Which medium uses water-soluble pigments applied to a wet surface?

The correct answer is watercolor, as this medium is characterized by the use of water-soluble pigments that are applied to a wet surface, typically paper. Watercolors are known for their transparent quality and fluidity, allowing artists to create delicate washes, subtle gradations, and vibrant hues.

When the pigment is mixed with water and applied to a wet substrate, it interacts with the surface, producing unique effects that are integral to the watercolor technique. This ability to manipulate the paint with water gives artists a wide range of expressive possibilities, from creating soft backgrounds to detailed imagery.

In contrast, acrylic paint is water-soluble when wet but becomes permanent when dry, and it can be used on various surfaces without the specific requirement of a wet background. Oil paint uses oil as a binder and does not rely on water for application, making it distinct from watercolors. Pastels are made from pure powdered pigments and a binder but do not involve the mixing with water for application. Thus, watercolor's defining characteristics align perfectly with the question's criteria.