Liquid
A state of matter with definite volume but no fixed shape.
Liquid is a state of matter that holds a fixed volume but lacks a set shape. When placed in a container and acted upon by a force like gravity, it takes on the container’s internal form in the direction of that force. Liquids are very difficult to compress, keeping their volume even under pressure. Their density is generally similar to that of a solid and much greater than that of a gas. They belong to the category of condensed matter (like solids) and are also a type of fluid (like gases).
A liquid consists of atoms or molecules connected by intermolecular bonds of moderate strength. These forces let the particles slide past one another while staying densely packed. In solids, strong intermolecular bonds lock particles into fixed positions, allowing only small vibrations. In gases, particles are far apart and move freely, with weak intermolecular forces.
When temperature rises, the molecules in a liquid vibrate more vigorously, increasing the gaps between them. At the boiling point, the cohesive forces can no longer hold the molecules together, and the liquid changes into a gas. When temperature drops, the molecules move closer. At the freezing point, they typically arrange into an ordered structure through crystallization, turning the liquid into a solid.
Even though liquid water is common on Earth, this state of matter is actually the rarest in the known universe, since liquids need a fairly specific range of temperature and pressure to exist. Most matter in the universe is either gas (like interstellar clouds) or plasma (like stars).
**Examples**
Only two elements are liquid under standard temperature and pressure: mercury and bromine. Four more have melting points just above room temperature: francium, caesium, gallium, and rubidium.
Pure substances that are liquid under normal conditions include water, ethanol, and many other organic solvents. Liquid water is essential in chemistry and biology and is required for all known life. Inorganic liquids in this category include nonaqueous inorganic solvents and many acids.
Mixtures that are liquid at room temperature include alloys like galinstan (a gallium-indium-tin alloy that melts at −19 °C or −2 °F) and some amalgams (mercury-based alloys). Certain mixtures, such as the sodium-potassium alloy NaK, are liquid at room temperature even though the individual elements are solid under the same conditions (this is called a eutectic mixture). Everyday liquid mixtures include aqueous solutions like household bleach, mixtures of different substances like mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like blood, and colloids like paint and milk.
Many gases can be turned into liquids by cooling, producing substances such as liquid oxygen, liquid nitrogen, liquid hydrogen, and liquid helium. However, not all gases can be liquefied at atmospheric pressure. Carbon dioxide, for example, turns directly into dry ice (a solid) rather than becoming a liquid, and it can only be liquefied at pressures above 5.1 atm. Most liquids solidify as temperature drops further. Liquid helium is an exception: it does not become solid even at absolute zero (0 K) under standard pressure, due to its quantum properties.
**Properties**
**Volume**
Quantities of liquid are measured in units of volume. These include the SI unit cubic meter (m³) and its subdivisions, especially the cubic decimeter, more commonly called the liter (1 dm³ = 1 L = 0.001 m³), and the cubic centimeter, also called milliliter (1 cm³ = 1 mL = 0.001 L = 10⁻⁶ m³).
The volume of a given amount of liquid is set by its temperature and pressure. Liquids generally expand when heated and contract when cooled. Water between 0 °C and 4 °C is a notable exception.
Liquids are also very hard to compress. For example, water compresses by only 46.4 parts per million for each additional atmosphere of pressure (bar). At around 4000 bar (400 megapascals or 58,000 psi) at room temperature, water’s volume decreases by just 11%. This incompressibility makes liquids suitable for transmitting hydraulic power, since a change in pressure at one point in a liquid is transmitted undiminished to every other part, with very little energy lost to compression.
However, this low compressibility also leads to other effects. The banging of pipes, known as water hammer, happens when a valve is closed suddenly, creating a huge pressure spike at the valve that travels backward through the system at nearly the speed of sound. Another effect is cavitation. Because liquids have little elasticity, they can be pulled apart in areas of high turbulence or sharp changes in direction, such as behind a boat propeller or at a sharp corner in a pipe. In a low-pressure (vacuum) region, the liquid vaporizes and forms bubbles, which then collapse as they enter high-pressure areas. This collapse forces liquid into the cavities left by the bubbles with tremendous localized force, eroding any adjacent solid surface.
**Pressure**
In a gravitational field, liquids exert pressure on the sides of a container and on anything inside the liquid itself. Liquid pressure is transmitted in all directions and increases with depth. If a liquid is at rest in a uniform gravitational field, the pressure \( p \) at depth \( z \) is given by \( p = p_0 + \rho g z \), where \( p_0 \) is the pressure at the surface, \( \rho \) is the density, and \( g \) is the acceleration due to gravity.
