What Is the Density for Water? Exact Value & Key Facts
The density of water is 1 gram per cubic centimeter (1 g/cm³) at 4°C (39.2°F), the temperature where water is most dense. One cubic centimeter of pure water at that temperature has a mass of exactly one gram. For most practical purposes, you can use 1 g/mL (since 1 mL equals 1 cm³) or 1000 kg/m³.
Why Water Density Changes with Temperature and Purity
Water’s density shifts with temperature and dissolved substances. As water warms above 4°C, molecules move faster and spread out, making it less dense. Ice is a clear example: because water expands when it freezes, ice has a density of about 0.92 g/cm³, which is why it floats. The USGS Water Science School explains this is why lakes freeze from the top down, insulating the water below.
Dissolved salts increase density. Seawater has an average density around 1.025 g/cm³ because of salt content. A quick example: 500 mL of pure water at 4°C has a mass of 500 grams. The same volume of seawater at the same temperature weighs about 512.5 grams. The National Institute of Standards and Technology provides reference tables for water density at different temperatures used in scientific measurements.
How to Measure Water Density in Practice
You can measure water density with a hydrometer, a weighted glass tube that floats higher in denser liquids. For laboratory precision, scientists use a pycnometer — a specialized flask with a fixed volume — and weigh it empty, then full of water at a controlled temperature. A common classroom method involves measuring the mass of a known volume of water on a digital scale. Fill a 250 mL graduated cylinder with distilled water at room temperature (about 20°C) and weigh it. You will find roughly 249.5 grams. Dividing mass by volume gives a density of about 0.998 g/mL at 20°C, slightly below the maximum at 4°C.
Key Takeaways
- Pure water reaches its maximum density of 1 g/cm³ at 4°C; density decreases as water warms or cools away from that point.
- Ice is less dense than liquid water (about 0.92 g/cm³), which is why ice floats and bodies of water freeze from the surface downward.
- Dissolved substances like salt increase water’s density — seawater is roughly 2.5% denser than pure fresh water.
