Liquid

Liquid is a state of matter that has a fixed volume but no set shape. When placed in a container and acted upon by a force like gravity, it takes on the container's internal shape in the direction of that force. Liquids are very difficult to compress, so they keep their volume even under pressure. Their density is generally similar to that of a solid and much greater than that of a gas. Liquids belong to the category of condensed matter (along with solids) and are also a type of fluid (along with gases). A liquid consists of atoms or molecules held together by intermolecular bonds of medium strength. These bonds let the particles slide past each other while staying close together. In solids, strong intermolecular forces keep particles tightly bound, limiting them to small vibrations in fixed spots. In gases, particles are far apart and move freely, with only weak forces between them. When temperature rises, the molecules in a liquid vibrate more strongly, pushing them farther apart. At the boiling point, the cohesive forces can no longer hold the molecules together, and the liquid turns into a gas. When temperature drops, the molecules move closer together. At the freezing point, they usually arrange into a structured pattern through crystallization, and the liquid becomes a solid. Even though liquid water is common on Earth, liquid is actually the least common state of matter in the known universe. This is because liquids need a fairly narrow 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 at 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 crucial for chemistry and biology and is necessary for all known life. Inorganic liquids in this group include nonaqueous 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 (alloys with mercury). Some 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 like 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 instead of 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 unusual because 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. The SI unit is the cubic metre (m³), with common subdivisions including the cubic decimetre (more often called the litre: 1 dm³ = 1 L = 0.001 m³) and the cubic centimetre (also called the millilitre: 1 cm³ = 1 mL = 0.001 L = 10⁻⁶ m³). The volume of a 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 not very compressible. For example, water compresses by only 46.4 parts per million for each additional atmosphere of pressure (bar). At about 4000 bar (400 megapascals or 58,000 psi) at room temperature, water’s volume decreases by just 11%. This incompressibility makes liquids useful for transmitting hydraulic power, because a pressure change at one point in a liquid is passed on unchanged to every other part, with very little energy lost to compression. However, this low compressibility also causes other effects. Water hammer—the banging of pipes—happens when a valve closes suddenly, creating a huge pressure spike at the valve that travels backward through the system at nearly the speed of sound. Another result is cavitation. Because liquids have little elasticity, they can be pulled apart in areas of high turbulence or sharp changes in direction, like the trailing edge of a boat propeller or a sharp pipe corner. In a low-pressure (vacuum) area, the liquid vaporizes and forms bubbles, which then collapse when they enter high-pressure areas. This collapse forces liquid into the cavities left by the bubbles with tremendous localized force, eroding any nearby solid surface.

**Pressure** In a gravitational field, liquids push against the sides of a container and against anything inside the liquid. This pressure is transmitted in all directions and increases with depth. If a liquid is still in a uniform gravitational field, the pressure \( p \) at a depth \( z \) is given by \( p = p_0 + \rho g z \), where \( p_0 \) is the pressure at the surface, \( \rho \) is the liquid’s density, and \( g \) is the acceleration due to gravity.