Density is the mass per unit volume of a substance and is simply the ratio of mass to volume. Unlike mass, density is a characteristic property of a material, meaning a property that has the same value for any size sample of a given substance. For example, a small piece of gold has much less mass than a large piece of gold but both pieces have the same density.
In the metric system, the standard unit for density is kg per cubic meter. The imperial system of units uses the weight (or force) unit, pounds, instead of the unfamiliar mass unit, slugs. The weight per unit volume of a substance is called the specific weight and has standard imperial units of lb per cubic foot. Commercial and industrial publications in the U.S. routinely refer to the specific weight of a material as its density.
Density, Mass and Volume
Density tells you how much matter is present in a unit volume of space. As density increases, the amount of matter contained in a unit volume also increases.
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Gold has a much greater density than aluminum. A 1-inch cube of gold weighs 11.2 oz., while a 1-inch cube of aluminum only weighs 1.6 oz. In terms of mass, there is much less aluminum in a 1-inch cube than there is gold.
If you have 5 oz. of gold and 5 oz. of aluminum in the form of a cube, the gold cube has sides of length 0.8 inches, while the aluminum cube has sides of 1.5 inches. In terms of mass, aluminum takes up more space than the same amount of gold.
Science teachers often include questions on their tests to make sure their students know how to use the term "density" properly. For example, is the statement that gold is heavier than aluminum true or false? This is false, since a big enough piece of aluminum can be heavier than a small piece of gold. The statement would be true if it also stated that the gold and aluminum pieces are the same size. A true statement is that gold is denser (not heavier) than aluminum.