density of water
Summary
The density of water is its mass per unit volume, about 1.00 g/mL or 1.00 × 10³ kg/m³ near room temperature. The most testable point is that ice is less dense than liquid water, because hydrogen bonds hold it in an open lattice, so ice floats.
Detail
Meaning: Density is mass per unit volume. Liquid water near room temperature is about 1.00 g/mL (1.00 g/cm³, or 1.00 × 10³ kg/m³), so 1 mL has a mass near 1 g and 1 L near 1 kg. Water is unusual: its liquid density peaks near 4 °C, and ice (about 0.92 g/cm³) is less dense because hydrogen bonds hold the molecules in an open lattice.
Equation: ρ = m/V, where ρ is density (kg/m³), m is mass (kg), and V is volume (m³).
On the MCAT: Water is the reference fluid. Use its density in pressure-at-depth, buoyant force, and floating-or-sinking questions: an object floats when its density is below the fluid's. Specific gravity is a substance's density divided by water's, so a value above 1 means it sinks in water. Passages may ask why ice floats or how dissolved salt changes density (it raises it).
Common confusion: In SI units water's density is 1000 kg/m³, not 1; the value 1 belongs to g/mL. Ice floats because freezing expands water and lowers its density, not because it is colder.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Physics (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.