Which of the following describes characteristics observed without altering a substance's composition?

Physical properties are characteristics that can be observed or measured without altering the substance's composition. These include attributes such as color, odor, density, melting point, boiling point, and state (solid, liquid, gas). These properties allow one to describe how a substance appears and behaves under certain conditions without changing its chemical structure or composition.

Chemical properties, in contrast, describe how a substance interacts with other substances, leading to a change in its chemical composition. Reactivity and combustibility are specific types of chemical properties that indicate how a substance may react with other substances or undergo burning, respectively. Unlike physical properties, observing these characteristics often involves a change in the substance itself, which is why they do not fit the original question's criteria regarding preservation of composition.