Water anomalies: A common and abundant molecule in the Universe

Molecular structure and physical properties of water — This **water droplet** illustrates the power of hydrogen bonds: thanks to them, water molecules attract each other, creating a surface tension that allows the droplet to maintain its spherical shape, minimizing its energy. Water droplets are round thanks to these bonds, which act like small magnets between molecules.
Image source: astronoo.com (new window) — AI-generated image, public domain.

Scientific summary

Astronoo's article presents a synthesis of the 70 physico-chemical anomalies of water, linking these singular behaviors to the structure of the hydrogen bond network. The analysis covers thermodynamic properties (maximum density at 4 °C, high specific heat), dynamic properties (non-monotonic viscosity, supercooled diffusion) and structural properties (ice polymorphism, expansion upon solidification). The article also explores microscopic mechanisms, such as tetrahedral network fluctuations and proton transport, which explain macroscopic phenomena such as extreme capillarity and the role of universal solvent. This approach shows how hydrogen bond cooperativity gives water unique properties essential to life and many planetary processes.

Why is water considered the most abnormal substance in the Universe ?

This article is dedicated to the anomalies of water, a seemingly ordinary molecule, but which possesses more than 70 abnormal properties listed. But why is water so unique? The answer lies in its hydrogen bond network, a directional and cooperative interaction between molecules. This network is the origin of counter-intuitive behaviors: water is denser at 4 °C than at 0 °C, it freezes faster when hot (Mpemba effect), and its viscosity has a minimum around 30 °C. These anomalies are not laboratory curiosities: they are fundamental for climate regulation, oceanic circulation, cell physiology and even prebiotic chemistry. Water is the universal solvent, capable of dissolving a multitude of compounds, and its dielectric and optical properties make it a unique medium. This article explores the four main families of anomalies (thermodynamic, dynamic, structural, optical), detailing the microscopic mechanisms and illustrating with concrete examples, such as extreme viscosity in interstellar space or molecular encapsulation. Understanding water means understanding the living and inert universe, because it is present everywhere, from terrestrial oceans to galactic molecular clouds.

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Water: A liquid with mysterious behaviors

Water is one of the rare substances that presents about 70 listed anomalies. These anomalies concern its thermodynamic, mechanical, structural and even acoustic properties. They arise from the cooperative nature of hydrogen bonds and explain why water is the basis of life as we know it.

N.B.:
Hydrogen bonds are directional dipole-dipole interactions between a hydrogen atom bonded to a highly electronegative atom (such as oxygen) and another electronegative atom. In water, each molecule can form up to four hydrogen bonds, creating a dynamic network. This network is the main cause of most of water's anomalies, including its maximum density, high specific heat and non-monotonic viscosity.

The four main families of anomalies

Thermodynamic: high specific heat, maximum density at 4 °C, enthalpy of vaporization (energy required for the liquid → vapor state change), unusual critical point, freezing point under pressure, high boiling point.
Dynamic: non-monotonic viscosity, increasing molecular diffusion in supercooled regime, supercooling, minimum compressibility.
Structural: unstable tetrahedral network, coexistence of dense and open micro-structures, abnormal expansion upon solidification, multiple ice polymorphs, very high surface tension.
Optical and dielectric: very high dielectric constant, particular infrared absorption, abnormal speed of sound, high solvation capacity.

Example of a fascinating anomaly: Non-monotonic viscosity of water and extreme behaviors

A viscosity minimum at 30 °C

The viscosity of water is one of its most intriguing anomalies. Unlike most liquids, its viscosity does not decrease linearly with temperature. It shows a minimum around 30 °C, then increases slightly as the temperature approaches 0 °C. This behavior, called non-monotonic viscosity, is due to the dynamic reorganization of the hydrogen bond network which strengthens at low temperature, slowing down molecular motion.

Extreme viscosity in interstellar space

In extreme conditions, such as in the cold and sparse regions of interstellar space, water molecules can form highly ordered structures even at very low density and temperature. Theoretical studies and simulations suggest that this interstellar water could have an extremely high viscosity, comparable or even greater than that of honey on Earth. This phenomenon is explained by the almost permanent presence of strongly correlated hydrogen bonds and the absence of thermal disturbances.

