States of Matter
Matter is normally found as a solid, a liquid or a gas, and at very high temperatures as plasma. The state depends on a contest between the attractions holding particles together and the particles' energy of motion, which rises with temperature. Melting, boiling and condensing change how the particles are arranged, not what they are, so they are physical changes rather than chemical reactions.
Phase behaviour of water
NIST's Chemistry WebBook gives water's boiling point as 373.17 ± 0.04 K, its triple-point pressure as about 0.0061 bar and its critical point near 647 K and 220.64 bar. Each line on a phase diagram is a set of pressure–temperature pairs at which two phases coexist, so boiling temperature depends on pressure. With NIST's Antoine fit, log₁₀(P/bar) = 5.08354 − 1663.125/(T − 45.622), P = 1.01325 bar gives T ≈ 373.15 K and P = 0.5 bar gives T ≈ 354.5 K, about 81 °C (ScienceVerse calculation).
In the kinetic picture, temperature is a measure of mean molecular kinetic energy. A state change occurs when that energy is comparable to the energy of the intermolecular attractions, which is why substances with stronger intermolecular forces have higher melting and boiling points.
Plasma, produced at very high temperatures as on the Sun, is an ionised (often only partially ionised) mixture of neutral atoms, free electrons and positive ions.
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Solids, liquids and gases
- Solid: keeps its own shape and volume; particles are closely packed and vibrate in place.
- Liquid: fixed volume but takes the shape of its container; particles are close together but can slide past each other.
- Gas: fills whatever container it is in; particles are far apart and move freely.
Liquids and gases can flow, so both are called fluids. In any fluid the particles are in constant, random motion, colliding with each other and with the container walls.
Why substances change state
Particles attract each other through intermolecular forces, which are much weaker than the chemical bonds inside molecules. Temperature measures the particles' average kinetic energy. Heat a solid and its particles eventually have enough energy to move past each other (it melts); heat further and they escape into a gas (it boils). Cooling reverses this: water vapour condenses on a cold glass.
Pressure matters too. Squeezing a gas pushes its particles together, so many gases can be liquefied by compression — the fuel in a disposable lighter is butane stored as a liquid this way. Each state change happens at particular combinations of pressure and temperature, so boiling points shift with pressure. Using NIST's vapour-pressure fit for water, water boils at about 81 °C when the pressure is halved to 0.5 bar, compared with 100 °C at standard atmospheric pressure (ScienceVerse calculation).
Worked example: predicting a state
Chlorine melts at −101.5 °C and boils at −34.04 °C. At 20 °C it is above its boiling point, so it is a gas. At −50 °C it is between the two, so it would be a liquid; below −101.5 °C it would be a solid.
The fourth state: plasma
At very high temperatures, like those on the Sun, electrons are stripped from atoms. The result — a mixture of neutral atoms, free electrons and positive ions — is plasma, which behaves differently from an ordinary gas.
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Sources and methodology
- Matter normally exists as a solid, a liquid or a gas: a solid holds its own shape and has a fixed volume, a liquid has a fixed volume but takes the shape of its container, and a gas fills its container, taking both its shape and its volume. (awaiting scientific review)
- Phases of Matter (Beginner's Guide to Aeronautics) — Government or standards body
- In solids, particles are closely packed and vibrate in place; in liquids they are still close together but can move past one another; in gases they are far apart and move freely. (awaiting scientific review)
- The state of a substance depends on the balance between the intermolecular forces holding its particles together and the particles' kinetic energy, which is set by the temperature. (awaiting scientific review)
- The temperature of a gas is a measure of the mean kinetic energy of its molecules, which are in constant random motion. (awaiting scientific review)
- Kinetic Theory of Gases (Beginner's Guide to Aeronautics) — Government or standards body
- A change of state is a physical change, not a chemical one: water vapour, liquid water and ice all have the same chemical composition, H₂O. (awaiting scientific review)
- Phases of Matter (Beginner's Guide to Aeronautics) — Government or standards body
- Plasma forms at very high temperatures, such as those on the Sun, when electrons are stripped from atoms, leaving a mixture of neutral atoms, free electrons and positively charged ions. (awaiting scientific review)
- Phases of Matter (Beginner's Guide to Aeronautics) — Government or standards body
- NIST's Chemistry WebBook lists the boiling point of water as 373.17 ± 0.04 K, which is about 100 °C. (awaiting scientific review)
- NIST Chemistry WebBook (SRD 69): Water — phase change data — Primary dataset
- Water's critical temperature is about 647 K and its critical pressure about 220.64 bar, while its triple-point pressure is about 0.0061 bar. (awaiting scientific review)
- NIST Chemistry WebBook (SRD 69): Water — phase change data — Primary dataset
- NIST lists Antoine-equation parameters for the vapour pressure of water, log₁₀(P/bar) = A − B/(T/K + C), with A = 5.08354, B = 1663.125 and C = −45.622 for 344–373 K (from Bridgeman and Aldrich, 1964). (awaiting scientific review)
- NIST Chemistry WebBook (SRD 69): Water — phase change data — Primary dataset
- The curves of a phase diagram give the temperature–pressure pairs at which two phases are in equilibrium, and the liquid–vapour curve ends at the critical point, above which the boundary between liquid and vapour disappears and the substance is a single phase called a supercritical fluid. (awaiting scientific review)
- Intermolecular forces, the attractions between molecules, are much weaker than the chemical bonds within molecules. (awaiting scientific review)
- Many gases can be liquefied by compressing them if the temperature is not too high; butane, the fuel in disposable lighters, is a gas at standard temperature and pressure but is held as a liquid in the lighter's fuel compartment. (awaiting scientific review)
- Sodium melts at 97.794 °C and boils at 882.940 °C. (awaiting scientific review)
- Sodium — Element information, properties and uses (Periodic Table) — Primary dataset
- Among the halogens, fluorine and chlorine are gases at room temperature, bromine is a liquid and iodine is a solid, because dispersion forces grow stronger as atoms and molecules get larger. (awaiting scientific review)
- Chlorine melts at −101.5 °C and boils at −34.04 °C, so it is a gas at 20 °C. (awaiting scientific review)
- Chlorine — Element information, properties and uses (Periodic Table) — Primary dataset
Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.
Content status: published 1 October 2026.
- Scientific review: this version has not yet been signed off by a scientific reviewer.
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