At 39°F (or 3.98°C to be exact) water is the most dense. This is because the molecules are closest together at this temperature. The closer the molecules, the heavier. The farther apart the molecules, the lighter.

roughly 1 gram per milliliter

1 gram per cubic centimeter

As the temperature of warm water decreases, the water molecules slow down and the density increases. At 4 °C, the clusters start forming. The molecules are still slowing down and coming closer together, but the formation of clusters makes the molecules be further apart. Thus, the density of water is a maximum at 4 °C.

water vapor

In general, when considering the states of matter, solids are more dense than liquids and liquids are more dense than gases. Water is a bit of a contrarian in this regard. When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape.

Cold water has a higher density than warm water. Water gets colder with depth because cold, salty ocean water sinks to the bottom of the ocean basins below the less dense warmer water near the surface.

You might also say: “Water is heavier than air.” True, a glass of liquid water weighs more than a glass filled only with air. But, humidity is water vapor, not liquid water, and water vapor molecules are lighter than the molecules of nitrogen and oxygen that make up approximately 99% of the atmosphere.

Since the chemical properties of the heavier hydrogen-nucleus-with-a-neutron are slightly different, heavy water starts to gum up all manner of body parts. Eventually, if you drank enough purified heavy water—more than 20 gallons, at least a quarter heavy—you’d die.

Water is heavier the O2 and H2 gases because the H20 molecule’s particular electron configuration cause it to be a liquid between 0 C and 100 C at 1 atmosphere of pressure. Liquid water is much denser that gaseous oxygen and oxygen with a density of about 1000 Kg per cubic meter.

Mercury