The physical properties of water
Density, viscosity, surface tension, specific heat, conductivity — the numbers, what they mean, and why so many of them are anomalous.
Water’s physical properties are the reference points against which other liquids are measured, largely because they were measured first. Several of them are also genuinely unusual, and the anomalies trace back to hydrogen bonding.
| Property | Value | Note |
|---|---|---|
| Density | 997.0 kg/m³ | Maximum is 999.97 kg/m³ at 3.98 °C, not at freezing |
| Density of ice | 917 kg/m³ | About 9% less dense than liquid water — so ice floats |
| Specific heat capacity | 4,184 J/kg/K | Among the highest of any common liquid |
| Latent heat of vaporisation | 2,257 kJ/kg | Exceptionally high; the basis of evaporative cooling |
| Latent heat of fusion | 334 kJ/kg | |
| Dynamic viscosity | 0.890 mPa·s | Falls sharply with temperature — about half at 50 °C |
| Surface tension | 71.99 mN/m | Highest of any common liquid except mercury |
| Thermal conductivity | 0.606 W/m/K | High for a liquid |
| Relative permittivity | 78.4 | Very high; this is what makes water dissolve ionic solids |
| Speed of sound | 1,497 m/s | About 4.4 times its speed in air |
| Refractive index | 1.333 | |
| Electrical resistivity, ultrapure | 18.2 MΩ·cm | The specification for semiconductor process water |
Table sources
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain
- International Association for the Properties of Water and Steam — IAPWS formulations for the properties of water. Public but restricted · Publisher terms — public access, reuse not clearly granted
Specific heat: why coasts have mild winters
Water takes 4,184 joules to raise one kilogram by one degree — roughly four times as much as the same mass of air, and about five times as much as sand. It therefore warms slowly and cools slowly.
At planetary scale this makes the ocean an enormous thermal flywheel. It is why coastal climates have milder winters and cooler summers than inland ones at the same latitude, why the ocean absorbs the great majority of the excess heat trapped by greenhouse gases, and why a hot water bottle stays warm all night.
Surface tension: why insects walk on water
Molecules in the bulk of a liquid are pulled equally in every direction. Molecules at the surface are pulled inward and sideways but not outward, so the surface behaves like a stretched elastic membrane.
Water’s surface tension of about 72 mN/m is far higher than most liquids — ethanol is around 22. It is why droplets are spherical, why water beads on a waxed surface, and why pond skaters can stand on it. Soap is a surfactant: it disrupts the hydrogen bonding at the surface and drops surface tension by around two thirds, which is what lets water wet a greasy surface instead of beading off it.
Viscosity and why it matters for treatment
Water’s viscosity falls by roughly half between 0 °C and 25 °C. That has a direct operational consequence: cold water is thicker, so particles settle more slowly and filters run at lower rates. Winter is measurably harder for a water treatment works than summer, entirely because of viscosity.
Sources
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain
- International Association for the Properties of Water and Steam — IAPWS formulations for the properties of water. Public but restricted · Publisher terms — public access, reuse not clearly granted