Final answer:
Density is a physical property that can be observed or measured without altering a substance's identity, defined as mass per unit volume, and is constant for a substance under consistent conditions.
Step-by-step explanation:
Density is a physical property of matter, which means it is a characteristic that can be observed or measured without changing the identity of the substance. Density is defined as the substance's mass divided by its volume, and it can be measured for all states of matter including solids, liquids, and gasses. This property is used to identify a substance and differentiate it from others, since each substance has a characteristic density. Furthermore, it is important to note that density does not change with the size of the sample; whether you have a small or large amount of a substance, the density will remain constant as long as conditions such as temperature and pressure are consistent.