Surface chemistry deals with the phenomenon that occurs at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash. For example, the inetrface between a solid and a gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases. The bulk phases that we come across in surface chemistry may be pure compounds or solutions. The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

Emulsions are type of colloids in which the physical state of dispersed phase and dispersion medium are respectively:

liquid, liquid

The correct answer is option 2. liquid, liquid.

Let us explore the concept of emulsions, a type of colloid, and why the correct description of the physical states in emulsions is "liquid, liquid."

A colloid is a mixture where one substance is dispersed in another, but the particles of the dispersed substance are larger than those in a true solution and smaller than those in a suspension.

The two components of a colloid are:

Dispersed phase: The substance that is dispersed (similar to the solute in a solution).

Dispersion medium: The substance in which the dispersed phase is distributed (similar to the solvent in a solution).

An emulsion is a specific type of colloid where both the dispersed phase and the dispersion medium are liquids. In an emulsion, tiny droplets of one liquid are suspended in another liquid with which it is immiscible (i.e., the two liquids do not normally mix). The droplets are small enough to remain suspended without settling out, thanks to the presence of an emulsifying agent, which helps stabilize the emulsion.

Types of Emulsions:

Oil-in-Water (O/W) Emulsion: The dispersed phase is oil (liquid), and the dispersion medium is water (liquid).

Example: Milk, where fat droplets are dispersed in water.

Water-in-Oil (W/O) Emulsion: The dispersed phase is water (liquid), and the dispersion medium is oil (liquid).

Example: Butter, where water droplets are dispersed in oil.

The defining characteristic of an emulsion is that both the dispersed phase and the dispersion medium are in the liquid state. This is what differentiates emulsions from other types of colloids, where the dispersed phase and dispersion medium could be solid, liquid, or gas.

Examples of Emulsions:

Milk: An example of an oil-in-water emulsion where fat droplets are dispersed in water.

Mayonnaise: An oil-in-water emulsion where oil droplets are dispersed in vinegar or water.

Cream: Another oil-in-water emulsion, where fat droplets are dispersed in water.

Emulsions are common in both natural and commercial products. They are found in food (e.g., salad dressings, sauces), cosmetics (e.g., lotions, creams), and even in pharmaceuticals.

The key characteristic of emulsions is that they involve two immiscible liquids, where one liquid is dispersed in the other. The physical states of both the dispersed phase and the dispersion medium in an emulsion are liquid.

The correct answer is option 2: liquid, liquid.