Density is a measurement describing how much mass is contained within a specific volume of a substance. It is a fundamental physical property calculated by dividing mass by volume. For pure water, this ratio is consistently close to \(1.0\) gram per milliliter (\(\text{g/mL}\)), which serves as a widely adopted reference point in various scientific fields. This convenient value is linked to the historical definition of metric units of mass and volume, which were originally based on the properties of water.
The Standard Density Value
The standard density of water is typically expressed in the metric units of grams (g) for mass and milliliters (mL) for volume. This \(1:1\) relationship means that one gram of pure water occupies precisely one milliliter of space, simplifying calculations in chemistry and physics.
A milliliter is equivalent to a cubic centimeter (\(\text{cm}^3\)), meaning the density can also be stated as \(1.0 \text{ g/cm}^3\). In the International System of Units (SI), the equivalent value is \(1000\) kilograms per cubic meter (\(\text{kg/m}^3\)). This standardized value is relevant for calculating a substance’s specific gravity, which compares its density to that of water.
How Temperature and Pressure Affect Density
The precise density of \(1.0 \text{ g/mL}\) holds true only under specific physical conditions. Temperature significantly influences the packing of water molecules, causing the density of liquid water to fluctuate. Pure water reaches its maximum density at approximately \(3.98\) degrees Celsius, often rounded to \(4^\circ\text{C}\).
As the temperature moves above or below \(4^\circ\text{C}\), the density of the liquid water decreases. For example, at room temperature (\(25^\circ\text{C}\)), the density is slightly lower at about \(0.997 \text{ g/mL}\).
Pressure also influences water’s density, but only marginally because liquid water is highly incompressible. Although density increases as pressure rises, a substantial change in pressure is required to see a noticeable effect. For instance, the pressure at the bottom of the deepest ocean trenches only increases water’s density by a few percent.
Water’s Unique Density Structure
Water exhibits an unusual physical property compared to most other substances: its solid form, ice, is less dense than its liquid form. While most materials contract and become denser when they solidify, water expands when it freezes. The density of ice at \(0^\circ\text{C}\) is approximately \(0.9167 \text{ g/mL}\), which is why ice cubes float.
This anomaly is caused by the structure of the water molecule and the presence of hydrogen bonds. As liquid water cools toward its freezing point, hydrogen bonds force the molecules into a rigid, open hexagonal lattice structure. This arrangement maximizes the distance between molecules, creating more empty space within the volume.
The resulting decrease in density upon freezing has profound implications for aquatic ecosystems. Ice forms an insulating layer on the surface of lakes and rivers. This surface layer prevents the deeper water from freezing, allowing aquatic life to survive winter conditions beneath the ice.