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— CH. 1 · THE RAREST COMMON THING —

Liquid

10 min listen · Ch. 1 of 8
8 sections

  • Liquid water fills the oceans, falls as rain, and runs through every living body on Earth. Yet this state of matter is the least common in the known universe. The reason is a matter of margins. A liquid only forms within a narrow band of temperature and pressure. Step outside that band and matter becomes something else. Most of the cosmos is gas drifting in interstellar clouds or plasma burning inside stars. Liquid is the exception that needs everything to be just right. A liquid holds a definite volume but no fixed shape. Pour it into a container under gravity and it takes the shape of whatever holds it, in the direction of the force. It is nearly impossible to squeeze, keeping its volume even under pressure. So what governs that narrow window where liquid exists? Why does heating a liquid eventually destroy it, and cooling it lock it into a solid? And why has the inner life of liquids resisted scientists for so long? The answers begin with the bonds between molecules.

  • Intermediate is the word that defines a liquid. Its atoms or molecules are held by intermolecular bonds of middling strength. Those forces let particles slide around one another while staying closely packed. This is the middle ground between two extremes. In solids, particles are bound by strong forces, limited to small vibrations in fixed positions. In gases, widely spaced particles move freely with only weak forces between them. Temperature is what tips a liquid out of its state. As it rises, molecules vibrate harder and the gaps between them widen. At the boiling point, the cohesive forces can no longer hold the molecules together, and the liquid turns to gas. Lower the temperature instead, and the molecules draw closer. At the freezing point, they usually settle into a structured order through crystallization, and the liquid becomes solid. The density of a liquid sits close to that of a solid and far above that of a gas. That density places it among condensed matter beside solids, even as it flows like a gas, making it a fluid as well.

  • Mercury and bromine stand alone. They are the only two elements that are liquid at standard conditions for temperature and pressure. Four more sit just past room temperature: francium, caesium, gallium, and rubidium melt only slightly above it. Among pure substances liquid under normal conditions are water, ethanol, and many other organic solvents. Liquid water matters more than most, essential to chemistry, to biology, and to every known form of life. The category also holds inorganic nonaqueous solvents and many acids. Mixtures stretch the rules further. Galinstan, an alloy of gallium, indium, and tin, melts at minus 19 degrees Celsius. The sodium-potassium alloy NaK stays liquid at room temperature even though both elements are solid on their own, an effect of being a eutectic mixture. Everyday liquids include household bleach, mineral oil and gasoline, emulsions like vinaigrette and mayonnaise, the suspension that is blood, and the colloids of paint and milk. Gases can be coaxed into liquid form by cooling, giving liquid oxygen, nitrogen, hydrogen, and helium. Not all cooperate. Carbon dioxide freezes straight into dry ice and only liquefies above 5.1 atmospheres. Liquid helium is stranger still. It refuses to become solid even at absolute zero under standard pressure, a consequence of its quantum properties.

  • Forty-six point four parts per million. That is how little water compresses for each unit increase in atmospheric pressure, measured in bar. Push it to around 4000 bar at room temperature and water shrinks by only 11 percent. This stubbornness has uses and dangers alike. Because a pressure change at one point in a liquid passes undiminished to every other part, liquids transmit hydraulic power with very little energy lost to compression. The same property bites back. Slam a valve shut and the moving liquid creates a pressure spike that races backward through the pipes at just under the speed of sound, the banging known as water hammer. Cavitation is the other edge. With little elasticity, a liquid can be pulled apart in zones of high turbulence, such as the trailing edge of a boat propeller or a sharp corner in a pipe. In a low-pressure pocket the liquid vaporizes into bubbles. When those bubbles reach high pressure they collapse, and the liquid rushes into the void with tremendous localized force, eroding any nearby solid surface. Volume itself is fixed by temperature and pressure, and liquids generally expand when heated and contract when cooled. Water between 0 and 4 degrees Celsius breaks even that rule, a notable exception to the ordinary order of things.

  • A molecule at the surface of a liquid is missing half its neighbors. It bonds with other molecules only on the inner side, which leaves a net force pulling it inward. That pull has a name and a number behind it: surface tension, measured as energy per unit area in joules per square metre. Liquids with strong intermolecular forces carry large surface tensions. The visible result is everywhere. Liquids shrink their surface area, pulling into spherical drops and bubbles unless something else intervenes. Surface tension also drives surface waves, capillary action, wetting, and ripples. At the nanoscale the effect dominates, because so much more of the liquid sits near a surface. Wettability follows directly from this. Most common liquids carry tensions in the tens of millijoules per square metre, so droplets of oil, water, or glue merge and cling to surfaces with ease. Mercury and other liquid metals reach into the hundreds, so their droplets resist combining and only wet under special conditions. Surface tensions stay within a narrow range as conditions shift, a steadiness that stands in sharp contrast to a property like viscosity, which varies enormously.

