The Magic of Water – Part 2
- kieronconway
- Mar 14
- 7 min read
© 2026 Kieron Conway - All rights reserved.
What is the physics of water?
In part 1 of this article, we looked at how water molecules have six unique properties that make it very special and vital to the evolution of life.
In this article, we're going to look at how water molecules interact with other water molecules to create a dynamic and chaotic structure when in liquid form to a highly organised and static structure when in solid form (ice).
This significant difference is beautifully demonstrated in the above image, where each molecule is represented by an oxygen atom (red) and four hydrogen bonds (white), which provide weak binding to other water molecules.
Finally, we'll summarise how physics results in each of the six magical properties of the water molecule as defined in part 1 of the article.
Electron orbitals in the water molecule
Diagram 1
A very simplistic view of the structure of a water molecule is shown in the LHS of diagram 1, which might be familiar from school science lessons.
Oxygen has a total of 8 electrons, 2 in the inner shell's 1s orbital, 2 in the outer shell's 2s orbital and 4 more electrons in the three 2p polar orbitals. Each single orbital can hold a maximum of 2 electrons.
Atomic orbitals are explained in chapter 4 of A Journey into Modern Physics, if you are not familiar with them.
When oxygen bonds with two hydrogen atoms to form H₂O, the outer orbitals of oxygen (known as the valence orbitals) hybridise to form sp hybrid molecular-orbitals arranged in a tetrahedral geometry.
('Hybridise' in this context, is the mixing of atomic orbitals to form new “hybrid" orbitals that are better suited for pairing electrons in chemical bonds.)
In water, two sp orbitals form covalent bonds with hydrogen. These are depicted in the RHS of diagram 1, where each covalent sp orbital contains one electron from hydrogen and one from oxygen.
The remaining two sp orbitals contain lone pairs of electrons.
Each sp hybrid lobe corresponds to the corners of the geometric shape as shown on the right of diagram 1.
What's the advantage of a tetrahedral structure?
This tetrahedral structure creates a tiny negative charge at the end of the molecule dominated by the two lone pairs of electrons.
A slight positive charge is created at each of the two protons situated at the ends of the covalent bonds.
These covalent bonds are very strong compared with the weak hydrogen bonds that bond molecules together and remain intact under normal physical conditions.
The distribution of positive and negative charges on the ends of the four polar orbitals of the tetrahedron provides the ability to form weak bonds with other molecules.
How does each lobe contain two electrons?
Each of water's sp orbitals contains 2 electrons, one with a spin-UP orientation and one with a spin-DOWN orientation.
A lobe represents a single quantum state, but the different orientations of the spins ensures that each electron is in a separate quantum state as demanded by the Pauli exclusion principle.
The big reveal
The strong covalent bonds of water create an angle of 105.5 degrees between the two protons and the remaining pair of orbitals are also splayed out at 109.5° in the tetrahedron.
This allows weak bonding to other water molecules' hydrogen bonds. The hydrogen bonds are about ten times weaker than the covalent bonds.
This is water's secret weapon: each molecule can bond to four other molecules.
The weak hydrogen bonds
Diagram 2
In the LHS of diagram 2, the strong covalent bonds between the oxygen atom and the two protons are shown as solid connections.
Hydrogen bonds form between the positive hydrogen atom of one water molecule and a lone pair of electrons on the oxygen atom of a neighbouring molecule.
And of course, the same water molecule's two hydrogen atoms can bond to two adjacent water molecules' oxygen atoms.
These weaker bonds are shown as broken lines in diagram 2, held together by a slight positive charge on the hydrogen end and a slight negative charge on the oxygen end.
Hydrogen bonds in water only last for minute fractions of a second from 1 to 10 picoseconds before they break and then reform almost immediately.
The liquid water's web of hydrogen bonds
So, each water molecule can bond to four other molecules, using the attraction of positive and negative charges on the molecules, but these hydrogen bonds are constantly being broken and reformed as the molecules move about in the liquid.
As a result, a web of chaotic, temporary bonds exists between all the water molecules in the liquid. The higher the temperature, the more chaotic the web of bonds becomes.
Water below 4°C
As water cools, the thermal energy of the water molecules becomes less and the molecules get closer together and the dipole bonds become a little stronger but continue to create a dynamic web of bonds in the liquid.
As the water cools, its density increases, reaching a maximum at 4°C.
When the liquid reaches this temperature, the hydrogen bonds, encountering less thermal agitation, start to form tetrahedral structures of molecules, where one molecule at the centre links to four others as shown on the RHS of diagram 2.
This tetrahedral structure takes up more space than the equivalent number of molecules above 4°C and the liquid starts to expand, reducing its density.
