IB Physics HL Book 2025; Papers 1 & 2 Practice 5 Sections65 Lessons54 Weeks
Gas laws
B.3 — Gas Laws
IB Physics HL • Topic B: The particulate nature of matter • SL and HL core content
Gas laws describe how the macroscopic properties of gases—pressure, volume, and temperature—arise from the microscopic behaviour of individual particles. By modelling gases as ideal systems, we can predict the behaviour of many real gases under suitable conditions.
Guiding ideas
- Gas pressure comes from particle collisions with container walls.
- The ideal gas model neglects particle volume and intermolecular forces between collisions.
- Empirical gas laws connect
,
, and
; kinetic theory explains these relationships. - For a fixed amount of ideal monatomic gas, internal energy depends only on temperature.
B.3.1 What are ideal gases?
Learning objectives
- Define pressure and explain it through particle momentum changes.
- State the assumptions of the ideal gas model.
- Explain when real gases behave approximately ideally.
- Use the mole, the Avogadro constant, and the number of particles.
Pressure: macroscopic definition
Pressure is the perpendicular force applied per unit area:
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Here,
is pressure in pascals,
is perpendicular force in newtons, and
is area in square metres.
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For example, the same applied force concentrated on the point of one nail produces greater local pressure than when the force is distributed over many nails.
Pressure in gases: microscopic view
- Gas particles move randomly in all directions.
- Collisions with container walls change particle momentum.
- The wall exerts a force on the particles, and the particles exert an equal and opposite force on the wall.
- The average total force per unit area from many collisions is the gas pressure.
Other conditions being comparable, higher pressure can result from:
- More frequent collisions with a given wall area.
- A larger momentum transfer per collision.
The ideal gas model
An ideal gas is a simplified model based on the following assumptions:
- The gas contains a very large number of particles.
- The volume of the particles is negligible compared with the container volume.
- There are no intermolecular forces between collisions.
- Particles move randomly, with no preferred direction in equilibrium.
- Collisions with one another and the container walls are perfectly elastic.
- Collision duration is negligible compared with the time between collisions.
- Intermolecular potential energy is taken to be zero.
No real gas follows this model perfectly, but many real gases behave approximately ideally under suitable conditions.
When is a real gas close to ideal?
A real gas behaves most like an ideal gas when it has:
- A sufficiently high temperature, well away from condensation.
- A low pressure.
- A low density.
At low density, particles are widely separated, so their own volume occupies only a small fraction of the container volume. At sufficiently high temperature, their kinetic energy is large compared with the effects of attractive intermolecular interactions.
The mole and the Avogadro constant
Because gases contain enormous numbers of particles, we measure the amount of substance using the mole, with SI unit mol.
The Avogadro constant gives the number of specified particles per mole:
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The relationship between amount of substance
and number of particles
is:
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The particles being counted may be atoms or molecules. These relationships also apply to substances in the liquid and solid states.
Worked example 1 — Pressure
A force of
acts uniformly on a surface area of
. Calculate the pressure.
Show answer and full working
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Calculated pressure:
.
To two significant figures, this is
.
Worked example 2 — Moles and molecules
A glass contains
of water. Calculate the number of water molecules in the glass.
Show answer and full working
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Answer: approximately
molecules to two significant figures.
Worked example 3 — Amount of substance
A sample contains
gas molecules. Calculate the amount of substance in moles.
Show answer and full working
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Answer:
.
B.3.2 How do ideal gases behave?
Learning objectives
- Relate experimental observations to Boyle’s law, Charles’ law, and the pressure law.
- Apply the empirical gas laws and the combined gas law.
- Use the ideal gas equation in macroscopic and molecular forms.
- Explain gas pressure using force, momentum, and collisions.
- Calculate internal energy for an ideal monatomic gas.
Use absolute pressure and kelvin
Gas laws use absolute pressure, measured relative to a vacuum. A gauge often measures pressure relative to the surrounding atmosphere.
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Temperatures must be in kelvin:
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Doubling a Celsius temperature does not double the absolute temperature.
Useful conversions:
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Empirical gas laws
For a fixed amount of gas, experimental observations show that pressure
, volume
, and absolute temperature
are related by simple laws.
Boyle’s law: constant temperature
For a fixed amount of gas at constant temperature:
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Comparing two states:
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Microscopic explanation: at constant temperature, average particle kinetic energy is unchanged. Decreasing the volume increases the collision rate per unit wall area, so pressure increases.
