What does luster refer to in terms of physical properties?

Luster refers specifically to the shininess or reflectivity of a substance. It describes how light interacts with the surface of a material, indicating its ability to reflect light to produce a shiny appearance. This property is particularly relevant for materials such as metals and gemstones, where the quality of the surface can enhance their visual appeal. Luster is an important characteristic used in classifying minerals and assessing their aesthetic value.

In the context of the other options, weight pertains to mass rather than surface quality. Chemical reactivity relates to how a substance interacts with other chemicals, not its physical appearance. Solubility in water measures how much of a substance can dissolve in water, which is also unrelated to the reflective properties of a material's surface. Thus, the definition of luster is clearly tied to the visual characteristics of a material rather than its mass, reactivity, or solubility.