- state
- Liquid
- key_property
- Definite volume, no fixed shape
- common_examples
- Water, ethanol, mercury, bromine
- compressibility
- Nearly incompressible
- density_relation
- Usually close to that of a solid, much higher than that of a gas
- surface_tension_range
- Tens to hundreds of mJ/m2
Lore & Background
Liquids adapt to the internal shape of their container when subject to a force such as gravity. They are nearly incompressible, maintaining volume even under pressure. The density of a liquid is usually close to that of a solid and much higher than that of a gas. As temperature increases, molecules in a liquid vibrate more intensely, increasing distances between them; at the boiling point, cohesive forces fail and the liquid transitions to a gas. As temperature decreases, molecules draw closer; at the freezing point, they typically arrange into a structured order via crystallization, transitioning to a solid.
Reader's Guide
Liquids are fundamental to chemistry and biology, with liquid water being necessary for all known forms of life. They are measured in units of volume, such as the litre or cubic metre. Liquids exhibit properties such as pressure transmission in all directions, buoyancy (Archimedes' principle), and surface tension, which causes spherical drops and capillary action. Their incompressibility makes them suitable for hydraulic power transmission, but also leads to phenomena like water hammer and cavitation. Only two elements are liquid at standard temperature and pressure: mercury and bromine. Many gases can be liquefied by cooling, though not all at atmospheric pressure; carbon dioxide, for example, solidifies directly into dry ice unless pressurized above 5.1 atm. Liquid helium is exceptional in that it does not solidify even at absolute zero under standard pressure due to quantum properties.
Did You Know?
- Only two elements are liquid at standard conditions for temperature and pressure: mercury and bromine.
- Water between 0 °C and 4 °C is a notable exception to the rule that liquids expand when heated and contract when cooled.
- Liquid helium does not become solid even at absolute zero (0 K) under standard pressure due to its quantum properties.
- Carbon dioxide cannot be liquefied at atmospheric pressure; it solidifies directly into dry ice.
The Classical Triad and Liquid's Place
Matter organizes into distinct phases or states depending on its constituents and the external factors acting upon it, particularly pressure and temperature. Under conditions that are not extreme, atoms arrange themselves into the three classical states: solid, liquid, and gas. A solid holds a definite shape and volume on its own, without requiring a container, because its particles are held very close to one another. A liquid is a mostly non-compressible fluid; it takes the shape of whatever vessel holds it yet retains a nearly constant volume independent of the pressure applied. A gas, being compressible, does both—it assumes the shape of its container and expands to fill the entire available space. While these three are the states most people encounter daily, they represent only the beginning of the remarkable diversity that matter can exhibit.
Between the Classical Boundaries
Beyond the classical triad, matter can occupy intermediate territories that blend properties of more familiar states. Liquid crystals, for instance, exhibit characteristics sitting between liquids and crystals—they are generally able to flow like a liquid while simultaneously maintaining long-range orientational order among their molecules. Plastic crystals represent another hybrid: a molecular solid in which long-range positional order is preserved, yet the constituent molecules retain rotational freedom. Supercritical fluids arise at sufficiently high temperatures and pressures, where the boundary between liquid and gas dissolves entirely, producing a fluid with properties intermediate to both. Complex molecules in particular can form various mesophases, creating material behaviors that resist simple categorization. These states demonstrate that the boundaries separating solid from liquid are not rigid walls but rather gradual, tunable transitions shaped by molecular structure and environmental conditions.
Ionization, Degeneracy, and the Quantum Extreme
When conditions become extreme, matter sheds its familiar atomic structure entirely. At high temperatures or under strong electromagnetic fields, atoms become ionized, giving rise to plasma—a state containing a significant number of free electrons and ionized atoms. Unlike ordinary gases composed of neutral atoms, plasma can self-generate magnetic fields, sustain electric currents, and respond strongly and collectively to electromagnetic forces. In the cores of certain stars and in the early universe, atoms break down into their fundamental constituents, producing degenerate matter or quark matter, phenomena studied within high-energy physics. At the opposite thermal extreme, near absolute zero, large numbers of bosons can collapse into the same quantum state, forming a Bose-Einstein condensate. This remarkable phase was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein but was not experimentally observed until 1995, when Eric Cornell and Carl Wieman achieved it in the laboratory. Fermionic condensates extend the concept to fermions, where pairs of fermions bind together and collectively behave like bosons.
The Twentieth-Century Explosion of Phases
The twentieth century witnessed an extraordinary expansion in the catalog of known phases of matter. Condensed matter physics revealed that electrons in solid materials can organize into superconducting states exhibiting vanishing resistivity, or into a rich taxonomy of magnetic configurations including ferromagnetism, antiferromagnetism, ferrimagnetism, altermagnetism, spin-density waves, helimagnetism, spin glass, and quantum spin liquid. Electronic ordering states such as ferroelectricity and antiferroelectricity added yet another dimension to the landscape. Superfluids, achieved by a few cryogenic liquids at extreme temperatures, flow without any friction and can even climb the sides of an open container, while supersolids combine frictionless motion with a rigid shape. The Landau theory further classifies different structural phases of polymorphic materials as distinct states of matter. Altogether, the twentieth century's deepened understanding of matter's properties led to the identification of an infinite number of qualitatively different ways in which matter can organize itself.
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