What are the states of matter? Solid, liquid, gas and plasma

The states of matter are the different forms the same substance can take, depending on what its particles are doing. In a solid they vibrate in fixed places; in a liquid they slide past each other; in a gas they fly about freely; in a plasma they move so violently that electrons are torn off their atoms. The four everyday states are solid, liquid, gas and plasma, and adding or removing energy moves matter between them.

In short

• A state of matter is a description of how particles move and how tightly they hold on to each other, not of what they are made of. Ice, water and steam are the same molecules.
• Changing state takes energy in or out, and while the change is happening the temperature stays put.
• Plasma, an electrically charged gas, is rare on Earth but makes up the stars, so it is the most common state of the ordinary matter that can be seen in the universe.

What are the four common states of matter?

Each state comes from a contest between two things: the attraction that pulls particles together, and their thermal motion, which shakes them apart. Turn up the motion, by heating, and the attractions lose their grip one level at a time.

State	What the particles do	Shape and volume	Everyday examples
Solid	Vibrate around fixed positions, held tightly by their neighbors	Keeps its own shape and volume	Ice, rock, metal, wood
Liquid	Stay close together but slide past each other	Takes the shape of its container, keeps its volume	Water, oil, blood
Gas	Far apart, fly freely in straight lines between collisions	Fills whatever container it is in	Air, steam, helium in a balloon
Plasma	A gas so hot or energized that electrons have been stripped from atoms, leaving a mix of charged ions and free electrons	Like a gas, but it conducts electricity and responds to magnetic fields	The Sun, lightning, neon signs, the aurora

In a gas, the molecules are typically about ten times farther apart than in a liquid: a liter of water boils into about 1,600 liters of steam at normal pressure. That expansion is what drove steam engines.

What is plasma?

Plasma is sometimes called the fourth state of matter. Heat a gas enough, or hit it with enough electrical energy, and collisions knock electrons free of their atoms. The result is still gas-like, but full of charged particles, so it carries current, glows and can be steered by magnets. The chemist Irving Langmuir gave it the name in 1928.

On Earth plasma is mostly fleeting: a lightning bolt, the inside of a fluorescent tube or a neon sign, the glow of the northern lights, where particles from the Sun strike the upper atmosphere. In space it is the rule. Stars are almost entirely plasma, which is why the Sun is described as hot ionized gas, and estimates put the share of the universe's ordinary matter in the plasma state at about 99.9%.

Beyond these four, physicists have made more exotic states in the lab. The best known is the Bose–Einstein condensate, in which atoms cooled to less than a millionth of a degree above absolute zero merge into a single quantum state. It was first made in 1995, and Eric Cornell, Wolfgang Ketterle and Carl Wieman shared the 2001 Nobel Prize in Physics for it.

What are the changes between states called?

From	To	Name	Example
Solid	Liquid	Melting	Ice cube in a drink
Liquid	Solid	Freezing	Water in a freezer
Liquid	Gas	Vaporization (boiling, or evaporation below the boiling point)	Puddle drying, kettle boiling
Gas	Liquid	Condensation	Drops on a cold glass
Solid	Gas	Sublimation	Dry ice, solid carbon dioxide, at −78.5 °C
Gas	Solid	Deposition	Frost forming on a window
Gas	Plasma	Ionization	A neon sign switching on

Every change toward a looser state (melting, vaporization, sublimation, ionization) takes energy in, and the reverse change gives the same energy back out. That is why sweating cools you: evaporating water carries heat away from your skin.

A worked example: turning 1 kg of ice at −10 °C into steam

Heating matter does two different jobs. Sometimes the energy speeds the particles up, and the temperature rises. During a change of state, it goes into breaking the attractions between particles instead, and the temperature stays flat. Follow 1 kilogram of ice from a freezer at −10 °C to steam at 100 °C, at normal sea-level pressure, using the values in OpenStax College Physics: specific heat of ice 2,090 J per kg per degree, of water 4,186 J per kg per degree, latent heat of melting 334,000 J/kg and of boiling 2,256,000 J/kg.

1. Warm the ice from −10 °C to 0 °C: 1 kg × 2,090 × 10 = 20,900 J. The temperature climbs.
2. Melt the ice at 0 °C: 1 kg × 334,000 = 334,000 J. The temperature stays at 0 °C the whole time.
3. Warm the water from 0 °C to 100 °C: 1 kg × 4,186 × 100 = 418,600 J. The temperature climbs again.
4. Boil the water at 100 °C: 1 kg × 2,256,000 = 2,256,000 J. The temperature stays at 100 °C until the last drop is gone.

Total: about 3,030,000 J, or 3.03 megajoules of energy. On a burner delivering a steady 2,000 watts with nothing wasted, the whole job would take about 25 minutes. Melting alone would take nearly 3 minutes, about as long as warming the meltwater 80 degrees, and the boiling stage would take about 19 minutes.

Three-quarters of all the energy goes into the last step, turning liquid water into gas, without raising its temperature by a single degree. The energy goes into pulling the molecules apart from one another, against the hydrogen bonds that hold liquid water together. The same energy comes back out when steam condenses, which is why a scald from steam is worse than one from boiling water: the steam releases 2,256 kJ per kilogram on your skin before it even starts to cool.

Does water always boil at 100 °C?

No. A substance's state depends on pressure as well as temperature. At lower pressure, molecules escape from a liquid more easily, so the boiling point drops: on the summit of Mount Everest, where air pressure is about a third of sea level's, water boils at about 70 °C, which is why food takes longer to cook at altitude. A pressure cooker does the opposite, raising the pressure so water stays liquid above 100 °C and cooks food faster.

Freezing can be delayed too. Very pure water with nothing for ice crystals to form on can stay liquid well below 0 °C, which is called supercooling. Clouds are full of supercooled droplets, and giving them something to freeze onto is the idea behind cloud seeding.

Common mix-ups about states of matter

• The white cloud above a kettle isn't steam. Steam, water as a gas, is invisible. The cloud is tiny droplets of liquid water that have already condensed in the cooler air.
• Glass isn't a liquid that flows very slowly. It is an amorphous solid, whose atoms are frozen in a disordered arrangement rather than a neat crystal. Old church windows thicker at the bottom were made unevenly, not drooped over centuries.
• Ice is the odd one out. Most substances are denser as solids than as liquids, but ice, at about 0.92 grams per cubic centimeter, is less dense than liquid water, so it floats. Lakes freeze from the top down, and the fish below survive the winter.
• Gases are not weightless. A cubic meter of air at sea level has a mass of about 1.2 kilograms.

The states of matter are where matter and energy meet: the same particles take on wholly different behavior depending only on how much energy they have and how hard they are squeezed.

Check yourself

• Without looking back, explain why the temperature of melting ice stays at 0 °C even while it is being heated.
• Which step in turning ice into steam takes the most energy, and what is that energy actually doing?
• What makes a plasma different from an ordinary gas?

Sources

• Phase change and latent heat — OpenStax, College Physics 2e
• States of matter — NASA Glenn Research Center
• The Nobel Prize in Physics 2001 — Nobel Prize Outreach
• Plasma (physics) — Wikipedia
• State of matter — Wikipedia