The physical world is described using a variety of properties that characterize matter and energy within a given system. These characteristics are often categorized based on how they relate to the amount of matter being observed. This classification helps scientists understand which properties are inherent to a substance and which depend on the sample size. The primary distinction divides these characteristics into extensive and intensive properties. A fundamental question arises when classifying one of the most common measurements: is the weight of a system an extensive or intensive property?
Defining Extensive and Intensive Properties
The classification of a physical property depends entirely on whether its value changes with the size or extent of the system. An extensive property is one whose magnitude is directly proportional to the amount of matter present. If two identical systems are combined, the value of the property will double. Examples include mass, volume, and total internal energy.
In contrast, an intensive property is a characteristic whose value does not depend on the quantity or size of the system being measured. This property is inherent to the substance itself, remaining constant regardless of how much material is present. If a large system is divided into two halves, the intensive properties of each half will be identical to the original system. Common examples are temperature, color, and melting point.
Weight as an Extensive Property
Weight is classified as an extensive property because its magnitude is directly dependent on the amount of matter in the system. Weight measures the force of gravity acting on an object’s mass, expressed by the formula \(W = mg\). Since mass (\(m\)) is an inherently extensive property, weight (\(W\)) must also be extensive because it is directly proportional to mass.
Consider a block of iron with a certain mass and corresponding weight. If that block is cut exactly in half, the mass of each resulting piece is half the original mass. Because the acceleration due to gravity (\(g\)) remains the same, the weight of each half is also reduced by half. This confirms that weight changes with the extent of the system, fitting the definition of an extensive property.
Comparing Weight to Other System Characteristics
The extensive nature of weight is clarified by contrasting it with related intensive properties. Density is calculated as the ratio of two extensive properties, mass and volume, but the resulting value is intensive. A small sample of pure water has the same density (about 1.0 g/cm\(^3\)) as a very large sample, even though the weight of the large sample is greater.
Temperature is another property that remains constant regardless of the system’s size. If a large container of water is at 25°C, a small cup taken from it will have the same temperature, despite the difference in total weight. Intensive properties, such as temperature and density, are useful for identifying a substance, while extensive properties convey information about the total amount present.