Molecular encapsulation thanks to the extreme viscosity of water

In conditions of extremely high viscosity, such as those observed in certain space environments or at low temperature, water can act as a true structuring solvent.

The network of strongly correlated hydrogen bonds gives water the ability to stick to neighboring molecules, stabilizing their relative position and limiting their diffusion.

This phenomenon allows water to trap and encapsulate molecules, forming protective micro-environments that can:

Preserve sensitive organic molecules against chemical or radiative degradation.
Facilitate chemical reactions by bringing reactants together in a confined space.
Serve as a matrix in the formation of amorphous ices and interstellar grains enriched in water.

N.B.:
Molecular encapsulation by water is a direct consequence of extreme viscosity and the structured hydrogen network. This property could play a fundamental role in prebiotic chemistry and the formation of the first molecular building blocks in the universe.

Non-exhaustive table of physical and chemical anomalies of water

Examples of physical and chemical anomalies of water
Anomaly	Family	Observation	Consequences	Comment
Maximum density (densest liquid at 4 °C)	Thermodynamic	Reaches 1,000 g/cm³ at 4 °C, then decreases at lower temperature	Stratification of lakes and oceans, protection of aquatic organisms during winter, local climate regulation	Tetrahedral structure of the hydrogen network
Non-monotonic viscosity (minimum around 30 °C)	Dynamic	Approximately 0.797 mPa·s at 25 °C, non-linear variation with temperature	Optimization of cellular transport, influence on ocean dynamics and nutrient circulation, impact on molecular diffusion of substances	Dynamic reorganization of hydrogen bonds
High specific heat (very large heat capacity)	Thermodynamic	≈ 4.18 J·g⁻¹·K⁻¹ at 25 °C, significantly higher than similar liquids	Thermal stabilization of aquatic and terrestrial ecosystems, global climate regulation, protection against sudden temperature variations	Energy needed to break the H-bond network
High dielectric constant (strong polarity)	Optical/Dielectric	≈ 78.5 at 25 °C, decreases with increasing temperature	Allows efficient solubilization of salts and polar molecules, influences chemical and biochemical reactions, impacts electrical properties of solutions	High polarity due to hydrogen bonds
Abnormal molecular diffusion (increases in supercooling)	Dynamic	Diffusion of ≈ 2.3×10⁻⁵ cm²/s at 25 °C, increases at temperatures below 0 °C	Importance in cryobiology, influence on amorphous ice formation, role in intracellular transport at low temperature	Rapid rearrangements of the hydrogen network
Minimum speed of sound (≈74 °C)	Thermodynamic	≈ 1402 m/s at 74 °C, varies non-linearly with temperature	Impact on acoustic propagation in oceans and ice, useful in geophysics and underwater sonar	Abnormal local density and compressibility
Very high surface tension (enhanced capillarity)	Structural	≈ 72.8 mN/m at 20 °C, higher than most simple liquids	Facilitates capillarity in plants and soils, allows certain animal locomotion on water, influences liquid-gas interfaces	Reinforcement of the hydrogen network at the surface
Minimum compressibility (46 °C)	Thermodynamic	≈ 4.6×10⁻¹⁰ Pa⁻¹, decreases with increasing temperature then increases again	Attenuation of pressure waves in oceans and organisms, role in the mechanical protection of cells and biological tissues	H-bond network resistant to compression
Ice polymorphs (≥17 forms)	Structural	Ice I to VII, different densities and crystal structures depending on pressure/temperature	Influence on the formation and stability of planetary ices, role in extraterrestrial geology and climatology	Different H-bond arrangements depending on pressure/temperature
Freezing point under pressure (decreases under pressure)	Thermodynamic	Decreases from 0 °C to -22 °C at 2000 atm	Melting ice in glaciers and underwater, impact on ice sheet dynamics and cryogenics	Hydrogen network destabilized by pressure
Expansion upon solidification (ice less dense)	Structural	Volume increases by ≈ 9 % upon freezing	Buoyancy of ice protecting aquatic life, impact on erosion and natural habitats	Fixed and open H-bond network in ice
High heat of vaporization (very large latent heat)	Thermodynamic	≈ 40.7 kJ/mol at 100 °C	Terrestrial thermal regulation, slow evaporation, stabilization of temperatures in ecosystems	Massive rupture of hydrogen bonds to become vapor
High boiling point (100 °C at 1 atm)	Thermodynamic	100 °C at 1 atm, significantly higher than comparable liquids	Maintains liquid water in various conditions, essential for life and industry	Cohesive H-bond network preventing rapid evaporation
High solvation capacity (universal solvent)	Optical/Dielectric	Significant solubility for most salts and polar molecules	Basis of aqueous chemistry and biology, allows dissolution and transport of nutrients and ions	Polarity and hydrogen bonds promote hydration
Supercooling	Dynamic	Water can remain liquid down to -40 °C under controlled conditions	Allows survival of certain cells and organisms, influences crystal formation in nature and industry	Flexible H-bond network delaying crystallization
Mpemba effect (hot water freezes faster than cold water)	Thermodynamic	Occasional, depends on initial temperature, convection and supercooling	Influences freezing in nature and laboratory experiments, shows the complexity of the H-bond network	Effect still partially misunderstood, linked to hydrogen bonds and evaporation
Extreme capillarity and adhesion	Structural	Rise of water in very thin tubes or plant xylems	Transport of water and nutrients in plants, allows certain animal locomotion on water	Strong surface tension effect and solid H-bond network
Exceptional ionic conductivity (Grotthuss mechanism)	Dynamic	Protons and hydroxide ions move at high speed, much faster than classical molecular diffusion	Acceleration of acid-base reactions, rapid charge transport in solutions	H-bonds facilitate proton "hopping" between molecules
Transparency over a wide spectrum	Optical	Low absorption in the visible, increases in the IR	Allows underwater photosynthesis, light penetration in the ocean	Molecular structure and low energy loss polarity
Thermal anomalies in deep oceans	Thermodynamic	Liquid water at T<0 °C under high pressure (≈1000–4000 atm)	Maintains liquid water in the abyss, impact on ocean circulation and deep ecosystems	H-bond network stabilized by pressure
Fluctuating local structure (dense and open micro-domains)	Structural	Coexistence of areas with slightly different density at the nanometric scale	Influences solubility, diffusion and chemical reactions in solution	Rapid rearrangements of H-bonds at the molecular scale
Theoretical molecular superfluidity	Dynamic	Simulation: quasi-frictionless movement of confined molecules	Facilitates encapsulation and selective mobility of certain molecules, possible role in prebiotic chemistry	Theoretical phenomenon linked to H-bond network and extreme confinement

