Understanding Emulsions

An emulsion is a mixture of two liquids that normally don't mix, like oil and water. One liquid is dispersed as tiny droplets within the other.

Emulsions are classified based on which liquid is dispersed (the dispersed phase) and which surrounds it (the continuous phase):

• Oil-in-Water ($O/W$) Emulsion: Oil droplets are dispersed in water.
• Water-in-Oil ($W/O$) Emulsion: Water droplets are dispersed in oil.

Analyzing the Options for Water-in-Oil Emulsion

The question asks for an example of a 'water - in - oil' ($W/O$) emulsion. Let's examine the options:

• Milk: This is a classic example of an oil-in-water ($O/W$) emulsion, where fat globules (oil) are dispersed in an aqueous (water) phase.
• Mayonnaise: This is also an oil-in-water ($O/W$) emulsion, typically made with oil, egg yolk (as an emulsifier), and vinegar or lemon juice (water-based).
• Egg yolk: Contains lecithin, a strong emulsifier, primarily used to stabilize $O/W$ emulsions like mayonnaise. While it has both oil and water components, it's known more as an emulsifier or base for $O/W$ systems.
• Butter: Butter is typically manufactured as a water-in-oil ($W/O$) emulsion. It consists of about 80-82% fat (the continuous phase) with 16-18% water dispersed as tiny droplets within the fat.

Conclusion

Based on the composition, Butter is the correct example of a water-in-oil ($W/O$) emulsion among the choices provided.