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. Molecules with weak attractive forces form crystals with low melting points, and crystals with stronger attractions melt at higher temperatures. 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. 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.

Matter is anything that has mass and takes up space. Solids, liquids and gases are the three states of matter commonly found on Earth. Matter can act like more than one state at once: a cloud is a mixture of air, a gas, and tiny drops of water, liquid or solid. Sand can be poured like a liquid even though every grain is a solid. In zero gravity, a liquid forms a ball shape instead of settling flat.

Dew is made of tiny water drops that form by condensation on cool surfaces such as grass and spider webs, and eyeglass lenses fog up when you walk from a cold room into warm, humid air, because water vapor condenses on them. On a global scale, the amount of water evaporating is about the same as the amount falling as precipitation. Over the oceans, evaporation is more common than precipitation. About 90 percent of the moisture in the atmosphere came from oceans, seas, lakes and rivers. Seawater freezes at about 28.4 degrees Fahrenheit, lower than fresh water's 32 degrees, because of the salt in it. When seawater freezes, the ice has very little salt in it, because only the water part freezes.

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. It has no surface tension, because it has no boundary between a liquid and a vapor. Supercritical fluids dissolve liquids and solids easily because of their high density. Carbon dioxide's critical point is at a much milder temperature and pressure than water's. 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.

The line separating liquid from gas on a phase diagram gives the boiling point at any pressure. 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. Ice floats because it is less dense than liquid water, but in most other substances the solid is denser than the liquid. Water expands when it freezes, so ice takes up more room than the liquid water it came from. A glacier's weight puts great pressure on its bottom, which can melt some ice into a layer of water it slides on. 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.

Most solids are crystalline, with particles in a regular pattern, but a liquid that freezes before its molecules line up becomes an amorphous solid. Candle wax is an amorphous solid made of large hydrocarbon molecules. Rapidly cooling molten silicon dioxide makes an amorphous solid called fused silica. Diamond, a crystal of carbon atoms, is one of the hardest substances known and melts above 3500 degrees Celsius. Graphite is very soft and conducts electricity, unlike most covalent solids, because its layers are held together by weaker forces. Metals are held by metallic bonding, which makes them good conductors and malleable, so they bend instead of shattering. Mercury is a liquid metal at room temperature, and alkali metals melt below 200 degrees Celsius, while transition metals melt above 1000 degrees Celsius. Solids can expand and contract a little when the temperature changes.

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, and 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. 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. 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 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. 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.

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. Physicists usually make BECs by trapping bosonic atoms in a magnetic trap and cooling them with laser beams. 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. NASA's Cold Atom Lab, launched in 2018, is housed in the Destiny module of the International Space Station and made BECs from rubidium-87 atoms, as of November 2026.

Oobleck is a mixture of cornstarch and water that is a non-Newtonian fluid: the harder you stir it, the thicker it gets. Hit with a hammer, oobleck hardens enough to shatter, and it is named after a gooey substance that fell from the sky in a Dr. Seuss story. A person can run across a pool of oobleck because the pressure of a foot hitting the surface makes the liquid thicken under it. Its thickening under force is called shear thickening, and it happens in mixtures of tiny solid particles in a fluid. Shear-thickening fluids are used in drilling mud and in one type of body armor that is flexible until a bullet's impact turns it solid. For decades scientists argued over why shear thickening happens: friction locking particles together, or liquid squeezed out between particles that locks them in clusters.

Words to know

• supercritical: above critical point
• amorphous: no set pattern
• condensate: cold atom gas
• viscosity: resistance to flow
• malleable: able to bend