Which type of property can be observed without changing the substance?

The correct answer is physical property, which refers to characteristics of a substance that can be observed or measured without changing its chemical composition. Examples of physical properties include color, melting point, boiling point, density, and solubility. When you measure or observe a physical property, such as observing the color of a substance or determining its state of matter, the substance itself remains unchanged.

In contrast, chemical properties involve the behavior of a substance during a chemical reaction, resulting in a change in composition. Some properties indicate how a substance reacts with others, which would fundamentally alter the substance. Reactive properties are those that indicate the extent to which a substance reacts with other chemicals, showcasing a change in its original form.

Mechanical properties typically refer to characteristics that describe how a substance responds to applied forces, such as tensile strength or elasticity, which also may involve some change in the substance's shape or structural integrity when stressed.

Understanding these distinctions is crucial in chemistry and materials science, reinforcing why physical properties are the only category that can be assessed without altering the substance itself.