References

FAQ: Everything you need to know about water anomalies

What is a water anomaly?

A water anomaly is a physical or chemical property that deviates from the behavior expected for a simple liquid, due to the particular structure of its hydrogen bond network. Unlike most liquids whose properties vary linearly and predictably with temperature, water exhibits non-monotonic, even counter-intuitive behaviors that defy the usual laws of physics.

How many anomalies does water have?

About 70 anomalies are listed, covering fields such as thermodynamics, dynamics, structure and optics. This exceptional number makes water the most abnormal substance known. New anomalies are still regularly discovered or refined by experimental research and numerical simulations, testifying to the inexhaustible complexity of this molecule.

Why is water denser at 4 °C than at 0 °C?

At 4 °C, the hydrogen bond network reaches an optimal configuration that minimizes volume, making water denser. Below this temperature, the network opens to form tetrahedral structures, increasing the volume. This property allows lakes to freeze on the surface while keeping liquid water at depth, thus protecting aquatic life during harsh winters.

What is the Mpemba effect?

It is the phenomenon by which hot water can freeze faster than cold water, under certain conditions. It is still partially understood, but involves differences in convection, evaporation, supercooling and hydrogen network structuring. The effect is named after Erasto Mpemba, a Tanzanian student who observed it in 1963, but it had already been mentioned by Aristotle and Francis Bacon.

Why is water a universal solvent?