Water freezes at 0°C
The hydrogen bonds start to get stronger and more permanent as the temperature plunges to 0 degrees as water freezes, and a solid lattice of tetrahedrons results.
This structure is less dense than water and this is well illustrated in the diagram at the top of the article, showing liquid water on the left and a slice of solid ice on the right.
Each water molecule in ice, is bonded to a total of four near-by molecules, two bonds created by its two lone pair lobes and two more created by its hydrogen protons binding to two separate oxygen atoms in adjacent molecules.
The molecules in ice create hexagonal channels throughout the lattice.
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The structure shown on the right of the first diagram at the top of the article, is repeated over and over as lateral bonding occurs and the empty hexagonal channels become more pronounced throughout the lattice.
Hydrogen bonding in water & ice
That's the story, water molecules form weak bonds to adjacent molecules allowing all the water molecules to become interconnected. The bonds are easily broken, but reform swiftly creating a dynamic, chaotic web of connections.
The higher the temperature, the more easily the bonds break and reform, but as the temperature drops, the density increases and reaches a maximum at 4°C.
As the temperature drops below 4°C, more open tetrahedral structures of multiple water molecules begin to form, which occupy more space and cause the density of water to decrease.
Below 0°C, water starts to form a lattice structure, which becomes a solid of tetrahedrons with empty, hexagonal channels that make the solid less dense than water and so, ice floats.
The two pictures in the image at the top of the article indicate the huge difference in structure between chaotic-water and structured-ice.
Wrapping up the six properties from part 1
The six properties of water that make it unique, defined in part 1 of this article are:
Water freezes at 0°C and boils at 100°C.
Water expands below 4°C and ice floats.
Water is a poor conductor of heat compared to many solids.
Water has a high thermal capacity.
Water has remarkable surface tension.
Water is an excellent solvent.
Let's take the six key properties in turn;
1) Water freezes at 0°C and boils at 100°C
Breaking apart the web of molecular hydrogen bonds requires heat energy before the temperature of the water can start to rise.
Breaking up the web accounts for the high boiling point and and the time it takes to reach boiling, as the heat energy required to break up the web slows down any rise in temperature.
2) Water expands below 4°C and ice floats
As water is cooled, the density increases, just like other liquids. At 4°C, water reaches its maximum density.
As it cools further, more permanent structures start to form, which result in expansion of the liquid as they take up more space than the loosely-bound water molecules.
At and below 0°C, the tetrahedral lattice structure is totally ordered and because of its expansion together with hexagonal channels running through the lattice, it is less dense than water and so floats.
3) Ice is a poor conductor of heat compared to other solids
Ice conducts heat relatively poorly compared with many solids, which helps to slow the transfer of heat between the water and the air.
The colder the surface temperature drops, the thicker the ice becomes creating a greater thermal barrier. So, water below the ice lid remains above 0°C, even when the air temperature drops well below 0°C.
4) Water has a high thermal capacity
Thermal capacity is the amount of heat energy required to raise the temperature of a substance by 1°C.
The web-like structure of interconnecting hydrogen bonds in water has to be broken up, requiring heat energy, before the temperature can be increased by 1°C.
As a result, water has a high thermal capacity compared to other liquids that don't require the breaking up of molecular bonds before raising temperature.
5) Water has remarkable surface tension
Surface tension arises from cohesive hydrogen bonding at the surface and this surface layer is anchored to the molecules immediately below it, forming a membrane-like structure.
Capillary action is created by a mixture of adhesion, cohesion and evaporation in large trees.
6) Water is an excellent solvent
Many compounds dissolve in water as their lattice structures, held together by ionic bonds, have these bonds pulled apart by the electric dipoles of water molecules surrounding the atoms of the salt.
The key takeaways
And there you have it.
The hybridisation of oxygen's outer orbitals (valence orbitals) produces four sp orbitals arranged in a tetrahedral geometry.
Two sp orbitals form covalent bonds with hydrogen atoms, while two contain lone pairs of electrons.
This asymmetric distribution of charges creates a permanent electric dipole, allowing each molecule to form up to four hydrogen bonds with neighbouring molecules.
The result: the six unique properties of water that have allowed life to evolve on this planet.
There are other liquids that have some similar properties to water, but none exhibit all six of the listed properties to the same extent, making water unique.
Some substances do show similar hydrogen bonding, such as ammonia (NH3), hydrogen fluoride (HF) and alcohols that contain an OH group.
However, no liquids can support life on earth, or anywhere else, like water can, all because of its ability to create weak hydrogen bonds.
Acknowledgement of image at start of article
Title of Image “Liquid water and ice”.
Licence: Wikimedia Commons. Licensed under CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).
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