Liquid

Liquid is a state of matter with a definite volume but no fixed shape. When placed in a container and affected by forces like gravity, liquids flow to match the container's shape. Unlike solids, which keep a fixed form, liquids change their shape to fit the space around them.

Liquids are made of tiny particles called atoms or molecules that are held together by intermolecular bonds. These bonds are strong enough to keep the particles close but weak enough to let them slide past one another. 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 actually rare in space, where most matter 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

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, making them useful for moving force in machines.

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

Microscopic structure

See also: Structure of liquids and glasses

Liquids have a complex internal structure. Their molecules are packed closely together but not in a regular pattern, unlike solids. This makes liquids different from both gases, where molecules are far apart, and solids, where molecules are arranged in a fixed pattern.

Liquids do not have long-range order like crystals, but they do have short-range order. This means that molecules are arranged in a somewhat ordered way over a few molecular distances. In simple liquids, this order comes from the way spheres pack together. In more complex liquids, the shapes and forces between molecules create local structures, such as networks in water. These structures are constantly changing due to temperature and movement.

Liquids behave differently depending on the balance between forces that pull molecules together and forces that keep them apart. This balance makes it hard to describe liquids using simple models, unlike gases or solids. Additionally, quantum effects can play a role in some liquids, especially at very low temperatures or with very light molecules like hydrogen and helium.

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, reaching a balance between the two. If the liquid is kept at or above its boiling point, it will usually turn into gas, unless special conditions stop this from happening.

Below the freezing point, a liquid will change into a solid. Unlike turning into a gas, this change does not balance out under constant pressure, so the liquid will eventually become solid entirely, unless special cooling keeps it liquid. This balance can happen only 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 form solutions. When two liquids can mix in any amount, they are called miscible. For example, water and ethanol (drinking alcohol) mix well, but water and gasoline do not. Sometimes, special mixtures called emulsions can be made where one liquid is broken into tiny droplets spread throughout 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

Liquids have many useful roles because of their ability to flow and take the shape of 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

See also: Computational materials science

There are different ways to figure out what liquids will do and what they are like. We can group these ways by how big or small the pieces we look at are.

• Macroscopic methods look at big-picture ideas like how a liquid moves or changes with temperature and pressure.
• Microscopic methods look at what individual tiny parts, called molecules, are doing.
• Mesoscopic methods sit in the middle, mixing ideas from both big and tiny views.

Macroscopic

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

We can also use rules that tell us 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 describe how liquids move and change over space and time. These theories use math rules to predict things like how fast a liquid flows.

Mesoscopic

Methods that sit between looking at tiny parts and big pictures mix 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 figure out the liquid’s overall traits. Sometimes we use simple rules for how these tiny parts push and pull each other. Other times, we use very 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

See also: Car–Parrinello molecular dynamics