  • Viscosity is the measure of a liquid's resistance to flow. More precisely, it gauges resistance to deformation at a given rate, such as when a liquid is sheared at finite velocity. Picture liquid moving through a pipe. It flows more slowly near the walls than at the center, so it undergoes shear deformation and pushes back with viscous resistance. To keep it moving, an external force is needed, like a pressure difference between the ends. Heat thins this resistance. The viscosity of liquids falls as temperature rises, which is why the lubrication industry cares so much about control. One method blends two or more liquids of differing viscosities in precise ratios. Additives can also reshape how a lubricating oil's viscosity responds to temperature, important for machinery that runs across a span of conditions. Liquids split into two flow personalities. A Newtonian liquid shows a linear strain-stress relationship, its viscosity unchanged by time, shear rate, or shear-rate history. Water, glycerin, motor oil, honey, and mercury behave this way. A non-Newtonian liquid does not. It thickens or thins under shear, as ketchup, custard, and starch solutions do. The same medium that resists deformation also carries sound, at 1.5 kilometres per second through water, set by a bulk modulus near 2.2 gigapascals and a density of 1000 kilograms per cubic metre.

  • No small parameter. That phrase captures why liquids have defied theorists for so long. Liquids are a dense, disordered packing of molecules, caught between the two phases scientists understand best. Gases are disordered but well-separated, interacting mainly through collisions. Solids are densely packed but fall into regular structures like a crystalline lattice, with glasses as the notable exception. A liquid is dense like a solid yet disordered like a gas, belonging fully to neither. There is order, but it is short-range. It persists over only a few molecular diameters. Excluded volume interactions create positional order in even the simplest cases, such as the monatomic liquids argon and krypton, which behave like disordered heaps of closely packed spheres. Where molecules are not spheres, their forces carry directionality, adding short-range orientational order. In hydrogen-bonded liquids like water, the strength and direction of those bonds build local networks and clusters that constantly deform, break, and reform. This difficulty has a precise cause. Gases can be modeled from the ideal gas using density as a small parameter, and solids from a perfect lattice using thermal motion and defects. Liquids have neither reference state, because attractive intermolecular forces and entropic forces are comparable. The binding energy between neighbors lands in the same order of magnitude as the thermal energy, so neither can be ignored in favor of the other.

  • The proton's small mass is what lets the quantum world intrude on something as ordinary as water. Under standard conditions, most of a liquid's behavior follows classical mechanics, with molecules treated as discrete entities obeying Newton's laws. The threshold is the thermal de Broglie wavelength. When it is small compared with the average distance between molecules, the classical picture holds. Typical values run about 0.01 to 0.1 nanometers. For hydrogen at 14.1 kelvin the wavelength reaches 0.33 nanometers, and the ratio to molecular spacing climbs to 0.97, deep in quantum territory. Krypton at 116 kelvin sits far lower, its ratio near 0.046. The pattern is clear: quantum effects grow at low temperatures and with small molecular mass. In water, hydrogen bonding brings in zero-point motion and tunneling because the proton is so light. At extreme cold, even the large-scale behavior of liquids departs from classical rules. Hydrogen and helium are the standout cases, their low temperature and mass giving them a thermal de Broglie wavelength comparable to the spacing between molecules. That same helium, refusing to freeze at absolute zero, marks the boundary where a liquid stops behaving like ordinary matter and starts obeying the rules of the quantum world.

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GasSolid

Common questions

What is a liquid in terms of states of matter?

A liquid is a state of matter with a definite volume but no fixed shape. It is nearly incompressible and adapts to the shape of its container in the direction of an applied force such as gravity. It is a form of condensed matter alongside solids and a form of fluid alongside gases.

Why is liquid the least common state of matter in the universe?

Liquid is the least common state of matter in the known universe because liquids require a relatively narrow temperature and pressure range to exist. Most known matter is either gaseous, as in interstellar clouds, or plasma, as in stars.

Which elements are liquid at room temperature?

Only mercury and bromine are liquid at standard conditions for temperature and pressure. Four more elements, francium, caesium, gallium, and rubidium, have melting points slightly above room temperature.

What is viscosity in a liquid?

Viscosity is the measure of a liquid's resistance to flow, or more technically its resistance to deformation at a given rate. The viscosity of liquids decreases as temperature increases. Newtonian liquids like water and honey keep a constant viscosity, while non-Newtonian liquids like ketchup and custard thicken or thin under shear.

What causes surface tension in a liquid?

