Table of Contents
Water Density Properties
Water has the unusual property of reaching maximum density at approximately 3.98 degrees Celsius, not at its freezing point. Below 4C, water actually becomes less dense as it cools further, which is why ice (917 kg/m3) floats on liquid water (1000 kg/m3). This anomalous behavior is crucial for aquatic life survival in winter.
The density of water also varies with pressure and dissolved minerals. Seawater is about 2.5% denser than freshwater due to dissolved salts. At high pressures found in deep ocean trenches, water can be compressed by about 5% at 1000 atm.
Density Formula
This polynomial approximation (IAPWS) gives water density at 1 atm as a function of temperature in degrees Celsius, accurate to better than 0.01 kg/m3.
Water Density vs Temperature
| Temp (°C) | Density (kg/m³) | Temp (°C) | Density (kg/m³) |
|---|---|---|---|
| 0 | 999.84 | 50 | 988.07 |
| 4 | 999.97 | 60 | 983.20 |
| 10 | 999.70 | 70 | 977.78 |
| 20 | 998.21 | 80 | 971.82 |
| 30 | 995.65 | 90 | 965.35 |
| 40 | 992.22 | 100 | 958.40 |
Frequently Asked Questions
Why is water densest at 4C?
Below 4C, hydrogen bonding forces water molecules into an open hexagonal structure (similar to ice), which takes up more volume. Above 4C, thermal expansion reduces density normally. The 4C maximum results from the competition between these two effects.
How does salt affect water density?
Dissolved salt increases density. Seawater (3.5% salt) has a density of about 1025 kg/m3. The Dead Sea (34% salt) has a density of about 1240 kg/m3, making it easy to float.
What is the density of steam?
At 100C and 1 atm, steam has a density of about 0.598 kg/m3, roughly 1/1600th that of liquid water. This enormous density difference drives the power of steam engines and turbines.