Extensive Property

An extensive property is a property whose value depends on the size or amount of the material being described. Double the material, and the value of an extensive property doubles with it.

In Merrill's framing, an extensive property is what you get when you take an observable feature whose value is a quantity that refers to a region of space. The value is tied to size. A region of space is a definite area or volume of the three-dimensional continuum, and an extensive property's value tracks how much of that region the material occupies. Mass and volume are the obvious cases. The mass of a glass of water is not the mass of water; it is the mass of this water, here, in this glass. Pour half of it out, and the mass is now half. The property has changed because the amount has changed.

This is the move that separates an extensive property from an intensive one. An extensive value answers the question "how much?" An intensive value answers a different question, one that does not depend on amount. Density of water at room temperature is the same in a thimble or in a swimming pool. Mass of water is not. The two questions are different, and the properties that answer them belong to different families.

Notice that extensive properties are exactly the kind that conservation talks about. When we say no material disappears, we are saying that the total mass before equals the total mass after. We can only say that because mass is extensive, because it adds up across parts. Without extensive properties there would be nothing to conserve, nothing to total, nothing to balance. The arithmetic of chemistry rides on properties that scale with size.