Surface tension arises because a molecule at the surface bonds with other liquid molecules only on the inner side, creating a net force that pulls surface molecules inward. It is measured as energy per unit area in joules per square metre, and liquids with strong intermolecular forces have larger surface tensions.

Why are liquids hard to model at the molecular level?

Liquids are hard to model because there is no small parameter from which to build a systematic theory, unlike the ideal gas reference for gases or the perfect lattice reference for solids. In liquids the attractive intermolecular forces and entropic forces are comparable, and the binding energy between molecules is the same order of magnitude as the thermal energy.

When do quantum effects matter for liquids?

Quantum effects become important for liquids at low temperatures and with small molecular mass, when the thermal de Broglie wavelength is comparable to the average distance between molecules. Hydrogen and helium are notable examples, and liquid helium does not become solid even at absolute zero under standard pressure.

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72 references cited across the entry

  1. 1BookThe elements: a visual exploration of every known atom in the universeTheodore Gray et al. — Black Dog & Leventhal — 2012
  2. 2JournalWater and astrobiologyMichael J. Mottl et al. — December 2007
  3. 3JournalAstrobiology: The Study of the Living UniverseChristopher F. Chyba et al. — 1 September 2005
  4. 4JournalVoltammetric analysis using a self-renewable non-mercury electrodePeter Surmann et al. — Springer Science and Business Media LLC — 2005-10-15
  5. 5JournalNaK alloy: underrated liquid metalSergei S. Leonchuk et al. — Royal Society of Chemistry (RSC) — 2022
  6. 6BookThe Properties of Liquid and Solid HeliumJ. Wilks — Clarendon Press — 1967
  7. 7BookPhysics for Scientists and Engineers: A Strategic Approach (With Modern Physics)Randall D. Knight — Addison-Wesley — 2008
  8. 8BookChemistry: The Molecular Nature of Matter and ChangeMartin S. Silberberg — McGraw-Hill Higher Education — 2009
  9. 9Compressibility of Liquids
  10. 10BookIntelligent Energy Field Manufacturing: Interdisciplinary Process InnovationsWenwu Zhang — CRC Press — 2011
  11. 11BookFluid Mechanics and Hydraulic MachinesS. C. Gupta — Dorling-Kindersley — 2006
  12. 12BookWetting of Real SurfacesEdward Yu. Bormashenko — De Gruyter — 5 November 2018
  13. 13BookFluid MechanicsL.D. Landau et al. — Pergamon Press — 1987
  14. 14BookTransport PhenomenaR. Byron Bird et al. — John Wiley & Sons, Inc. — 2007
  15. 15Viscosity Blending EquationsBoris Zhmud — 2014
  16. 16Viscosity IndexAnton Paar
  17. 17BookHoney in Traditional and Modern MedicineLaid Boukraa — CRC Press — 2014
  18. 18BookClassical MechanicsJohn R. Taylor — University Science Books — 2005
  19. 19JournalFrom 50 Years Ago, the Birth of Modern Liquid-State ScienceDavid Chandler — Annual Reviews — 2017-05-05
  20. 20JournalCollective modes and thermodynamics of the liquid stateK Trachenko et al. — IOP Publishing — 2015-12-22
  21. 21BookMolecular theory of water and aqueous solutions. Part 1, Understanding waterArieh Ben-Naim — World Scientific — 2009
  22. 22JournalStructure of Neat Liquids Consisting of (Perfect and Nearly) Tetrahedral MoleculesSzilvia Pothoczki et al. — American Chemical Society (ACS) — 2015-12-01
  23. 23BookIntermolecular forces: their origin and determinationGeoffrey C. Maitland et al. — Oxford University Press — 1981
  24. 24BookPhysics of liquid matterPaola Gallo et al. — Springer — 2021
  25. 25BookIntroduction to modern statistical mechanicsDavid Chandler — Oxford University Press — 1987
  26. 26JournalBernal's road to random packing and the structure of liquidsJohn L. Finney — Informa UK Limited — 2013-02-22
  27. 27BookWater: a very short introductionJ. L. Finney — Oxford University Press — 2015
  28. 28JournalThe Structure of Liquid MethanolRalf Ludwig — Wiley — 2005-07-11
  29. 29JournalIntroduction to liquid crystalsDenis Andrienko — October 2018
  30. 30JournalLiquids: Condensed, disordered, and sometimes complexDavid Chandler — 2009-09-08
  31. 31BookTheory of simple liquids: with applications to soft matterJean-Pierre Hansen et al. — Academic Press — 2013
