Liquid
Liquid is a state of matter with a definite volume but no fixed shape. When confined in a container and subjected to a force such as gravity, liquids will adapt to the internal shape of the container in the direction of the force. Liquids are nearly incompressible, maintaining their 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. Liquids are a form of condensed matter alongside solids, and a form of fluid alongside gases.
A liquid is composed of atoms or molecules held together by intermolecular bonds of intermediate strength. These forces allow the particles to move around one another while remaining closely packed. In contrast, solids have particles that are tightly bound by strong intermolecular forces, limiting their movement to small vibrations in fixed positions. Gases, on the other hand, consist of widely spaced, freely moving particles with only weak intermolecular forces.
As temperature increases, the molecules in a liquid vibrate more intensely, causing the distances between them to increase. At the boiling point, the cohesive forces between the molecules are no longer sufficient to keep them together, and the liquid transitions into a gaseous state. Conversely, as temperature decreases, the distance between molecules shrinks. At the freezing point, the molecules typically arrange into a structured order in a process called crystallization, and the liquid transitions into a solid state.
Although liquid water is abundant on Earth, this state of matter is actually the least common in the known universe, because liquids require a relatively narrow temperature/pressure range to exist. Most known matter in the universe is either gaseous or plasma.
Examples
Only two elements are liquid at standard conditions for temperature and pressure: mercury and bromine. Four more elements have melting points slightly 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 of vital importance in chemistry and biology, and it is necessary for all known forms of life. Inorganic liquids in this category include inorganic nonaqueous solvents and many acids.
Mixtures that are liquid at room temperature include alloys such as galinstan and some amalgams. Certain mixtures, such as the sodium-potassium metal alloy NaK, are liquid at room temperature even though the individual elements are solid under the same conditions. Everyday liquid mixtures include aqueous solutions like household bleach, other mixtures of different substances such as mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like blood, and colloids like paint and milk.
Many gases can be liquefied by cooling, producing liquids 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, solidifies directly into dry ice rather than becoming a liquid, and it can only be liquified at pressures above 5.1 atm. Most liquids solidify as the temperature is decreased further. Liquid helium is exceptional in that it does not become solid even at absolute zero under standard pressure due to its quantum properties.
Properties
Volume
Quantities of liquids are measured in units of volume. These include the SI unit cubic metre and its divisions, in particular the cubic decimeter, more commonly called the litre, and the cubic centimetre, also called millilitre.The volume of a quantity of liquid is fixed 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.
On the other hand, liquids have little compressibility. Water, for example, will compress by only 46.4 parts per million for every unit increase in atmospheric pressure. At around 4000 bar of pressure at room temperature water experiences only an 11% decrease in volume. Incompressibility makes liquids suitable for transmitting hydraulic power, because a change in pressure at one point in a liquid is transmitted undiminished to every other part of the liquid and very little energy is lost in the form of compression.
However, the negligible compressibility does lead to other phenomena. The banging of pipes, called water hammer, occurs when a valve is suddenly closed, creating a huge pressure-spike at the valve that travels backward through the system at just under the speed of sound. Another phenomenon caused by liquid's incompressibility is cavitation. Because liquids have little elasticity they can literally be pulled apart in areas of high turbulence or dramatic change in direction, such as the trailing edge of a boat propeller or a sharp corner in a pipe. A liquid in an area of low pressure vaporizes and forms bubbles, which then collapse as they enter high pressure areas. This causes liquid to fill 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 as well as on anything within 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 at depth is given bywhere:
For a body of water open to the air, would be the atmospheric pressure.
Buoyancy
Static liquids in uniform gravitational fields also exhibit the phenomenon of buoyancy, where objects immersed in the liquid experience a net force due to the pressure variation with depth. The magnitude of the force is equal to the weight of the liquid displaced by the object, and the direction of the force depends on the average density of the immersed object. If the density is smaller than that of the liquid, the buoyant force points upward and the object floats, whereas if the density is larger, the buoyant force points downward and the object sinks. This is known as Archimedes' principle.Surfaces
Unless the volume of a liquid exactly matches the volume of its container, one or more surfaces are observed. The presence of a surface introduces new phenomena which are not present in a bulk liquid. This is because a molecule at a surface possesses bonds with other liquid molecules only on the inner side of the surface, which implies a net force pulling surface molecules inward. Equivalently, this force can be described in terms of energy: there is a fixed amount of energy associated with forming a surface of a given area. This quantity is a material property called the surface tension, in units of energy per unit area. Liquids with strong intermolecular forces tend to have large surface tensions.A practical implication of surface tension is that liquids tend to minimize their surface area, forming spherical drops and bubbles unless other constraints are present. Surface tension is responsible for a range of other phenomena as well, including surface waves, capillary action, wetting, and ripples. In liquids under nanoscale confinement, surface effects can play a dominating role since – compared with a macroscopic sample of liquid – a much greater fraction of molecules are located near a surface.
The surface tension of a liquid directly affects its wettability. Most common liquids have tensions ranging in the tens of mJ/m2, so droplets of oil, water, or glue can easily merge and adhere to other surfaces, whereas liquid metals such as mercury may have tensions ranging in the hundreds of mJ/m2, thus droplets do not combine easily and surfaces may only wet under specific conditions.
The surface tensions of common liquids occupy a relatively narrow range of values when exposed to changing conditions such as temperature, which contrasts strongly with the enormous variation seen in other mechanical properties, such as viscosity.
Flow
An important physical property characterizing the flow of liquids is viscosity. Intuitively, viscosity describes the resistance of a liquid to flow. More technically, viscosity measures the resistance of a liquid to deformation at a given rate, such as when it is being sheared at finite velocity. A specific example is a liquid flowing through apipe: in this case the liquid undergoes shear deformation since it flows more slowly near the walls of the pipe
than near the center. As a result, it exhibits viscous resistance to flow. In order to maintain flow, an external force must be applied, such as a pressure difference between the ends of the pipe.
The viscosity of liquids decreases with increasing temperature.
Precise control of viscosity is important in many applications, particularly the lubrication industry. One way to achieve such control is by blending two or more liquids of differing viscosities in precise ratios.
In addition, various additives exist which can modulate the temperature-dependence of the
viscosity of lubricating oils. This capability is important since machinery often operate over a range of
temperatures.
The viscous behavior of a liquid can be either Newtonian or non-Newtonian. A Newtonian liquid exhibits a linear strain/stress curve, meaning its viscosity is independent of time, shear rate, or shear-rate history. Examples of Newtonian liquids include water, glycerin, motor oil, honey, or mercury. A non-Newtonian liquid is one where the viscosity is not independent of these factors and either thickens or thins under shear. Examples of non-Newtonian liquids include ketchup, custard, or starch solutions.