Liquid

Liquid is a state of matter that has a definite volume but no fixed shape. When you put a liquid in a container, it flows to match the shape of that container. Unlike solids, which keep the same shape, liquids change to fit the space around them.

The formation of a spherical droplet of liquid water minimizes the surface area, which is the natural result of surface tension in liquids.

Liquids are made of tiny particles called atoms or molecules. These particles are held together by intermolecular bonds. The bonds are strong enough to keep the particles close, but weak enough to let them slide past each other. This is why liquids can flow. In solids, particles are locked in place, while in gases, particles move freely and are far apart.

Liquids change with temperature. When heated, they can turn into gases at their boiling point. When cooled, they can become solids at their freezing point. Water is a familiar liquid, but liquids are rare in space. Most matter in space is found as gas in interstellar clouds or as glowing plasma in stars.

Examples

Only two elements are liquid at normal temperature and pressure: mercury and bromine. Other elements like francium, caesium, gallium, and rubidium become liquid just above room temperature.

Many everyday liquids are mixtures. For example, alloys such as galinstan stay liquid at room temperature. Common liquids include water, ethanol, household bleach, and milk. Gases like liquid oxygen and liquid nitrogen can be turned into liquids by cooling.

Properties

Cavitation in water from a boat propeller

Liquids have a fixed volume but can change shape to fit their container. We measure liquid amounts using units like liters and milliliters. Liquids stay the same size even when squeezed, which helps them move force in machines.

In liquids, pressure increases with depth. This pressure pushes in all directions and can make objects float or sink. Surface tension lets liquids form droplets and changes how they spread on surfaces. Viscosity tells us how easily a liquid flows, and it changes with temperature.

Microscopic structure

Liquids have a special internal structure. Their tiny parts, called molecules, are close together but not in a neat pattern, unlike solids. This makes liquids different from gases, where molecules are far apart, and solids, where molecules are in a fixed pattern.

Liquids do not have a strict order like crystals, but they have a bit of order over short distances. This means that molecules are arranged in a somewhat ordered way only for a few molecular distances. In simple liquids, this order comes from how spheres fit together. In more complex liquids, the shapes and forces between molecules create local structures, like networks in water. These structures change all the time because of temperature and movement.

Liquids act differently based on the balance between forces that pull molecules together and forces that keep them apart. This balance makes it tricky to describe liquids using simple models, unlike gases or solids. Also, special effects from physics can affect some liquids, especially at very low temperatures or with very light molecules like hydrogen and helium.

Table 1: Thermal de Broglie wavelengths Λ {\displaystyle \Lambda } of selected liquids. Quantum effects are negligible when the ratio Λ / a {\displaystyle \Lambda /a} is small, where a {\displaystyle a} is the average distance between molecules.
Liquid	Temperature (K)	Λ {\displaystyle \Lambda } (nm)	Λ / a {\displaystyle \Lambda /a}
Hydrogen (H₂)	14.1	0.33	0.97
Neon	24.5	0.078	0.26
Krypton	116	0.018	0.046
Carbon tetrachloride (CCl₄)	250	0.009	0.017

Phase transitions

Main articles: Boiling, Boiling point, Melting, and Melting point

When a liquid is below its boiling point, it can turn into a gas and then back into a liquid. If the liquid is kept at or above its boiling point, it will usually turn into gas.

Below the freezing point, a liquid will change into a solid. This change will keep going until the liquid becomes solid, unless special cooling keeps it liquid. This balance can only happen under certain conditions, like in a strong, closed container.

Because space has almost no pressure, liquids cannot stay liquid there. They will either turn to gas or freeze, depending on the temperature. Near Earth, water in space will freeze if it’s in shadow and turn to vapor when sunlight hits it. On the Moon, ice can only exist in dark areas where the sun doesn’t shine. Far from the Sun, like near Saturn, ice stays because the sunlight is too weak to change it.

Solutions

Main article: Solution (chemistry)

Liquids can mix with gases, solids, and other liquids to make solutions. When two liquids mix in any amount, they are called miscible. For example, water and ethanol mix well, but water and gasoline do not. Some special mixtures are called emulsions. In emulsions, one liquid is broken into tiny droplets spread in another liquid. This usually needs a surfactant to keep the droplets stable. A common example is mayonnaise, made from water and oil and kept together by lecithin found in egg yolks.

Applications

A lava lamp contains two immiscible liquids (a molten wax and a watery solution) which add movement due to convection. In addition to the top surface, surfaces also form between the liquids, requiring a tension breaker to recombine the wax droplets at the bottom.

Liquids are very useful because they can flow and change shape to fit their container. They are often used as lubricants, like oil in engines and machines, to help parts move smoothly. Liquids can also dissolve other materials, making them useful as solvents in paints, cleaning products, and many everyday items.

Liquids help control temperature because they move well and can carry away heat. For example, water cools car engines, and sweat helps cool our bodies. In cooking, liquids help spread heat evenly, making food cook properly. Liquids are also important in hydraulic systems, where fluids like oil create force to power machines, brakes, and more.

Prediction of liquid properties

There are different ways to learn about what liquids will do and what they are like. We can sort these ways by how big or small the pieces we study are.

Macroscopic methods look at big ideas like how a liquid moves or changes with temperature and pressure.
Microscopic methods look at what tiny parts, called molecules, are doing.
Mesoscopic methods mix ideas from both big and tiny views.

Macroscopic

One easy way to guess a liquid’s traits is by using simple math rules that match what we see in tests. For example, we can use a math rule to guess how thick a liquid is at different temperatures.

We can also use rules to learn about the balanced state of a liquid — like how much space it takes up or how much energy it has — by looking at its pressure and temperature.

Another way is to use theories that explain how liquids move and change over space and time. These theories use math rules to guess things like how fast a liquid flows.

Mesoscopic

Methods that mix tiny parts and big pictures use ideas from both. One example is a method called the lattice Boltzmann method, which pretends the liquid is made of tiny pretend particles moving on a grid.

Microscopic

We can also watch what each tiny part of a liquid does, moving step by step, to learn the liquid’s overall traits. Sometimes we use simple rules for how these tiny parts push and pull each other. Other times, we use careful science rules that look at how atoms and electrons behave, but this takes a lot of computing power.

Main articles: Molecular dynamics and Molecular mechanics