  32. 32BookStatistical physics of particlesMehran Kardar — Cambridge University Press — 2007
  33. 33BookTheory of molecular fluidsC. G. Gray et al. — Oxford University Press — 1984–2011
  34. 34BookAb initio molecular dynamics: basic theory and advanced methodsDominik Marx et al. — Cambridge University Press — 2012
  35. 35Statistical Theory of LiquidsI.Z. Fisher — The University of Chicago Press — 1964
  36. 36JournalNuclear quantum effects and hydrogen bond fluctuations in waterMichele Ceriotti et al. — 2013-09-06
  37. 37JournalNuclear quantum effects enter the mainstreamThomas E. Markland et al. — Springer Science and Business Media LLC — 2018-02-28
  38. 38JournalQuantum nature of the hydrogen bondXin-Zheng Li et al. — 2011-04-04
  39. 39JournalOn the stability of crystal latticesMax Born — 1940
  40. 40JournalThermodynamics of Crystals and MeltingMax Born — 1939
  41. 41BookIntroduction to Liquid State PhysicsN.H. March et al. — World Scientific — 2002
  42. 42Does water freeze or boil in space?Ethan Siegel — 2014-12-11
  43. 43BookLiquid rules: The Delightful and Dangerous Substances that Flow Through Our LivesMark Miodownik — Houghton Mifflin Harcourt — 2019
  44. 44BookLubricants and LubricationTheo Mang et al. — John Wiley & Sons — 2007
  45. 45BookHandbook of SolventsGeorge Wypych — ChemTec Publishing — 2001
  46. 46BookHandbook of thermal conductivity of liquids and gasesCRC press — 1994
  47. 47BookAutomotive Technology: A Systems ApproachJack Erjavec — Thomson/Delmar Learning — 2005
  48. 48BookThe prospects of nuclear power and technologyGerald Wendt — D. Van Nostrand Company — 1957
  49. 49BookModern Engineering for Design of Liquid-Propellant Rocket EnginesDieter K. Huzel et al. — American Institute of Aeronautics and Astronautics — 2000
  50. 50BookHVAC principles and applications manualThomas E. Mull — McGraw-Hill — 1998
  51. 51BookUnit operations in food processingR. L. Earle — Pergamon Press — 1983
  52. 52BookFluid Power DynamicsR. Keith Mobley — Elsevier — 1999
  53. 53JournalStretchable and Soft Electronics using Liquid MetalsMichael D. Dickey — Wiley — 2017-04-18
  54. 54JournalLiquid Metal Enabled BiodevicesTim Cole et al. — Wiley — 2021-05-04
  55. 55JournalGallium Liquid Metal: The Devil's ElixirShi-Yang Tang et al. — Annual Reviews — 2021-07-26
  56. 56BookInstrument Engineers' Handbook, Volume Two: Process Control and OptimizationBela G. Liptak — CRC Press — 2018
  57. 57JournalUBC/Laval 2.7 meter liquid mirror telescopePaul Hickson et al. — American Astronomical Society — 1994
  58. 58JournalImage Quality of Liquid-Mirror TelescopesPaul Hickson et al. — IOP Publishing — 2007
  59. 59BookThe lattice Boltzmann method: principles and practiceTimm Krüger et al. — Springer — 2016
  60. 60BookComputational multiscale modeling of fluids and solids: theory and applicationsM. O. Steinhauser — Springer — 2022
  61. 61BookThe properties of gases and liquidsBruce E. Poling et al. — McGraw-Hill — 2001
  62. 62BookTransport PhenomenaR. Byron Bird et al. — John Wiley & Sons, Inc. — 2007
  63. 63BookMultiparameter Equations of State: An Accurate Source of Thermodynamic Property DataR. Span — Springer — 2000
  64. 64JournalThe NIST REFPROP Database for Highly Accurate Properties of Industrially Important FluidsMarcia L. Huber et al. — American Chemical Society (ACS) — 2022-06-22
  65. 65JournalA Helmholtz Free Energy Formulation of the Thermodynamic Properties of the Mixture {Water + Ammonia}Reiner Tillner-Roth et al. — AIP Publishing — 1998
  66. 66Fluid DynamicsH.K. Moffatt — Princeton University Press — 2015
  67. 67BookComputational fluid dynamics: an introductionJohn F. Wendt et al. — Springer — 2008
  68. 68BookIntroduction to theoretical and computational fluid dynamicsC. Pozrikidis — Oxford University Press — 2011
  69. 69JournalSmoothed particle hydrodynamicsJ J Monaghan — IOP Publishing — 2005-07-05
  70. 70JournalReview of smoothed particle hydrodynamics: towards converged Lagrangian flow modellingSteven J. Lind et al. — The Royal Society — 2020
  71. 71JournalPerspective: Dissipative particle dynamicsPep Español et al. — AIP Publishing — 2017-04-21
  72. 72BookAdvanced Computer Simulation Approaches for Soft Matter Sciences IIIG. Gompper et al. — Springer Berlin Heidelberg — 2009