Aluminum and silver, when polished, share a bright, metallic sheen that makes casual identification challenging. Despite their similar appearance, these two elements possess vastly different material properties. Silver is a precious metal often used in jewelry, while aluminum is a lightweight, abundant industrial metal. Learning to reliably tell them apart requires moving beyond simple looks and employing specific physical observations. The following methods offer practical, non-destructive ways to confirm the identity of an unknown metallic object.
Differences in Visual Appearance and Surface Reaction
When freshly polished, silver typically displays a brighter, whiter luster that is highly reflective. Aluminum, due to a naturally occurring, thin oxide layer, often presents a slightly duller appearance with a subtle blue-gray tint. This oxide layer forms almost instantly when aluminum is exposed to air and provides protection against further corrosion.
The way each metal reacts over time provides a strong clue regarding its identity. Silver reacts chemically with sulfur compounds in the atmosphere to form silver sulfide. This manifests as the familiar black or dark gray tarnish that requires polishing to remove.
Aluminum’s reaction with oxygen creates a thin film of aluminum oxide immediately. This protective film may appear chalky or slightly gray, contributing to the metal’s duller look. Crucially, this oxide layer does not flake or turn black like the sulfide tarnish.
Using Density and Weight for Identification
The most conclusive method for distinction relies on the significant difference in density between the two elements. Silver is classified as a heavy metal, possessing a density of approximately 10.5 grams per cubic centimeter. In contrast, aluminum is a remarkably light element with a density of only about 2.7 grams per cubic centimeter.
This four-fold difference in density allows for a reliable “feel test” when comparing objects of similar size. A piece of silver will feel substantially heavier than a piece of aluminum that occupies the same volume. It is important to compare items based on their volume, not just their overall shape, to confirm this difference.
For a more definitive confirmation, one can calculate the object’s approximate density using a scale and the water displacement method. By measuring the object’s mass and then finding its volume through submerging it in water, the resulting calculation (mass divided by volume) yields a clear result. The difference between 2.7 and 10.5 grams per cubic centimeter makes this physical property an unmistakable identifier.
Simple Physical Tests for Confirmation
Beyond density, other physical properties provide supplementary evidence for identification. Silver is a relatively soft metal, ranking between 2.5 and 3.0 on the Mohs hardness scale, especially when pure. Aluminum, though often alloyed, generally exhibits greater resistance to deformation. Gently attempting to mark an inconspicuous spot on the object can reveal this difference in material strength.
Another test involves comparing thermal conductivity. While both metals are excellent conductors of heat, silver has the highest thermal conductivity of any element. This means silver transfers thermal energy much more quickly than aluminum does.
A simple way to test this is by placing a small ice cube on the surface of the metallic object. The ice on the silver item will melt noticeably faster because the silver rapidly draws heat away from the ice cube. The aluminum object, while still conductive, will not transfer the heat at the same rate.
Finally, a gentle tap can distinguish high-purity silver from aluminum due to the resulting resonance. Silver, particularly when formed into specific shapes like coins or hollowware, tends to produce a higher-pitched, sustained ringing sound, often described as a “ping.” Aluminum typically yields a duller, shorter-lived thud when struck.