The study of matter involves classifying its characteristics, known as physical properties, to better understand and identify substances. Scientists organize these properties based on whether they change when the amount of substance is varied, dividing them into two main categories: intensive and extensive. Density is definitively categorized as an intensive property. Understanding this classification requires distinguishing between properties that depend on the sample size and those that do not.
Properties That Depend on Sample Size
Properties that vary directly in proportion to the amount of matter present in a sample are known as extensive properties. If two identical samples are combined, the total value of the property is doubled. Extensive properties convey information about the extent or size of the system being measured.
Mass is a primary example, measuring the amount of matter in an object. If a 10-gram sample is combined with another 10-gram sample, the resulting total mass is 20 grams. Volume is another common extensive property, representing the space an object occupies. Doubling the amount of a substance will double its volume.
Other characteristics, such as total internal energy or length, are also classified as extensive properties. Because their values are directly dependent on the sample size, they are not reliable for identifying a substance. For instance, a small piece of gold and a large gold bar have different masses and volumes, making these measurements inadequate for confirming they are the same material.
Properties That Are Independent of Sample Size
Intensive properties are the characteristics of a substance that remain constant regardless of the sample size. These properties are intrinsic to the substance itself and are particularly useful for identifying and distinguishing different types of matter.
Temperature is a familiar example of an intensive property. A small cup of boiling water will register the same temperature as a large pot of boiling water. Similarly, the boiling point and melting point of a pure substance remain the same regardless of how much of the substance is present.
Other characteristics such as color, odor, and hardness are also intensive properties. A small diamond chip has the same inherent hardness as a large diamond jewel, and a single drop of milk has the same color as a gallon. These properties reflect the substance’s composition and structure, making them constant throughout the material. Scientists often use intensive properties to determine the identity of an unknown material.
Why Density Is an Intensive Property
Density is formally defined as the ratio of an object’s mass to its volume (Density = Mass / Volume). This definition is why density is categorized as an intensive property, even though it is calculated using two extensive properties. Density represents the amount of matter packed into a given unit of space, which is a characteristic of the material itself, not the size of the sample.
When the amount of a substance is changed, both its mass and its volume change in a proportional manner. For example, if a solid block of aluminum is cut into two equal pieces, the mass is halved, and the volume is also halved. This proportional decrease in both the numerator (mass) and the denominator (volume) of the density equation ensures that the resulting ratio remains mathematically identical.
The original block and the smaller pieces will therefore have the exact same density value. This constancy under changes in sample size is the defining feature of an intensive property. Because density is a ratio of two properties that scale equally with size, it becomes independent of size.
This classification allows density to serve as a reliable physical constant for identifying a pure substance. Pure water, for instance, has a density of approximately 1.0 gram per milliliter at room temperature, whether the sample is a single drop or a large tank. The consistent value of this property makes it a highly useful tool in fields such as chemistry and materials science.