Thanks to its high polarity and ability to form hydrogen bonds, water can dissolve a wide variety of polar and ionic substances, thus facilitating chemical and biological reactions. Its high dielectric constant (≈ 78.5 at 25 °C) allows it to separate the electrical charges of solutes, keeping them in solution. This property is the basis of aqueous chemistry and life as we know it.

What is the role of water anomalies in life?

Water anomalies are essential to life: they regulate temperature through high specific heat, allow nutrient circulation in organisms, protect aquatic ecosystems via thermal stratification of lakes, and influence cellular processes through molecular diffusion and ionic transport. Without these anomalies, life on Earth probably could not have emerged or been maintained.

What is the non-monotonic viscosity of water?

Non-monotonic viscosity is the abnormal behavior by which the viscosity of water has a minimum around 30 °C, then increases slightly as the temperature approaches 0 °C. Unlike most liquids whose viscosity decreases regularly with temperature, water's viscosity increases at low temperature due to the strengthening of the hydrogen bond network, which slows molecular movements.

What is the Grotthuss mechanism in water?

The Grotthuss mechanism is a proton transport process in water where protons (H⁺ ions) move much faster than classical molecular diffusion. Hydrogen bonds allow protons to "hop" from molecule to molecule via reorganization of the network, creating exceptional ionic conductivity. This mechanism is fundamental in acid-base reactions and biochemical processes.

Why does seawater freeze at a lower temperature than pure water?

Seawater freezes at approximately -2 °C, which is lower than 0 °C. This phenomenon, called freezing-point depression, is due to the presence of dissolved salts (mainly sodium chloride) that disrupt the hydrogen bond network. Ions intercalate between water molecules, preventing the formation of the ordered crystalline structure of ice, which lowers the freezing point.

Are there water anomalies in interstellar space?

Yes, in the cold and sparse regions of interstellar space, water molecules can form highly ordered structures despite very low temperature and density. Simulations suggest that this interstellar water could have extremely high viscosity, comparable to honey, due to strongly correlated hydrogen bonds and the absence of thermal disturbances. This property could play a role in prebiotic chemistry and the formation of interstellar grains.

What is molecular encapsulation by water?

Molecular encapsulation is water's ability to trap and stabilize molecules in its hydrogen bond network, particularly under conditions of extreme viscosity (low temperature or space environment). This phenomenon can preserve organic molecules against degradation, facilitate chemical reactions by bringing reactants together, and serve as a matrix for the formation of amorphous ices enriched in organic compounds.

Why does ice float on water?

Ice floats because it is less dense than liquid water. When water freezes, molecules organize into a tetrahedral crystalline network that occupies about 9 % more volume than liquid water. This open structure, maintained by hydrogen bonds, makes ice lighter. Without this property, oceans would freeze from the bottom up, making aquatic life impossible.

What is the difference between ice polymorphs?

Ice polymorphs are different crystalline structures that ice can adopt depending on pressure and temperature. More than 17 forms have been identified (ice I to XVII), ranging from common hexagonal ice (ice Ih) to denser structures like ice VII. These polymorphs are important in planetology to understand the formation of ices on planets, icy moons, and comets.

How do water anomalies influence the Earth's climate?

Water anomalies play a major climatic role: the high specific heat of oceans buffers temperature variations, stratification of lakes and oceans influences ocean currents, water vapor is the main greenhouse gas, and ice (via its albedo) reflects solar radiation. The abnormal expansion upon freezing also protects ecosystems by forming an insulating layer on the surface of water bodies.

Does water have memory?

No, water does not have memory in the sense that some pseudoscientific beliefs claim. However, the hydrogen bond network is dynamic and can maintain structural correlations over very short time scales (a few picoseconds to nanoseconds). These rapid fluctuations, although real, do not allow for lasting "memory" or long-term information transmission. Water's structure is dictated by the physico-chemical conditions of the moment (temperature, pressure, solutes).

Why does water absorb infrared light?

Water strongly absorbs in the infrared due to the vibrations of its chemical bonds. The stretching vibration modes (symmetric and asymmetric) of the O-H bonds and the angular deformation (bending) mode correspond to frequencies in the mid and near infrared. This property is essential for the greenhouse effect, spectroscopy, and drying technologies. Water's transparency in the visible (low absorption) allows underwater photosynthesis.