Question

A post-1982 penny is made with zinc, but its density is greater than that of zinc. Why?

Short answer

Penny's copper coating, which is denser than zinc, increases its overall density.

Step-by-step solution

Understanding Penny Composition

A post-1982 penny is primarily made of zinc with a thin outer layer of copper. The core of the penny is zinc, while the outer coating is copper.

Comparing Densities of Materials

The density of zinc is approximately 7.14 g/cm³, whereas the density of copper is about 8.96 g/cm³. Copper has a higher density than zinc.

Calculating Overall Density

Despite the core being zinc, the copper coating increases the overall density of the penny. The presence of copper, with its higher density, results in the penny having a density greater than that of zinc alone.

Key concepts

Penny Composition

A significant change in the composition of pennies occurred in 1982. This change was mainly due to the rising cost of copper. Prior to 1982, pennies were made almost entirely of copper. However, post-1982 pennies consist predominantly of a different metal. These newer pennies feature a core made of zinc, which is much cheaper and more abundant than copper.
However, to maintain the classic appearance and color of pennies, these zinc cores are coated with a thin layer of copper. This copper coating preserves the familiar look of pre-1982 pennies.
In summary, a post-1982 penny is primarily zinc, with copper accounting for just the surface layer.

Zinc

Zinc is a bluish-white metal that is not only more affordable but also lighter compared to copper. Its density is lower, measured at approximately 7.14 grams per cubic centimeter (g/cm³).
Zinc has become a favored choice for coinage due to its abundance and lower costs of production. It provides enough heft to the pennies, although not as much as copper would.
It's important to note that while zinc is used widely for such applications, it does need additional layers when appearance and corrosion resistance, like in pennies, are crucial. Hence, the necessity for a copper coating.

Copper

Copper, traditionally used in coin manufacture, is highly regarded for its characteristic reddish-brown color. Its role in pennies is primarily stylistic and protective in nature.
Its density is significantly higher than zinc, at approximately 8.96 g/cm³. Thanks to this property, even a tiny amount of copper can have a substantial influence on the overall density of an object.
Due to copper's higher density, the copper coating on post-1982 pennies increases their overall density, despite being just a surface layer on a core of lighter zinc.

Material Density

Density is a property that describes how much mass is contained within a given volume. It can be thought of as how 'packed' or 'dense' a material's atoms are.
The formula to calculate density is given by:\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
When different materials are combined, as in the case of modern pennies, the density of the resulting object is affected by the densities of the constituent materials and their proportions.
Thus, when a penny combines zinc (low density) and copper (high density), the overall density will be a weighted average based on the volume of each material. This is why a modern penny ends up having a density greater than that of zinc alone.