The Wonderful World of Water

Dec 21, 2020 - 4 min read

Around 3% of the Earth’s atmosphere is water vapour – a molecular gas comprising one oxygen atom bonded to two hydrogen atoms at an angle of 104.5°. Water vapour accounts for approximately 0.001% of the total amount of water on Earth.

Water is an amazing material. It makes up at least half of every living organism; it’s a very good solvent (nicknamed “the universal solvent”); and for many species, it’s the medium from which they absorb life-giving oxygen.

Solid water

One of water’s unusual features is that, as it freezes, its density decreases, effecting an increase in volume. In the case of most substances, density increases and volume decreases in the solid phase. So, why is water different?

As liquid water becomes cooler, it contains less thermal energy, so its molecules are less active. At around 4°C, water reaches a point where the molecules aren’t moving around enough to be pulled away from bonding in the way they prefer: hydrogen atom to oxygen atom. The result is that each hydrogen atom is between the oxygen atom in its own molecule and an oxygen atom of another molecule, creating an open, crystalline structure.

Due to the spaces in the molecular structure created by this particular bonding pattern, solid water has a lower density than liquid water.

Liquid water

It’s not until water warms up to that magic 4°C that the intermolecular bonds between hydrogen and oxygen atoms can be broken. Thermal energy makes the liquid water more dynamic, and the H2O molecules start to move around, repeatedly forming and breaking bonds. In the case of most materials, this would mean decreased density and increased volume; for water, it’s the opposite.

The problem with ‘steam’

‘Steam’ usually refers to ‘aerosol’ (abbreviation of aero + solution), otherwise known as ‘wet steam’. Aerosol describes liquid water droplets suspended within a gas – in most cases, air. Mist and cloud are examples of aerosols, and so is the hot mist that hangs over a pan of boiling water or the spout of a kettle.

Most of us would probably agree that we can see steam; that ‘steam’ is the same as ‘aerosol’.

The trouble is, the word ‘steam’ has wriggled its way into another definition. ‘Steam’ is often used to describe the gas phase of water, and this is where meaning and understanding start to break down. Most of us have a basic understanding of the science behind phase changes of water; but problems arise in communication.

The culprit is ‘steam’.

Maybe we should refrain from telling children that when water boils, it becomes steam. Yes, the liquid water does, eventually, become steam (aerosol). But only after it has turned into a gas at a temperature of 100°C and one atmosphere of pressure.

Rolling these two concepts into one word is misleading. (A bit like the word ‘love’, which is one word for many different concepts.)

Water vapour

The gas phase of H2O is ‘water vapour’.

Through evaporation, H2O molecules from the surface of liquid water evaporate into the atmosphere as they absorb thermal energy and gain dynamism.

Liquid water also changes to water vapour through boiling – when it reaches 100°C at one atmosphere (atm) of pressure (one atm is Earth’s approximate atmospheric pressure at sea-level) or a few degrees lower at a lower pressure.

When water vapour cools and condenses into aerosol, this thermal energy is released.

The Earth’s temperature control system is all about water. Very much like our own!

Deposition and Sublimation

Usually, a phase transition takes place from solid to liquid and vice versa, and from liquid to gas and vice versa. Sometimes, however, a transition is made from gas to solid or from solid to gas, without passing through the liquid phase. Gas to solid is ‘deposition’, and solid to gas is ‘sublimation’.

Frost, that thin, dainty layer of ice on the ground, is formed when water vapour comes into contact with a solid that’s at a sub-zero temperature. A phase transition occurs as the water vapour turns to ice – a transition from gas to solid, without going through the liquid phase. This is an example of deposition.

Instant coffee: gotta love it! Thin layers of frozen coffee liquor are heated at a very low pressure, and the solid water becomes vapour without passing through the liquid phase. Freeze-dried coffee is an example of sublimation.


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