General Chemistry
States of Matter
Solid, liquid, gas, plasma — different arrangements of particles
States of matter are distinct forms of matter based on particle arrangement and motion. Three classical: solid (fixed shape and volume), liquid (fixed volume, takes container shape), gas (no fixed shape or volume). Plasma: ionized gas (4th state). Modern additions: Bose-Einstein condensate (extreme cold), supercritical fluid (high T+P), liquid crystals. Phase transitions: melting (solid→liquid), boiling (liquid→gas), sublimation (solid→gas), deposition (gas→solid). Driven by temperature and pressure changes.
- SolidFixed shape, fixed volume, vibrate about positions
- LiquidFixed volume, flows; particles move past each other
- GasNo fixed shape or volume; high kinetic energy
- PlasmaIonized gas; abundant in stars
- Phase transitionsMelting, freezing, boiling, condensation, sublimation
- Triple pointT+P where all three states coexist (water: 0.01°C, 611 Pa)
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Why states matter
- Temperature regulation. Phase changes absorb/release heat.
- Materials. Selection by temperature range.
- Industry. Distillation, crystallization.
- Biology. Most biology in liquid water.
- Astrophysics. Plasma in stars.
- Quantum technology. BEC for sensors, computing.
- Energy. Latent heat (steam, refrigeration).
Common misconceptions
- Three states only. Plasma, BEC, supercritical fluids exist.
- Plasma is rare. Most matter in universe.
- Sublimation is unusual. Common (dry ice, snow disappearing without melting).
- States are absolute. Continuous transitions in some cases.
- Ice is denser than water. Less dense; ice floats.
- Gas particles touch. Mostly empty space.
Frequently asked questions
What distinguishes the three classical states?
Particle motion and arrangement. Solid: particles in fixed positions, only vibrate. Held tightly. Liquid: particles moving but still close — flow but stay in container. Gas: particles moving freely with much space between — fill any volume. Driven by kinetic energy (temperature). Higher T → more disorder.
What's a phase transition?
Change between states. Solid→liquid: melting (endothermic, energy input). Liquid→gas: boiling/vaporization (endothermic). Gas→liquid: condensation (exothermic). Liquid→solid: freezing (exothermic). Solid→gas: sublimation (e.g., dry ice — solid CO₂ → CO₂ gas without melting). Gas→solid: deposition.
What's plasma?
4th state of matter. Ionized gas — electrons stripped from atoms; charged particles. Conducts electricity. Examples: lightning, neon signs, Sun's interior, fluorescent lights. ~99% of visible matter in universe is plasma. Distinguished by: high temperature OR strong electromagnetic fields needed to ionize.
What's a Bose-Einstein condensate?
5th state. Extreme cold (~nanokelvin) reduces particles' quantum states until many occupy same lowest state. Behave as single quantum entity. First observed 1995 (Cornell, Wieman, Ketterle — Nobel 2001). Useful for: studying quantum mechanics on macroscopic scale, super-fluidity, atom lasers.
What's a supercritical fluid?
Above critical T and P, distinction between liquid and gas disappears. Density of liquid + diffusivity of gas. Useful as solvents (CO₂ for decaffeination, dry cleaning). Critical point of water: 374°C, 22 MPa. Appears in: deep oceans, industrial processes, near volcanic vents.
What's the triple point?
Specific T+P where all three states (solid, liquid, gas) coexist in equilibrium. Water: 0.01°C, 611 Pa. Used to define Kelvin temperature scale (was 1/273.16 of triple point until 2019 redefinition). Each substance has unique triple point. Phase diagram shows P vs T regions for each phase.
How do IMFs determine state?
Strong IMFs → particles stay close → solid or high-BP liquid. Weak IMFs → particles separate easily → low-BP liquid or gas. At given temperature, IMF strength determines state. Water (strong H-bonds): liquid at 25°C. Methane (weak London): gas at 25°C. Ice (more order): below 0°C, H-bonds lock particles.