The density of aluminum is approximately 2.7 grams per cubic centimeter (g/cm^3) at room temperature. The density of water is approximately 1 g/cm^3 at room temperature. Since the density of aluminum is greater than the density of water, aluminum will sink in water.

To understand why density is important in determining whether an object will sink or float, consider the following: when an object is placed in a fluid (such as water), it displaces an amount of fluid equal to its own volume. The buoyant force acting on the object is equal to the weight of the fluid displaced by the object. If the buoyant force is greater than the weight of the object, the object will float. If the buoyant force is less than the weight of the object, the object will sink.

In the case of aluminum, the buoyant force is less than the weight of the aluminum, so aluminum sinks in water.