What Is the Density of Water?

The density of water depends on temperature and purity.
The density of water depends on temperature and purity. (copyright: Anne Helmenstine)

The density of water is the mass of water per unit of volume. For all practical purposes, this is the same as the weight of water per unit of volume. The density of water is about 1 gram per milliliter (g/ml), 1 gram per cubic centimeter (g/cm3), 1000 kg/m3, or 62 pounds per cubic foot (lb/ft3). The exact value is actually slightly lower and depends on temperature. The maximum density of water is 0.9998395 g/ml at 4.0° C (39.2° F).

Effect of Temperature on the Density of Pure Water

Sometimes rounding the value for the density of water to 1 g/ml isn’t good enough. Fortunately, there are tables of density values for different temperatures:

Density of pure water at different temperatures.

Other Factors That Affect Water Density

In addition to temperature, other factors affect the density of water. Pressure affects density, but because water isn’t very compressible, it isn’t usually a significant factor. Density also depends on how much dissolved material is in the water. Dissolved gases might make water less dense. Dissolved salt, minerals, and other chemicals can make water more dense. For example, sea water is more dense than pure water.

How to Find Density of a Liquid

The easiest way to measure the density of a liquid, including water, is to use a hydrometer. A typical hydrometer consists of a weighted bulb with a cylindrical stem. Lines marked on the stem show how far the bulb sinks in the liquid. The lower the bulb sinks, the lower the density; the higher the bulb floats, the higher the density of the liquid. The lines are calibrating by floating the hydrometer in a liquid with known density. Usually the standard is water because it has a specific gravity of 1.000 at about 4°C.

Measuring mass and volume is another way to find density of a liquid.

  1. Weigh a graduated cylinder or beaker.
  2. Pour in liquid and record the volume measurement.
  3. Weigh the glassware with the liquid.
  4. Find the mass of liquid. Take the mass of liquid plus glass and subtract the weight of the glass.
  5. Find the density by taking the mass of liquid and dividing it by the volume. Be sure to record the units of mass and volume.

Ice Is Less Dense Than Water

Usually, the solid form of a compound is heavier or more dense than its liquid. This is not the case with water. Ice is about 9% less dense than water. Ice cubes float in a glass of water and icebergs float in the sea. The reason water is more dense than ice has to do with hydrogen bonding. The attraction between the positive-charged hydrogen atoms of a water molecule to the negative-charged oxygen atoms of neighbor water molecules draws particles of the liquid very close together. The rigid crystalline structure of solid water (ice) holds the molecules slightly further apart.

This property is significant for life. If ice was heavier than water, it would sink to the bottom of rivers and lakes and they would freeze from the bottom up. Because water is an excellent thermal insulator, deep lakes might never thaw in the summer and the ecosystem would be very different.

Does Heavy Water Ice Float or Sink?

Heavy ice sinks in water because it has a higher density.
Heavy ice sinks in water because it has a higher density. (photo: Mike Walker)

In heavy water, the usual hydrogen atoms are replaced with deuterium atoms. Regular hydrogen is the isotope called protium, in which atoms have one proton and one electron. Deuterium is the hydrogen isotope in which atoms have one proton, one neutron, and one electron. The formula for heavy water is written D2O to reflect the difference. Adding a neutron to each hydrogen atom makes deuterium 10.6% more dense than normal water. Ice made from heavy water floats in heavy heavy water, but sinks in normal water.


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