In the chemistry of paints and art materials, tempera is a type of paint in which the vehicle (liquid medium) is present in which physical form?

Introduction / Context:
This question connects chemistry with art materials, specifically traditional paint formulations. Tempera is an ancient painting medium that was widely used before the advent of modern oil paints. In tempera, the colouring matter (pigment) is dispersed in a binding medium that is typically an emulsion, often made from egg yolk mixed with water and sometimes oil. Understanding that tempera paint uses an emulsion as its vehicle helps illustrate how basic concepts from colloid chemistry and emulsions apply in artistic and historical contexts.

Given Data / Assumptions:
- The paint type is tempera, used historically in fine art and icon painting.
- The question focuses on the vehicle, meaning the liquid medium that carries the pigment and helps it adhere to the surface.
- Options include emulsion, pigment, impasto, and ground, which refer to different aspects of painting materials and techniques.
- It is assumed that you know pigments are solid colour particles and that emulsions are mixtures of immiscible liquids dispersed in one another.

Concept / Approach:
In tempera painting, finely ground pigments are mixed with a binding medium that is usually an emulsion of water and oil or an egg yolk and water mixture. An emulsion is a colloidal dispersion of tiny droplets of one liquid in another immiscible liquid, stabilised by natural emulsifiers like lecithin in egg yolk. This emulsion acts as the vehicle: it carries the pigment particles, allows them to be applied smoothly, and then dries to form a durable film. Impasto refers to a thick application of paint, not the composition of the vehicle, and ground refers to the prepared surface on which the paint is applied. Pigment is the colouring component, not the liquid vehicle itself. Therefore, the correct description is that in tempera paint, the vehicle is in the form of an emulsion.

Step-by-Step Solution:
Step 1: Identify tempera as a traditional paint medium in which pigments are mixed with a binder, commonly egg yolk and water. Step 2: Recognise that egg yolk contains natural emulsifiers that allow oil and water to mix, forming a stable emulsion. Step 3: Understand that this emulsion of water and oil, or egg and water, serves as the vehicle that carries and binds pigment particles to the painting surface. Step 4: Note that pigment itself is the solid colouring powder and cannot be the vehicle. Step 5: Recall that impasto describes a technique of applying paint thickly, and ground refers to the primed surface; neither term describes the physical state of the vehicle. Step 6: Conclude that the tempera vehicle is best described as an emulsion.

Verification / Alternative check:
Art history and materials science references describe egg tempera as a mixture of pigment with an emulsion of egg yolk and water. They explain that the mixture creates a stable water based paint that dries quickly to a matte finish. Descriptions of oil paints and acrylics also mention that these media often involve emulsions or polymer dispersions as vehicles, reinforcing the idea that emulsions are common paint vehicles. However, pigment is always distinguished from the binder or vehicle, and impasto and ground are used to describe techniques and surfaces rather than the medium. This confirms that emulsion is the correct answer.

Why Other Options Are Wrong:
Pigment is the solid material that gives paint its colour, not the liquid vehicle; calling the vehicle a pigment would confuse the colouring component with the binder. Impasto refers to a thick, textured application of paint, which can be used with various media but is not the name of the vehicle. Ground is the prepared surface or priming layer on a support such as canvas or wood, not the liquid medium in the paint. None of these terms describe the physical form of the binding medium in tempera paint. Only an emulsion correctly captures the nature of the tempera vehicle.

Common Pitfalls:
Students unfamiliar with art terminology may confuse pigment with the entire mixture of paint, or think that impasto or ground refer to the ingredients rather than to artistic techniques and surfaces. Others might not recall that emulsions are relevant outside food chemistry and pharmacy, appearing in paints, cosmetics, and other materials. To avoid these misunderstandings, remember that pigment is colour, vehicle is the liquid binding medium, and in tempera, that medium is an emulsion of water and other components, making emulsion the correct choice.

Final Answer:
Technically, tempera is a paint in which the vehicle or binding medium is in the form of an emulsion.