Density is a fundamental physical property of matter that helps us understand how much “stuff” is packed into a given space. It is an intrinsic property, meaning a substance’s density remains constant regardless of its amount.
What Density Means
Density quantifies the relationship between an object’s mass and the space it occupies. Mass is the amount of matter an object contains, measured in grams (g) or kilograms (kg). Volume is the three-dimensional space an object occupies. Density is expressed by the formula: Density = Mass / Volume.
Standard units for density include grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL) for liquids and solids, and kilograms per cubic meter (kg/m³) in the International System of Units (SI). For instance, water has a density of approximately 1 g/cm³. This means that one cubic centimeter of water has a mass of one gram.
How Density is Measured
Determining the density of an object involves measuring both its mass and its volume. Mass is typically measured using a balance or a scale. Placing the object on the balance provides its mass in grams or kilograms.
Measuring volume depends on the object’s shape. For regularly shaped objects, such as a cube or a rectangular prism, volume can be calculated by multiplying its length, width, and height. For example, a block measuring 5 cm x 2 cm x 3 cm would have a volume of 30 cm³.
For irregularly shaped objects, the water displacement method is used. This involves submerging the object in a known volume of water in a graduated cylinder; the rise in the water level indicates the object’s volume. If an object has a mass of 100 g and displaces 50 mL of water, its volume is 50 cm³. Dividing the mass (100 g) by the volume (50 cm³) yields a density of 2 g/cm³.
Density All Around Us
Density plays a role in many everyday phenomena, explaining why certain objects float or sink. An object floats if it is less dense than the fluid it is in, while it sinks if it is more dense. For example, wood floats in water because its density is less than water’s 1 g/cm³, whereas a metal coin sinks because its density is greater. This principle applies to liquids as well, which is why oil floats on water, as oil is generally less dense than water.
Hot air balloons offer another example of density at work. Heating the air inside the balloon causes the air molecules to spread out, making the hot air less dense than the cooler air outside the balloon. According to Archimedes’ principle, this difference in density creates an upward buoyant force, allowing the balloon to rise.
Comparing materials like lead and aluminum highlights density differences. Lead, with a density of about 11.3 g/cm³, is much denser than aluminum, which has a density of approximately 2.7 g/cm³. This means a small piece of lead will be significantly heavier than a much larger piece of aluminum if they have the same volume, or conversely, a small volume of lead will weigh as much as a larger volume of aluminum.