Particles in solids, liquids and gases

Nitrogen molecules in the air around you at 30 °C move at about 519 m/s, faster than a passenger jet.

Drag to turn. Scroll or pinch to zoom.

Nitrogen at −180 °C is a gas. Drag down past −196 °C and it condenses; below −210 °C it freezes.

Pour water into a glass and it settles at the bottom with a level surface. Let the same water boil and the steam spreads through the room. The water molecules have not changed; H₂O is H₂O in ice, water and steam. What changes is how far apart the molecules are, how strongly they hold on to one another and how fast they move. The particle model on this page draws the same 40 particles in each state so that you can compare them directly.

How the three states compare

State	Particles	Shape and volume
Solid	Packed in a regular pattern; vibrate in place	Own shape and volume; hardly shrinks when squeezed
Liquid	Packed but disordered; slide past each other	Takes the container's shape, keeps its volume; hardly shrinks
Gas	Far apart; fly in straight lines	Fills the container; shrinks a lot when squeezed

Because the particles in solids and liquids already touch, pushing harder cannot squeeze them much closer. That is why a hydraulic jack can pass a push through oil without the oil shrinking. In a gas most of the volume is empty space between the molecules, so a bicycle pump can squeeze air into a tyre. The forces between molecules also explain why liquids form drops and solids keep their shape: in both, the particles attract one another strongly enough to stay together. In a gas they are moving too fast and are too far apart for those attractions to hold them.

Water breaks the usual rule

For most substances the solid is denser than the liquid, so a piece of the solid sinks in its own melt. Water is different. Liquid water at 0 °C has a density of 0.99987 g/cm³, and when it freezes the density drops by about 9% (USGS). Ice floats with most of its volume under the surface, which is why most of an iceberg is hidden. The open arrangement of molecules in an ice crystal takes more room than the jumbled arrangement in the liquid. Liquid water itself is densest at 4 °C, at 1.00000 g/cm³, and becomes less dense again as it warms, down to 0.95865 g/cm³ at 100 °C.

Temperature is particle motion

The Celsius scale puts 0 at the freezing point of water, which is convenient but has nothing to do with the particles. The kelvin scale starts at absolute zero, the lowest possible temperature, which is −273.15 °C. To convert, add 273.15: 25 °C is 298.15 K. On this scale the average kinetic energy of the particles is directly proportional to the temperature, so a gas at 600 K has twice the average kinetic energy of the same gas at 300 K.

For a gas the speed follows from that energy. The root mean square speed is u = √(3RT/M), where R is 8.314 J/(mol K), T is in kelvins and M is the molar mass in kilograms per mole. For nitrogen at 30 °C, OpenStax works out √(3 × 8.314 × 303 ÷ 0.028) = 519 m/s. Heavier molecules move more slowly at the same temperature: an oxygen molecule at −23 °C averages about 441 m/s. The particles in the picture above are slowed down by a factor of millions so that you can follow them.

Kinetic energy is shared out unevenly. At any moment some molecules are much faster than the average and some much slower. That spread matters for evaporation in a later unit.

Kelvin	°C + 273.15; 0 K is absolute zero
rms speed	u = √(3RT/M) with M in kg/mol
Density of ice	About 9% lower than liquid water, so ice floats
Densest water	At 4 °C: 1.00000 g/cm³

Convert 25 °C to kelvins.
Liquid nitrogen boils at 77.34 K (NIST). What is that in °C (1 decimal place)?
A gas is heated from 300 K to 600 K. Its average kinetic energy…
- doubles
- stays the same
- goes up four times
Find the rms speed of an N₂ molecule (M = 0.028 kg/mol) at 303 K, to the nearest m/s.
At the same temperature, which moves faster on average?
- N₂ (28 g/mol)
- O₂ (32 g/mol)
- They are the same speed
Why can air be squeezed into a tyre but water hardly at all?
- Gas molecules are far apart; liquid molecules already touch
- Water molecules are heavier
- Air is not matter
Water at 0 °C has density 0.99987 g/cm³. Ice is about 9% less dense. Estimate the density of ice (2 decimal places).
At what temperature is liquid water densest?
- 4 °C
- 0 °C
- 100 °C
In which state do particles touch but slide past one another?
- Liquid
- Solid
- Gas
Order the states from most tightly held particles to most free.

Key points

The molecules stay the same; spacing, attraction and speed change.
Ice is about 9% less dense than water, so it floats; water is densest at 4 °C.
K = °C + 273.15; average kinetic energy ∝ T in kelvins.
Gas speed u = √(3RT/M): N₂ at 30 °C ≈ 519 m/s.

Sources for this course (8)

OpenStax Chemistry 2e (openstax.org/books/chemistry-2e): 1.2 Phases and Classification of Matter (matter, three states, plasma, mass and weight, Moon 1/6, conservation of matter, sand and clouds)
9.2 gas laws (Boyle's law P1V1 = P2V2, syringe 15.0 mL 13.0 psi to 7.5 mL 26 psi, breathing, divers: every 33 ft of sea water adds 1 atm and halves the air volume at 33 ft, Amontons ~1700, Gay-Lussac ~1800, Charles, STP 273.15 K and 101.325 kPa, -273 °C absolute zero)
9.5 kinetic-molecular theory (average kinetic energy proportional to kelvin temperature, KEavg = 3/2 RT, urms = sqrt(3RT/M), N2 at 30 °C 519 m/s, O2 at -23 °C 441 m/s)
10.3 phase transitions (vapor pressure, normal boiling point at 1 atm = 101.3 kPa, Leadville 68 kPa about 90 °C, ΔHvap 44.01 kJ/mol at 25 °C and 40.67 kJ/mol at 100 °C, 1.5 L sweat 3600 kJ, ΔHfus 6.01 kJ/mol, ice at -12 °C staying at 0 °C while melting, CO2 sublimation 26.1 kJ/mol, frost is deposition, heating curve example 135 g ice -15 °C to steam 120 °C = 416 kJ with c ice 2.09, water 4.18, steam 1.86 J/g °C, burner turned up does not raise boiling temperature)
NIST Chemistry WebBook phase change data (webbook.nist.gov: ethanol Tfus 159 K, Tboil 351.5 K; nitrogen Tfus 63.3 K, Tboil 77.34 K; oxygen Tfus 54.8 K, Tboil 90.2 K; mercury Tfus 234.31 K, Tboil 629.81 K; CO2 triple point 216.58 K at 5.185 bar, solid vapour-pressure equation Giauque and Egan 1937 giving 194.7 K at 1 atm)
USGS Water Science School, Water Density (0.99987 g/cm³ at 0 °C, 1.00000 at 4 °C, 0.95865 at 100 °C; ice about 9% less dense)
Britannica, Boyle's law (Robert Boyle 1662, air compressed by mercury; Edme Mariotte 1676)
Boyle's 1662 table (48 spaces at 29 2/16 in, 12 spaces at 117 9/16 in of mercury) as reprinted in J. Appl. Physiol. 87 (1999) 1543.

Checked: 2026-10-17