Water density is the mass of water per unit of volume. It is important to note that water does not have a fixed density because water density is dependent on the water temperature.
Water is unusual in the way that it has a negative thermal expansion. While other substances in its solid form will be heavier than the liquid form, water is the exact opposite. One can see an example of this in ice, water’s solid form. Ice floes and ice cubes float in water rather than sink, and this is because liquid water is more dense than ice water.
Water achieves a maximum density of 1 gram per cubic centimeter. This means that every cubic centimeter of water will weigh 1 gram at most in any given circumstance. Pure water will achieve this water density only at 4 degrees Celsius, any lower and it will expand and have a lower water density. When water reaches 0 degrees Celsius it then turns into its solid state which is ice.
It’s also important to note that the composition of water will affect its maximum density, as contaminants in the water will have its own specific density as well. Examples are fresh water, salt water, mineral water etc. Salt water or sea water has a higher density than pure water, because of the salt mass which makes it denser but does not make a significant change on volume.