Matter: Extensive versus Intensive Properties

Extensive properties are characteristics of matter that depend on the size of the sample rather than its identity. Mass and volume, are examples of extensive properties. When we weigh two different samples of the same substance, we get two different mass measurements; the larger mass is associated with the bigger sample. Extensive properties are useful when we try to determine the composition of a mixture or the efficiency of a process, such as a titration experiment or finding the per cent yield.

If we measure the mass and volume of an object then divide the mass by the volume, the result is the object's density, which gives us an indication of how tightly packed the particles that make up that object are. Density is an intensive property of matter. Intensive properties are the characteristics of a sample of matter that do not depend on its size. When you measure the density of aluminum, you will find it is 2.70 grams per cubic centimeter, no matter what sized piece of aluminum you use. Intensive properties are most useful when trying to identify a substance. Density, color, and boiling point are examples of intensive properties that can be used to identify unknown chemical compounds.

Note that physical properties of matter can be either extensive (like mass) or intensive (like density). That is because physical/chemical and extensive/intensive are two different classification systems.

Concept Check: The purpose of an experiment is to make aspirin. Near the end of the procedure, you are instructed to take the melting point of your product. What type of property is melting point, and what will it tell you?

Answer: Melting point is an intensive property; a snowflake and an iceberg both melt at 0 C. The melting point of aspirin is 284 C. The melting point of salicylic acid, the starting material used to make aspirin, is 157 C. Though the two compounds look the same, you can tell which is which by the melting point. You can also tell if the product you made is pure or if it is a mixture. Recall that the physical properties of a mixture are a blend of the properties of its components. The melting point of a sample that has impurities will be somewhat lower that the melting point of a pure sample; the greater the amount of impurities, the farther the melting point will be. If your sample has a melting point of 275 C, it must be aspirin because it can't be salicylic acid. Unfortunately, it isn't very pure, either.

This site was last updated 08/01/06