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. 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. The speed of sound, about 340 meters per second at room temperature, reflects the high rms speed of air molecules. Liquids have short-range order only: their molecules pack tightly but move rapidly past one another. Molecules in liquids diffuse much more slowly than in gases, because a molecule cannot go far before colliding with another. A liquid has a definite volume that stays the same whatever the size or shape of its container.

Liquids with strong attractions between molecules, like water with its hydrogen bonds, have lower vapor pressures than liquids with weak attractions. Molecules with weak attractive forces form crystals with low melting points, and crystals with stronger attractions melt at higher temperatures. The melting point of a solid and the freezing point of its liquid are the same temperature: where solid and liquid are in equilibrium. 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. 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. At normal air pressure, water boils at 100 degrees Celsius. High up in the mountains, water boils lower, at about 90 degrees in Leadville, Colorado, elevation 10,200 feet. The line separating liquid from gas gives the boiling point at any pressure. 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. Ice placed in a vacuum with a pressure below 0.20 kPa will sublime instead of melting. Scientists track phase changes with heating curves: temperature climbs, then levels off in flat plateaus wherever the substance melts or boils.

The triple point is the one temperature and pressure where the solid, liquid and gas phases of a substance exist together in equilibrium. Below water's triple-point pressure, water cannot be a liquid at any temperature. Carbon dioxide's triple point is well above 1 atmosphere, so cooling the gas at 1 atmosphere makes solid directly, without a liquid. 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. 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 has been 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 gas is easy to squeeze into a smaller space, but liquids and solids hardly change their size when squeezed. Squeezing a gas can also turn it into a liquid, if the gas is not too hot. Gases can be turned into liquids by cooling them, and liquids into solids by cooling them more. But 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. 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.

Glass is neither a liquid nor an ordinary solid: it is an amorphous solid, a state somewhere between the two. Glass is made by cooling a liquid so quickly that it does not crystallize. Below its glass-transition temperature, its atoms nearly stop moving. The word glass covers any material that turns into an amorphous solid. Obsidian is a natural glass, and some metals can be made glassy. The myth that glass flows comes from old window panes that are thicker at the bottom than the top. Glass atoms move too slowly for the liquid-like properties of glass to make windows visibly thicker at the bottom. For practical purposes, such as holding a drink, glass is like a solid, although a disorganized one.

Plasma is an ionized state of matter made of positively charged ions and negatively charged electrons. In plasma, some electrons separate from their atoms. In space, gravity can squeeze gas so hard that the pressure superheats it and turns it into plasma, which is how a star begins. 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. 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.

Words to know

• kinetic: moving energy
• sublime: solid to gas
• supercritical: above critical point
• amorphous: no set pattern
• plasma: charged gas
• condensate: cold atom gas