Water is necessary for life to exist.
It is a very simple chemical compound that most people are familiar with: H2O. A water molecule is made up of two Hydrogen atoms (chemical symbol “H”) and one Oxygen atom (chemical symbol “O”).
Water molecules have relatively unique characteristics that allow the formation of it’s various states. The structure of a water molecule is bent due to the aspects of the hydrogen and oxygen molecules.
The hydrogen molecules each have a slightly positive charge and the oxygen molecule has a slightly negative charge. For these reasons, water is said to be a polar molecule. It is this polarity that attracts water molecules to each other to form liquid water.
Basically, the partial positive charge of the water molecule’s hydrogen atom is attracted to another water molecule’s partially negative oxygen atom, and a bond is formed.
Each water molecule can participate in up to four Hydrogen Bonds by donating two hydrogen atoms, and accepting two bonds using the lone pairs on the oxygen atom.
Hydrogen bonds are weaker than ionic or covalent bonds and are responsible for many of the unique properties of water, such as its ability to break apart and reform easily.
Water’s hydrogen bonds are what ultimately determines its viscosity (thickness and consistency). Ambient temperature has a significant effect on the formation and strength of hydrogen bonds, so much so, for example, a 20°C / 68°F decrease in temperature doubles water’s viscosity. This is one of the reasons why swimming organisms are required to expend more energy to move through cold water compared to warmer water.
Like most liquids, water has what is referred to as “Surface Tension” which is often explained as a very thin film on the surface of the water.
Water has a relatively high surface tension due to its polar properties. In fact, all things being equal, water has the highest surface tension of all the common non-ionic and non-metallic liquids.
Water’s surface tension is also directly affected by temperature, the lower the temperature, the higher the surface tension. Surface tension is why it is possible for bacteria, protozoa, fish eggs and even some jellyfish stay within the “film” on the surface, which form the beginning of the planet’s massive food chain.
Density is defined as “the mass per unit volume of an element” and should not be confused with viscosity, which is defined as “the degree of friction that exists within a fluid.”
Water density is important because it plays a role in the physiological characteristics of sea life.
For example, large marine mammals, also known as cetaceans, have relatively light bone structures in comparison to earthbound mammals. Underwater plants do not require the same hard trunks, or root systems that their landlocked counterparts do.
Like water viscosity, water density is affected by temperature changes.
Maximum water density is reached at approximately 4°C / 39°F. Approximately one quart (1.06 quart) of distilled water at that temperature weighs exactly 1 liter or 2.2 lbs weighs exactly 1kg.
Below 4°C / 39°F, water molecules start to form crystals, and actually move further apart from each other, causing an increase in volume. This is why ice floats on water; it is less dense than liquid water.




Advanced Open Water Diver

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