Last updated September 1, 2025

Water Anomalies: A Common and Abundant Molecule in the Universe

Molecular structure and physical properties of water

Water: A Liquid with Mysterious Behaviors

Water is one of the few substances that exhibits about 70 documented 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 primary cause of most of water's anomalies, including its maximum density, high specific heat, and non-monotonic viscosity.

The Four Major Families of Anomalies

Thermodynamic: high specific heat, maximum density at 4 °C, enthalpy of vaporization, unusual critical point, freezing point under pressure, high boiling point.
Dynamic: non-monotonic viscosity, increasing molecular diffusion in supercooled regime, supercooling, minimal compressibility.
Structural: unstable tetrahedral network, coexistence of dense and open microstructures, abnormal expansion during 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

The viscosity of water is one of its most intriguing anomalies. Unlike most liquids, its viscosity does not decrease linearly with temperature. It exhibits a minimum around 30 °C, then slightly increases 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 temperatures, slowing molecular movement.

Under extreme conditions, such as in cold and low-density 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 exhibit extremely high viscosity, comparable to or even greater than that of honey on Earth. This phenomenon is explained by the quasi-permanent presence of strongly correlated hydrogen bonds and the absence of thermal disturbances.

Molecular Encapsulation Due to Extreme Viscosity of Water

Under conditions of extremely high viscosity, such as those observed in certain space environments or at low temperatures, 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 from 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 with 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 temperatures	Stratification of lakes and oceans, protection of aquatic organisms during winter, local climate regulation	Tetrahedral structure of the hydrogen bond 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 changes	Energy required to break the H-bond network
High dielectric constant (strong polarity)	Optical/Dielectric	≈ 78.5 at 25 °C, decreases with increasing temperature	Allows efficient dissolution 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 bond 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, enables certain animal locomotion on water, influences liquid-gas interfaces	Strengthening of the hydrogen bond 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 mechanical protection of cells and biological tissues	H-bond network structure resistant to compression
Anomaly	Family	Observation	Consequences	Comment
Ice polymorphs (≥17 forms)	Structural	Ice I to VII, different densities and crystalline 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 (decrease under pressure)	Thermodynamic	Decreases from 0 °C to -22 °C at 2000 atm	Melting ice in glaciers and underwater, impact on the dynamics of ice caps and cryogenics	Hydrogen bond network destabilized by pressure
Expansion during solidification (ice less dense)	Structural	Volume increases by ≈ 9% during 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	Earth's thermal regulation, slow evaporation, temperature stabilization in ecosystems	Massive breaking of hydrogen bonds to transition to vapor
High boiling point (100 °C at 1 atm)	Thermodynamic	100 °C at 1 atm, significantly higher than comparable liquids	Maintenance of liquid water under 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 favor 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
Anomaly	Family	Observation	Consequences	Comment
Extreme capillarity and adhesion	Structural	Rise of water in very fine tubes or plant xylems	Transport of water and nutrients in plants, enables 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 transport of electrical charges in solutions	H-bonds facilitate the "jump" of protons 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 polarity result in low energy losses
Supercooling	Dynamic	Remains liquid down to -40 °C under controlled conditions	Allows survival of cells and organisms, influences natural and industrial crystallization	Flexible H-bond network delaying ice formation
Thermal anomalies in deep oceans	Thermodynamic	Liquid water at T<0 °C under high pressure (≈1000–4000 atm)	Maintenance of 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 densities 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 the H-bond network and extreme confinement
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
Anomaly	Family	Observation	Consequences	Comment
Extreme capillarity and adhesion	Structural	Rise of water in very fine tubes or plant xylems	Transport of water and nutrients in plants, enables 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 transport of electrical charges in solutions	H-bonds facilitate the "jump" of protons 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 polarity result in low energy losses
Supercooling	Dynamic	Remains liquid down to -40 °C under controlled conditions	Allows survival of cells and organisms, influences natural and industrial crystallization	Flexible H-bond network delaying ice formation
Thermal anomalies in deep oceans	Thermodynamic	Liquid water at T<0 °C under high pressure (≈1000–4000 atm)	Maintenance of 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 densities 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 the H-bond network and extreme confinement