What Are States of Matter?

Matter (the stuff everything is made of) comes in different states, depending on how its particles are arranged and how much energy they have. The three states you meet every day are solid, liquid and gas. Scientists now also recognise a fourth state called plasma (think lightning and the Sun) and a fifth, ultra-cold state called the Bose-Einstein condensate. The same substance (water, for example) can change between states just by adding or removing energy. Heat ice, you get liquid water. Heat water, you get steam. Cool steam, you get water again.

  • Three everyday statesSolid, liquid, gasPlus plasma and BEC
  • What changesParticle motion + spacingParticles themselves stay the same
  • SolidFixed shape, fixed volumeParticles vibrate in place
  • LiquidVariable shape, fixed volumeParticles slide past each other
  • GasVariable shape, variable volumeParticles fly free
  • PlasmaHot ionised gasStars, lightning, neon lights

What the states mean

The state of a substance depends on two things: how strongly its particles attract each other, and how much energy they have. Heat up a substance and its particles vibrate, move and break free of their bonds, changing it from solid to liquid to gas.

  • Solid: particles are packed closely and held tight by strong attractions. They vibrate in place but cannot wander. The substance has a fixed shape and volume.
  • Liquid: particles still close together but have enough energy to slide past one another. The substance has a fixed volume but takes the shape of its container.
  • Gas: particles have so much energy that they break free of attractions and fly apart, bouncing off the walls of any container. A gas has neither a fixed shape nor a fixed volume.
  • Plasma: even more energy. The atoms are ripped apart into a soup of electrons and positive nuclei. Plasma conducts electricity, glows brightly and responds to magnetic fields.
  • Bose-Einstein condensate: at the other extreme of cold. Within a few billionths of a degree above absolute zero, certain atoms can merge into a single quantum blob that behaves like one big particle. Only made in special labs.
Fact The atoms or molecules of a substance do not change when it shifts state. Water is always H2O whether it is ice, liquid water or steam. Only the arrangement and motion of the molecules changes. That is why melting, boiling and freezing are physical changes, not chemical reactions.

Why a substance changes state

Substances change state when their temperature or pressure crosses a threshold. The key thresholds for water are:

  • 0 degrees Celsius: water freezes to ice, or ice melts to water.
  • 100 degrees Celsius: water boils to steam, or steam condenses to water.

These thresholds depend on pressure. On the top of Mount Everest the air pressure is much lower, and water boils at around 70 degrees Celsius. Inside a pressure cooker, water boils much hotter than 100 degrees, cooking food faster.

The state changes have names

Each change between states has its own name:

  • Solid -> liquid: melting
  • Liquid -> solid: freezing (or solidifying)
  • Liquid -> gas: boiling or evaporating
  • Gas -> liquid: condensing
  • Solid -> gas (skipping liquid): subliming
  • Gas -> solid (skipping liquid): depositing

See phase changes for more.

Examples around you

  • Ice in your drink: solid melts to liquid as it absorbs heat from the drink.
  • Steam from a kettle: liquid water turns to invisible water vapour, then condenses back into the visible white "steam" cloud you can see.
  • Candle flame: solid wax melts to liquid wax, which evaporates near the wick and burns as a gas. Three states at once.
  • Frost on a cold morning: water vapour deposits directly onto cold grass as ice crystals.
  • Lightning: a brief, hot column of plasma in the sky.
  • Dry ice: solid CO2 that sublimes directly into gas at -78 degrees Celsius, creating the famous theatrical fog.
Did you know? Plasma is the most common state of matter in the universe, even though it is rare on Earth. Over 99 per cent of all the matter we can see (stars, nebulae, the gas between stars) is in plasma form. We live on a tiny solid planet in an overwhelmingly plasma universe.

Other unusual states

Beyond the basic five, scientists have identified or proposed many other states of matter in special conditions:

  • Superfluids: liquid helium near absolute zero, with zero viscosity. It can flow up the walls of its container.
  • Superconductors: certain materials at very low temperatures conduct electricity with no resistance at all.
  • Liquid crystals: somewhere between liquid and solid, used in LCD screens.
  • Quark-gluon plasma: existed in the first microseconds after the Big Bang, only briefly recreated in giant particle accelerators.
  • Neutron-degenerate matter: found inside neutron stars, where atoms are so squashed that they merge into a sea of neutrons.
Try this Take an ice cube from the freezer and place it in a small bowl. Watch it slowly melt to liquid water (solid -> liquid). Then warm the liquid in a saucepan until it boils and steam rises (liquid -> gas). Hold a cold metal spoon above the steam: drops of water will form on it (gas -> liquid). Put one of those drops in the freezer and you can watch it freeze (liquid -> solid). One substance, three states, the full cycle.
Deeper dive: what is absolute zero?

How cold can things get? The lowest possible temperature is called absolute zero: minus 273.15 degrees Celsius, or 0 kelvin on the absolute scale.

At absolute zero, all classical motion of atoms and molecules would stop. Particles would settle into their lowest possible energy state, and could not give up any more heat. (In real quantum physics, particles still have a tiny "zero-point energy", but classical motion is at minimum.) Nothing can be colder, because there is no more thermal energy to remove.

Reaching absolute zero is physically impossible, but scientists have come amazingly close. The coldest temperature ever produced in a laboratory was about 38 picokelvin (3.8 x 10^-11 K), achieved using a Bose-Einstein condensate in a German lab. That is colder than anything ever found anywhere else in the known universe. Even the empty space between galaxies is around 2.7 kelvin (warmed by the leftover Big Bang radiation).

Approaching absolute zero is not just a curiosity. New states of matter (Bose-Einstein condensates, superfluids, superconductors) appear only in extreme cold. Quantum computers rely on it. Ultra-precise atomic clocks need it. Studying matter at the edge of absolute zero has won several Nobel Prizes and continues to reveal new physics.

For more, see solid, liquid and gas.