Why Is Density Considered a Physical Property Rather Than a Chemical Property of Matter?

Learning to tell the difference between physical and chemical properties is a cornerstone of all basic chemistry courses. Some properties, however, are often difficult for beginning students to place in one category or the other. Density is one of those properties. With just a little clarification, students can gain the confidence to tackle not only density but other tricky cases as well.

Physical properties are those that do not change the chemical composition of the substance being described. In other words, measuring a physical property does not cause a chemical reaction. Length, mass, index of refraction and melting point are all examples of physical properties because none of these involves a chemical reaction. Measuring melting point involves changing the phase of a substance, but it does not change it into another substance. (In other words, melting ice still leaves you with H2O.)

Chemical properties are the opposite of physical properties. Measuring a chemical property involves a chemical reaction. Rate of oxidation, flammability and acidity (pH) are examples of chemical properties because measuring each of these properties involves a chemical reaction. To determine the rate of oxidation, for example, the substance must oxidize.

Density is defined as mass per unit volume. In other words, it's how much the mass is confined or spread out in a substance. It helps demonstrate whether atoms are packed closely together or spread so far apart they don't even interact with each other (as is the case for an ideal gas). To measure density, simply measure the mass on a balance, calculate volume from measured lengths and divide the two. This process involves no chemical change; therefore, density is a physical property. Another way to measure density is by using its displacement of liquid. As a consequence of Archimedes' principle, the volume of the liquid after the object of interest is submerged minus its original volume will give the volume of the object. Mass is again measured on a balance, and density is calculated. This process still involves no chemical change in the substance, so density is a physical property.

Every substance has its own density; this property changes when the chemical substance changes. Don't confuse this with the definition of a chemical change, which is that the measurement itself alters the chemical nature of the substance. In the first case, you are simply identifying a chemical based on its properties (either physical or chemical). In the second case, a chemical reaction is taking place.