What does kinetic molecular theory say about gases?

Kinetic molecular theory models a gas as a large number of tiny particles in constant, random motion, with negligible particle volume and no attractive forces, undergoing elastic collisions, and with average kinetic energy proportional to the Kelvin temperature. This explains why gases exert pressure (collisions with the walls), compress easily (mostly empty space) and mix freely. Real gases behave most ideally at high temperature and low pressure.

Why does temperature stay constant during a phase change?

During melting or boiling, the energy added goes into overcoming the attractions between particles and changing their arrangement, not into increasing their average kinetic energy, which is what temperature measures. So the temperature stays constant until the change is complete. On a heating curve this appears as a flat plateau at the melting point and again at the boiling point.

Which phase changes absorb energy and which release it?

Changes toward a less ordered state absorb energy: melting (solid to liquid), vaporization (liquid to gas) and sublimation (solid to gas) are endothermic. Changes toward a more ordered state release energy: freezing, condensation and deposition are exothermic. Each change has a partner running the opposite way at the same temperature.

How do I choose and use the right gas law?

Pick the law from what is held constant. Boyle's law (PV constant) relates pressure and volume inversely at constant temperature. Charles's law relates volume and Kelvin temperature directly at constant pressure. Gay-Lussac's law relates pressure and Kelvin temperature directly at constant volume. The combined gas law handles all three at once. Temperature must always be in kelvin. Use the ideal gas law, PV = nRT, when the number of moles is involved.

What is the molar volume of a gas at STP?

At standard temperature and pressure (0 degrees Celsius and 1 atmosphere), one mole of any ideal gas occupies 22.4 liters. This molar volume is the same for every gas because kinetic molecular theory treats particle volume and forces as negligible, so only the number of particles sets the volume. It lets you convert between moles of a gas and its volume at STP without the full ideal gas law.

VirginiaChemistry

Virginia SOL Chemistry phases of matter and gas laws: a complete skills guide to kinetic molecular theory, phase changes, heating curves and the gas laws

A deep-dive Virginia SOL Chemistry guide to the phases of matter and the gas laws (CH.4): the particle model of solids, liquids and gases, kinetic molecular theory, phase changes and heating curves, Boyle's, Charles's, Gay-Lussac's and the combined gas law, and the ideal gas law with molar volume, with worked calculations and SOL exam technique.

Generated by Claude Opus 4.816 min readCH.4Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Why the particle model unifies this reporting category
States of matter and KMT
Phase changes and heating curves
The gas laws
The ideal gas law and molar volume
Check your knowledge

Why the particle model unifies this reporting category

The reporting category Phases of Matter, Kinetic Molecular Theory and Gas Laws rests on one idea: matter is made of moving particles, and how fast they move and how strongly they attract sets everything from the state of a substance to the pressure of a gas. Kinetic molecular theory explains phase changes, heating curves and the gas laws together. This guide ties together the matching dot-point pages, each with its own practice: states of matter and kinetic molecular theory, phase changes and heating curves, the gas laws, and the ideal gas law and molar volume.

States of matter and KMT

In a solid, particles are fixed and vibrate in place (definite shape and volume); in a liquid, they are close but slide (definite volume, no fixed shape); in a gas, they are far apart and move freely (no fixed shape or volume, easily compressed). Temperature measures the average kinetic energy of the particles, so heating speeds them up. Kinetic molecular theory models an ideal gas as many tiny particles in constant random motion, with negligible volume, no forces and elastic collisions, behaving ideally at high temperature and low pressure.

Phase changes and heating curves

The phase changes are melting, freezing, vaporization, condensation, sublimation and deposition. Changes to a less ordered state (melting, vaporization, sublimation) absorb energy; changes to a more ordered state (freezing, condensation, deposition) release it. On a heating curve, sloped sections show a single phase warming and flat plateaus show a phase change at constant temperature, where energy breaks particle attractions instead of raising kinetic energy.

The gas laws

For a fixed amount of gas, with temperature in kelvin:

Law	Relationship	Held constant
Boyle's	$P_1 V_1 = P_2 V_2$ (inverse)	temperature
Charles's	$\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}$ (direct)	pressure
Gay-Lussac's	$\dfrac{P_1}{T_1} = \dfrac{P_2}{T_2}$ (direct)	volume
Combined	$\dfrac{P_1 V_1}{T_1} = \dfrac{P_2 V_2}{T_2}$	amount of gas

The combined gas law reduces to each of the others when one quantity is held constant.

The ideal gas law and molar volume

When the amount of gas matters, use the ideal gas law $PV = nRT$ with $R = 0.0821\ \text{L·atm/(mol·K)}$ (so pressure in atm, volume in L, temperature in K). At STP, one mole of any gas occupies $22.4$ L, which you can derive from $PV = nRT$ and use to convert moles of gas to volume.

A combined gas law calculation

A gas occupies $3.0$ L at $1.0$ atm and $300$ K. Find its volume at $2.0$ atm and $600$ K.

step 1 Choose the law

Pressure, volume and temperature all change, so use the combined gas law $\dfrac{P_1 V_1}{T_1} = \dfrac{P_2 V_2}{T_2}$ .

step 2 Confirm kelvin

$300$ K and $600$ K are absolute temperatures, so no conversion is needed.

step 3 Rearrange and substitute

$V_2 = V_1 \times \dfrac{P_1}{P_2} \times \dfrac{T_2}{T_1} = 3.0 \times \dfrac{1.0}{2.0} \times \dfrac{600}{300} = 3.0 \times 0.5 \times 2 = 3.0$ L.

step 4 Interpret

Doubling the pressure halves the volume, but doubling the Kelvin temperature doubles it again, so the two effects cancel and the volume is unchanged.

Check your knowledge

Attempt these under timed conditions, then check the solutions.

Describe the particle arrangement and motion in a gas. (2 marks)
Name the phase change from solid directly to gas, and state whether it absorbs or releases energy. (2 marks)
A gas at $1.0$ atm and $6.0$ L is compressed to $3.0$ L at constant temperature. Find the new pressure. (2 marks)
A gas at $2.0$ L and $200$ K is heated to $400$ K at constant pressure. Find the new volume. (2 marks)
What volume does $3.0$ mol of gas occupy at STP? (1 mark)
Using $PV = nRT$ with $R = 0.0821$ , find the pressure of $1.0$ mol of gas in a $24.6$ L container at $300$ K. (2 marks)

Sources & how we know this

2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
Chemistry Curriculum Framework — Virginia Department of Education (2018)