Matter possesses characteristics, known as properties, that scientists use to describe and classify it. These properties are fundamental to the study of chemistry and physics, helping to distinguish one material from another. Properties are categorized based on how they relate to the quantity of the substance being observed. This categorization establishes whether a property’s value changes when the amount of material changes.
Defining Extensive Properties
An extensive property is a physical or chemical characteristic whose measured magnitude is directly proportional to the amount of matter in a system. If the size or quantity of the substance increases, the value of the extensive property increases by the same factor. This relationship means that the property is dependent on the extent of the sample being examined.
A defining feature of these properties is their additive nature for subsystems. If a large system is divided into smaller parts, the extensive property of the total system equals the sum of that property in all its individual parts. For instance, if two separate containers of water are combined, the total value is the sum of the initial values. This additivity makes extensive properties useful for quantifying the absolute size or quantity of a substance.
Common Examples
One of the most familiar examples of an extensive property is mass, which is a measure of the amount of matter contained within an object. If a 10-gram sample of a metal is placed next to an identical 10-gram sample, the resulting system has a total mass of 20 grams, clearly demonstrating the additive principle. Similarly, volume is an extensive property because it measures the amount of three-dimensional space that a substance occupies.
Internal energy is another extensive property, particularly in the study of thermodynamics. It represents the total energy contained within a system, including the kinetic energy of its molecules and the potential energy stored in chemical bonds. If the amount of a substance is doubled, the total stored energy will also double. This makes internal energy directly dependent on the sample size.
Comparing Extensive and Intensive Properties
The concept of an extensive property is best understood when contrasted with its counterpart, the intensive property. Intensive properties are those whose values remain constant regardless of the amount of substance present. Properties such as temperature, color, and melting point are intensive because they are inherent to the substance’s identity, not its quantity.
For example, a cup of boiling water and a bathtub full of boiling water will both have a temperature of 100 degrees Celsius. The ratio of two extensive properties results in a new, intensive property. Density illustrates this principle, as it is calculated by dividing mass (extensive) by volume (extensive). This yields a value that is constant for a pure substance, making density a characteristic value used to identify the substance itself.