Density is a fundamental physical property used for identifying unknown materials. It is defined as the ratio of a substance’s mass to the volume it occupies, frequently expressed in units like grams per cubic centimeter \((\text{g}/\text{cm}^3)\) or grams per milliliter \((\text{g}/\text{mL})\). Measuring this ratio determines how tightly packed the matter is within a given space. Because this relationship is constant for a pure substance under specified conditions, it provides a unique numerical signature that confirms a material’s identity.
Density as a Unique Identifier
Density’s ability to identify a substance stems from its nature as an intensive property of matter. Intensive properties do not change regardless of the amount of substance present. For instance, a small drop of pure water has the same density as a swimming pool full of water, assuming the same temperature and pressure.
This characteristic contrasts with extensive properties, such as mass and volume, which directly depend on the quantity of matter. If you double the amount of a substance, both its mass and its volume will double. However, since density is calculated by dividing mass by volume, the ratio remains unchanged, making it a reliable characteristic property for identification. Every pure substance has a distinct density value, allowing scientists to distinguish one material from another.
Practical Steps for Measurement
Determining the density of an unknown substance requires accurately measuring both its mass and its volume. Mass, a measure of the amount of matter, is typically found using a precise instrument like an electronic balance or a scale. This straightforward measurement provides the numerator for the density calculation.
Measuring the volume depends on the material’s shape. For a liquid, volume is measured directly using calibrated laboratory glass, such as a graduated cylinder. For solids with regular shapes, like a cube or cylinder, the volume is calculated using standard geometric formulas based on measurements of length, width, and height.
For irregularly shaped solids, the water displacement method, based on Archimedes’ principle, is necessary. This technique involves submerging the solid into a known initial volume of liquid, typically water, in a graduated cylinder. The object displaces a volume of water equal to its own volume, causing the water level to rise. The difference between the final and initial water levels provides the accurate volume of the solid.
Identifying the Substance
Once the mass and volume are precisely measured, the experimental density is calculated using the formula: density equals mass divided by volume \((D=M/V)\). This calculated value, expressed with the correct units, represents the density of the unknown sample under the measured conditions.
The identification process involves comparing this calculated experimental density to established reference tables of known density values for pure substances. If the experimental value is very close to the standard density of a known substance, the material’s identity is confirmed.
Density is sensitive to temperature because most substances expand when heated, which increases volume and decreases density. For an accurate comparison, the temperature at which the measurement was taken must be recorded and matched to the temperature specified in the reference tables. The presence of impurities can also cause the measured density to deviate slightly from the standard value. Despite these variations, a strong match between the experimental density and a reference value confirms the material’s identity.