Matter possesses measurable physical characteristics, or properties, that allow scientists to describe and differentiate substances. These properties are classified based on how they relate to the quantity of material being observed. This classification helps define the state and behavior of a system. The fundamental distinction lies in whether a property changes with the amount of substance present.
What Defines an Extensive Property
An extensive property is a physical quantity whose value is directly proportional to the amount of matter in a sample. If you were to double the quantity of a substance, the value of that extensive property would also double. Extensive properties are often described as being additive in nature. If a system is divided into two parts, the value of the extensive property for the original system is the sum of the values for the two smaller subsystems. For example, combining two separate containers of gas results in a total volume that is the addition of the two initial volumes.
Common Examples of Extensive Properties
Mass is the most straightforward example of an extensive property, as it is a direct measure of the amount of matter contained in an object. A small sample of material will have a small mass, while a larger sample of the same material will have a proportionally greater mass. If a 1-kilogram block of wood is cut exactly in half, each resulting piece will have a mass of 0.5 kilograms. Other common extensive properties include:
- Volume, the measure of the three-dimensional space occupied by a substance.
- Total energy, including internal energy, which is dependent on the quantity of matter it contains.
- The number of moles, which represents the amount of substance.
The Essential Difference: Extensive vs. Intensive Properties
Understanding extensive properties is made clearer by contrasting them with intensive properties, which are independent of the amount of substance. An intensive property is determined by the substance’s inherent nature, remaining constant whether the sample is large or small. Examples of intensive properties include temperature, color, and melting point.
Density, defined as mass divided by volume, is a significant intensive property because it is the ratio of two extensive properties. A drop of water and a swimming pool of water both have the same density, illustrating that density is fixed for a pure substance regardless of its quantity.