are dispersed in the other, stabilized by an emulsifying agent like soap or egg yolk.

liquid-liquid systems

Liquid-liquid systems are intriguing mixtures involving two or more liquids. These systems can behave in different ways depending on the mutual solubility of the liquids involved. If the liquids are miscible, they mix thoroughly, forming a clear solution. If they are immiscible, they separate into distinct phases, as seen in emulsions.
In an emulsion, one liquid forms tiny droplets within another, facilitating the mixing of immiscible liquids. Cooking is an area where liquid-liquid systems are common, such as in vinaigrettes, where oil droplets are dispersed in vinegar with the help of a bit of mustard.
These systems also rely heavily on the concept of surface tension to maintain stability. So next time you're enjoying a creamy sauce or blending a smoothie, it's liquid-liquid systems and emulsions at work. Understanding these systems helps us appreciate the chemistry behind many daily experiences.