Scientists describe the state of a substance as a balance between the kinetic energy of its particles, which keeps them apart and moving, and the forces that pull them together. Forces between molecules are much weaker than the bonds inside a molecule: pulling HCl molecules apart takes about 17 kJ per mole, but breaking its bond takes about 430. Bigger, heavier atoms and molecules attract each other more strongly: fluorine and chlorine are gases at room temperature, bromine is a liquid and iodine is a solid. Liquids with strong attractions between molecules, like water with its hydrogen bonds, have lower vapor pressures than liquids with weak attractions. Vaporizing a liquid takes energy: turning water to vapor at standard temperature takes 44.01 kilojoules per mole. Melting takes less energy than boiling, because only part of the attractions between particles are overcome when a solid becomes a liquid, but all of them when it becomes a gas.
In a gas, the particles are far apart and move around on their own. The average kinetic energy of a gas molecule is directly proportional to its absolute temperature. At the same temperature, helium and xenon gas molecules have the same average kinetic energy, even though their masses are very different. The speed of sound, about 340 m/s at room temperature, reflects the high rms speed of air molecules. Gas molecules collide with one another, and those collisions keep their motion random. Heating a gas speeds up its molecules, and gas pressure comes from molecules colliding with the walls of their container. The more gas molecules there are in a container, the more collisions there are with the walls and the higher the pressure. Pressure is force divided by the area it pushes on, so the same force pushes harder on a smaller area. One kilopascal is 1000 pascals, and one bar is 100,000 pascals.
A liquid boils when its vapor pressure rises to equal the pressure of the air around it. The normal boiling point of a liquid is its boiling point when the surrounding pressure is 1 atmosphere, or 101.3 kPa. As temperature rises, more molecules in a liquid have enough kinetic energy to escape into the gas phase, so its vapor pressure goes up. In a closed container, liquid and vapor reach a dynamic equilibrium: molecules keep moving both ways at equal rates, so the amounts stop changing. Scientists track phase changes with heating curves: temperature climbs, then levels off in flat plateaus wherever the substance melts or boils. During a phase change, added heat goes into weakening the attractions between particles instead of making them move faster. A phase diagram plots pressure against temperature and shows which state a substance is in under each condition. The triple point is the one temperature and pressure where the solid, liquid and gas phases of a substance exist together in equilibrium.
Until 2019, the kelvin temperature unit was defined as 1/273.16 of the temperature of the triple point of water. Since 2019 the kelvin is defined using the Boltzmann constant instead of water, but as of November 2026 the triple point of water is still used to calibrate thermometers. A triple-point cell is a sealed glass cylinder nearly full of very pure water, used to make a thermometer-calibrating temperature. Tiny differences in the hydrogen and oxygen isotopes in water change its triple-point temperature, so labs use one agreed formulation, Vienna Standard Mean Ocean Water. Water's melting point falls slightly as pressure rises, which is unusual because for most substances it rises. A glacier's weight puts great pressure on its bottom, which can melt some ice into a layer of water it slides on.
Above its critical temperature, a gas cannot be turned into a liquid no matter how much pressure is applied. Water's critical point is 374 degrees Celsius and 218 atmospheres, and above it water is a supercritical fluid with properties between those of liquid and gas. A supercritical fluid has the low viscosity of a gas and the high density of a liquid, and there is no boundary between its liquid and gas. It is possible to go from gas to liquid without ever crossing the boiling line, by going around the critical point through the supercritical range. Supercritical fluids were discovered in 1822 by Baron Charles Cagniard de la Tour, who listened to a flint ball in a sealed cannon barrel. Supercritical water forms naturally near deep underwater volcanoes, from the huge pressure of the depth and the heat of the vents. Carbon dioxide's critical point is at a much milder temperature and pressure than water's.
Is glass a liquid? The myth that glass flows comes from old window panes that are thicker at the bottom than the top. About half of the glass in old stained glass windows is thicker at the bottom and the other half is thicker elsewhere, which flowing would not explain. If old panes were thicker at the bottom because glass flows, glass from Egyptian tombs would be a puddle, a glass chemist pointed out. Glass is neither a liquid nor an ordinary solid: it is an amorphous solid, a state somewhere between the two. It is made by cooling a liquid so quickly that it does not crystallize, and below its glass-transition temperature its atoms nearly stop moving. Glass atoms move too slowly for the liquid-like properties of glass to make windows visibly thicker at the bottom. Over very long times the molecules of glass can shift into a more stable, crystal-like arrangement.
