Density is a physical property of matter that can be used to identify an unknown substance. Every pure material possesses a unique density value, defined as its mass per unit volume (D = M/V). Finding the density of a sample involves two distinct measurements and a simple calculation, allowing comparison to reference tables of known substances. However, this method is only successful when the measurement is performed under carefully controlled conditions and the substance is confirmed to be pure.
Density as a Unique Material Property
Density is considered a characteristic property of a substance because it does not depend on the amount of matter present. This quality is known as being an “intensive property,” meaning a small drop of pure water has the exact same density as a large volume of water. In contrast, properties like mass and volume are “extensive” because they change with the size of the sample.
The fixed density value of a pure substance acts like a distinct fingerprint, consistent regardless of the sample’s shape or size. For example, pure gold consistently has a density of approximately 19.3 grams per cubic centimeter, while iron’s density is about 7.87 grams per cubic centimeter. This difference makes density a powerful tool for distinguishing between materials that might otherwise look similar.
The Process of Density Measurement
Identifying a substance using density requires accurately determining two values: mass and volume. The mass of the unknown sample is measured using a balance or scale and is typically recorded in grams. The method used for volume measurement depends entirely on the sample’s physical form.
If the sample is a regularly shaped solid, such as a block, its volume is calculated by measuring its dimensions and applying the appropriate geometric formula. For irregularly shaped solids, the volume is found using the water displacement method. This technique involves submerging the object in a known volume of liquid, usually in a graduated cylinder, and noting the rise in the liquid level. The difference between the initial and final volumes represents the object’s volume.
Once both the mass and volume are known, the density is calculated by dividing the mass by the volume. The calculated density is then compared to a table of known values for pure substances to tentatively identify the unknown material. This process is used across various fields, including geology for identifying minerals and in the oil industry to assess crude oil quality.
Limitations and Necessary Controls
While density is a useful characteristic, its reliability depends on controlling certain environmental factors. Both temperature and pressure affect a substance’s volume, which alters its calculated density. Increasing the temperature causes a substance to expand, increasing its volume and lowering its density.
The effect of temperature is especially pronounced in gases, but it also affects liquids and solids. For this reason, density tables list values at a standard reference temperature, often 20 degrees Celsius. Pressure significantly affects the density of gases, though it has a much smaller impact on liquids and solids. Therefore, accurate density measurements must be taken under specified temperature and pressure conditions to be comparable to reference data.
The most significant constraint is that density is only a reliable identifier for pure substances. A pure substance is composed of a single type of molecule, resulting in a constant density. Conversely, a mixture, such as an alloy or saltwater, has a variable density that depends on the proportions of its components. Because a mixture’s density will fall somewhere between the densities of the substances it contains, a single density measurement cannot definitively identify a mixed or impure sample.