Magnesium (Mg) and Aluminum (Al) are two of the most commonly used lightweight metals in modern manufacturing. Both metals share a silvery-white color and are prized for their low mass, making them common in the aerospace, automotive, and consumer electronics industries. Since visual inspection is insufficient for proper identification, practical testing methods are necessary. These methods range from simple physical comparisons to more definitive chemical and thermal reactions that exploit the distinct properties of each metal.
Utilizing Density and Weight for Identification
Comparing densities is the primary method for differentiating Magnesium from Aluminum. Magnesium is the lightest structural metal, possessing a density of approximately 1.74 grams per cubic centimeter (g/cm³). Aluminum is significantly denser, with a density of about 2.70 g/cm³, meaning it is roughly 35% heavier for the same volume.
If two pieces of metal are identical in shape and size, the heavier piece is Aluminum. For irregularly shaped objects, density calculation requires determining the object’s mass using a scale. Volume is found through the water displacement method by submerging the object and recording the volume increase. Dividing the mass by the calculated volume provides the density, allowing for precise differentiation between the 1.74 g/cm³ of Magnesium and the 2.70 g/cm³ of Aluminum.
Non-Destructive Visual and Mechanical Tests
A quick mechanical test provides a strong indication of the metal’s identity. Aluminum alloys exhibit higher stiffness and hardness compared to Magnesium alloys. Aluminum has a modulus of elasticity around 68 to 70 GigaPascals (GPa), while Magnesium is lower, around 45 GPa.
This difference means Magnesium is easier to scratch or dent than Aluminum. A scratch test using a material of known hardness will show that the Magnesium sample yields more easily. Additionally, when tapped, Magnesium’s lower stiffness often results in a duller, less resonant sound compared to the sharper, metallic ring produced by Aluminum. These subtle differences are suggestive and should support a more definitive test like density measurement.
Definitive Chemical and Thermal Reactions
The most conclusive tests exploit Magnesium’s higher chemical reactivity. Magnesium reacts vigorously with mild acids, such as household vinegar. Applying a few drops will cause Magnesium to readily produce visible bubbles of hydrogen gas. Aluminum reacts much more slowly due to its naturally protective oxide layer.
Magnesium also exhibits a distinct thermal reaction compared to Aluminum. Aluminum melts at its melting point of about 660°C but does not easily ignite under normal conditions. Magnesium, however, is highly flammable, especially when in thin strips, shavings, or powder form. If heated, Magnesium will ignite and burn with an intensely bright white light, a reaction that does not occur with Aluminum. Safety Note: Due to the extreme heat and brightness, this test should only be performed in a controlled environment with proper safety equipment.