Plasma is an ionized state of matter made of positively charged ions and negatively charged electrons. In plasma, some electrons separate from their atoms, and these free electrons make plasma different from other states of matter. Unlike a gas, plasma can conduct electricity. Plasma makes up about 99 percent of the visible matter in the universe, much of it in space, such as in nebulae and stars. In space, gravity can squeeze gas so hard that the pressure superheats it and turns it into plasma, which is how a star begins. Most plasma in the universe is high-temperature plasma, which can be hotter than 10,000 degrees Fahrenheit, but low-temperature plasma can be as cool as room temperature. In a lab, scientists make plasma by blasting atoms with high-voltage electricity, lasers or electromagnetic fields. Studying plasma helps scientists toward fusion energy, and plasma research has improved the making of the semiconductors in phones and computers.
Just after the Big Bang, the universe was filled with a hot, dense quark-gluon plasma where quarks and gluons moved freely. CERN and other labs recreate quark-gluon plasma by smashing heavy ions such as gold or lead nuclei together head-on. In a heavy-ion collision the nuclei melt into a tiny fireball that cools instantly and recombines into ordinary matter such as protons and neutrons. Jets in heavy-ion collisions were first seen in 2003 at Brookhaven's RHIC, where one of the two back-to-back jets was always quenched, sometimes weakened and sometimes extinguished. As of November 2026, the ALICE, ATLAS and CMS experiments at the Large Hadron Collider have confirmed jet quenching in heavy-ion collisions.
A Bose-Einstein condensate, or BEC, is called the fifth state of matter. It is a dilute gas of bosonic atoms so cold that the atoms all occupy the same quantum state and act in unison as a superfluid. Bosons are particles with whole-number spin and tend to gather in the lowest-energy quantum state at low temperatures. The first Bose-Einstein condensate in a dilute gas was made in June 1995 by Eric Cornell and Carl Wieman's group at JILA, using rubidium-87. The 2001 Nobel Prize in Physics was shared by Eric Cornell, Wolfgang Ketterle and Carl Wieman for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms. A BEC was reached by laser cooling the atoms, moving them to a purely magnetic trap, and then applying evaporative cooling.
On Earth, gravity pulls BEC atoms into the apparatus within a fraction of a second after release, so scientists use drop towers, parabolic flights and rockets for free fall. In orbit, objects are in continuous free fall, so a space BEC can be released slowly and cooled to even lower temperatures. As of November 2026, NASA's Cold Atom Lab, launched in 2018 and housed in the Destiny module of the International Space Station, made BECs from rubidium-87 atoms. In a 2020 report, its condensates were detectable for up to 1.18 seconds, which one outside physicist called comparable with the best Earth-based labs. In the Cold Atom Lab, atoms are cooled to less than a billionth of a kelvin using an atom chip and radio waves. As of November 2026, the lab is operated remotely, fits in just 0.4 cubic metres and cost $70 million.
Oobleck is a mixture of cornstarch and water that is a non-Newtonian fluid: the harder you stir it, the thicker it gets. Its thickening under force is called shear thickening, and it happens in mixtures of tiny solid particles in a fluid. For decades scientists argued over why shear thickening happens: friction locking particles together, or liquid squeezed out between particles that locks them in clusters. In a Cornell experiment that suddenly reversed the shear, there was negligible resistance pulling back, which the researchers took to mean that friction between particles is the primary mechanism of shear thickening. The researchers used a lab-grade suspension of spherical microparticles so only particle interactions would matter. Cornell researcher Neil Lin said that knowing the mechanism lets engineers tune a shear-thickening fluid by adjusting the friction of its particles, or by varying their concentration.
Words to know
- kinetic: moving energy
- supercritical: above critical point
- amorphous: no set pattern
- condensate: cold atom gas
- quenched: weakened, wiped out
- viscosity: resistance to flow
