Density describes a fundamental physical property of matter, indicating how much “stuff” is condensed into a given space. It measures the compactness of a substance, helping distinguish one material from another. Density remains constant for a pure substance under defined conditions, regardless of the sample size.
What is Density?
Density is a derived property that quantifies the relationship between a substance’s mass and the volume it occupies. Mass is the amount of matter an object contains, and volume is the three-dimensional space it fills. Density is determined by dividing mass by volume.
This relationship explains why two objects of the same size can feel significantly different when lifted. For instance, a small block of lead and a small block of wood, identical in volume, contain vastly different amounts of matter. The lead block has greater density because its atoms are packed more closely together, resulting in a larger mass within the same space.
How Density Differences Work
To be “less dense” means a substance contains less mass packed into the same volume compared to another substance. Alternatively, a less dense object takes up a larger volume while containing the same mass as a more dense one. This property is always comparative; a material is only less dense relative to something else.
Consider a piece of foam and a rock of equal size. The foam is less dense because its structure incorporates empty space, meaning it contains less actual material than the solid rock occupying the same volume. Wood is less dense than water, which is why it floats, but water is far less dense than iron. Air is a low-density material, but a gas like helium is even less dense than the surrounding air.
Why Things Float and Sink
Density differences determine why some objects float while others sink, a phenomenon governed by buoyancy. Buoyancy is the upward force exerted by a fluid that opposes an object’s weight. An object placed in a fluid will float if its average density is less than the density of the fluid it is submerged in.
If the object’s average density is greater than the surrounding fluid, the downward force of its mass outweighs the upward buoyant force, causing it to sink. The upward force is equal to the weight of the fluid the object displaces. This is why a massive ocean liner floats; its hollow steel hull, filled mostly with air, gives the entire ship an average density much less than the surrounding seawater.
Ice floating in liquid water is an unusual example for a solid in its own liquid. Solid ice is less dense than liquid water because the water molecules form a rigid, open crystalline structure when frozen, which takes up more space. Similarly, oil floats on water because its molecular structure results in a lower density. This principle of stratification means that in any mixture of non-mixing fluids, the least dense material will always rise to the top.
How Temperature and Pressure Affect Density
The density of a substance can be altered by changes in external conditions like temperature and pressure. For most materials, increasing the temperature causes the substance to expand as its molecules move faster and push farther apart. This expansion increases the volume while the mass remains constant, resulting in a decrease in density.
This effect is the underlying principle for the operation of a hot air balloon, where heating the air inside the envelope makes it less dense than the cooler air outside, causing the balloon to rise. Conversely, decreasing the temperature causes the volume to contract and the density to increase. Pressure primarily affects the density of gases, which are highly compressible. Increasing the pressure on a gas forces the molecules closer together, decreasing the volume and significantly increasing the density.