Which statement best describes chemical properties of matter? Chemical properties, such as density, must be observed when a substance is in its natural state at room temperature and normal atmospheric pressure. Chemical properties, such as reactivity, must be observed when a substance is in its natural state at room temperature and normal atmospheric pressure. Chemical properties, such as combustibility, are generally observed as the identity of a substance changes and one or more new substances form. Chemical properties, such as boiling point, are generally observed as the identity of a substance changes and one or more new substances form.