Problem 27 - Chemistry The Science in Context

Question

Can an extensive property be used to identify a substance? Explain why or why not.

Step-by-Step Solution

Answer

Explain your reasoning. Answer: No, extensive properties cannot be used to uniquely identify a substance. This is because extensive properties depend on the amount of the substance present, and therefore change as the amount of material changes. To identify a substance, it is essential to rely on intensive properties, which are characteristics of the material itself and do not depend on its size or quantity.

Step 1: Definition of Extensive Properties

An extensive property is a physical property of a substance that depends on the amount of the substance present. Examples of extensive properties include mass, volume, and heat capacity.

Step 2: Intensive Properties

Intensive properties, on the other hand, are properties that do not depend on the amount of substance present. Examples of intensive properties include density, melting point, boiling point, and specific heat capacity.

Step 3: Identifying Substances Using Properties

When identifying a substance, it is important to rely on intensive properties, as they are characteristics of the material itself, independent of its size or quantity. In general, an intensive property can be considered a "fingerprint" of a substance, as it can be used to uniquely identify it.

Step 4: Extensive Properties for Identification

Extensive properties, by definition, depend on the amount of substance being considered. Therefore, they are not suitable for identifying substances by themselves because they change as the amount of material changes. For example, the mass of one mole of a substance will be different from the mass of two moles of the same substance.

Step 5: Conclusion

In summary, extensive properties cannot be used to identify a substance, as they depend on the amount of the substance present. Intensive properties, which do not depend on the size or quantity of the substance, should be used to uniquely identify a substance.

Key Concepts

Substance Identification

Identifying a substance accurately is a crucial task in chemistry and material science. Knowing what a substance is comprised of allows scientists to predict how it will behave and how it can be used. The identification of substances relies on specific properties that are inherent to the substance itself.

• Intensive properties: Characterize the essence of the substance and remain constant regardless of the amount of material.
• Extensive properties: Change with the quantity of material and hence, cannot serve solely for identification purposes.

The key is to focus on properties that will remain unchanged whether you have a pinch or a mountain of the substance. This is why intensive properties, such as density and melting point, are used to identify substances.

Intensive Properties

Intensive properties are a fascinating aspect of material characteristics. These are the properties that do not alter with the size of the sample. Think of them as the 'personality traits' of a substance.

• Properties like density, which is mass per unit volume, remain constant as they describe the nature of the material itself.
• Other examples include melting point and boiling point, which remain the same for a substance regardless of the quantity you have.
• Each substance has its own unique set of intensive properties, often thought of as its chemical fingerprint.

These properties make it feasible to identify what a substance is without needing to worry about how much of it you are examining. Therefore, when you want to determine what a substance is, you should look at its intensive properties.

Extensive Properties

Extensive properties are dependent on the amount of material present. Understanding these properties helps in calculating quantities but is less useful for identification.

• Mass: This is an extensive property, as it increases with the amount of substance.
• Volume: Much like mass, it depends on the quantity of material, so it's another extensive property.
• Because of their dependency on amount, extensive properties cannot provide the unique "signature" needed to recognize a substance.

Though extensive properties are essential for understanding the scale and capacity of a substance, they do not aid in identifying what the substance itself is. They tell you how much is present, not what it is.