Charles’ law: constant pressure
For a fixed amount of gas at constant pressure:
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Comparing two states:
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Microscopic explanation: higher temperature means faster particles. The gas must expand to maintain the same pressure, reducing the collision rate per unit wall area sufficiently to compensate.
Pressure law: constant volume
For a fixed amount of gas at constant volume:
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Comparing two states:
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Microscopic explanation: higher temperature means faster particles. They collide with the walls more frequently and transfer more momentum per collision, increasing the average force and pressure.
Recognising gas-law graphs
- At fixed temperature and amount of gas, pressure against volume is a decreasing hyperbola.
- At fixed temperature and amount of gas, pressure against reciprocal volume is a straight line through the origin.
- At fixed pressure and amount of gas, volume against kelvin temperature is a straight line through the origin in the ideal model.
- At fixed volume and amount of gas, pressure against kelvin temperature is a straight line through the origin in the ideal model.
- The corresponding Celsius-temperature lines extrapolate to approximately −273 °C, although real gases cease to behave ideally before reaching this limit.
Combined gas law: fixed amount of gas
When pressure, volume, and temperature all change for the same fixed amount of ideal gas:
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Comparing two states:
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Worked example 4 — Pressure change at constant volume
A gas is contained in a sealed rigid cylinder of volume
at
and an absolute pressure of
. It is heated to
. Calculate the new pressure.
Show answer and full working
Step 1: Convert temperatures to kelvin.
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Step 2: Use the pressure law.
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Step 3: Substitute.
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Answer:
.
The cylinder volume does not enter the calculation because it remains constant.
The ideal gas equation
The empirical gas laws are represented by the ideal gas equation:
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— absolute pressure in
.
— volume in
.
— amount of substance in
.
— absolute temperature in
.
— molar gas constant.
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Molecular form of the ideal gas equation
Using the number of gas particles
, which may be atoms or molecules:
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Here,
is the Boltzmann constant:
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The constants are related by:
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Pressure and molecular speed
Kinetic theory links pressure to particle motion:
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Here,
is gas density and
is the mean square speed of the particles. At fixed density, a larger mean square speed produces a higher pressure.
Mean square speed and root mean square speed
Mean square speed is the average of the squared particle speeds:
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Root mean square speed is the square root of this quantity:
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Therefore:
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Here,
is the mass of one particle.
Important: root mean square speed is not the same as mean speed. Also,
is not generally equal to
.
Internal energy of an ideal monatomic gas
In the ideal monatomic gas model, intermolecular potential energy is neglected. Internal energy is the total random translational kinetic energy of the atoms.
The average translational kinetic energy per particle is:
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Therefore:
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For a fixed amount of ideal monatomic gas, internal energy depends only on temperature.
Ideal molecular gases may also store rotational and vibrational energy, so the factor
does not generally give their total internal energy.
Worked example 5 — Ideal gas equation
A gas occupies
at an absolute pressure of
and a temperature of
. Calculate the amount of gas.
Show answer and full working
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Answer: approximately
to two significant figures.
Worked example 6 — Heating at constant volume
A fixed amount of gas in a rigid container has an absolute pressure of
at
. It is heated to
. Calculate the new pressure.
Show answer and full working
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Answer:
.
Worked example 7 — Internal energy
Calculate the internal energy of
of an ideal monatomic gas at
.
Show answer and full working
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Answer:
.
Ideal gases and gas pressure: force and momentum
Microscopic explanation using momentum
- A particle of mass
has velocity component
perpendicular to a wall. - In an elastic collision with a stationary smooth wall, the perpendicular velocity component reverses.
- Velocity components parallel to the wall remain unchanged.
- The magnitude of momentum transferred to the wall is
. - The average force is the total momentum transferred to the wall per unit time.
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The subscript “mom” distinguishes momentum from the symbol
used for pressure.
Making connections
- Mechanics: momentum change explains the force on container walls.
- Thermal physics: absolute temperature is proportional to average translational kinetic energy.
- Energy: internal energy can change through heating or work.
- Applications: expanding gases can push a piston and do work.
Kinetic theory — deriving the pressure equation
Consider
particles, each of mass
, moving in a cube of side length
. For one particle, let
be its velocity component perpendicular to a chosen wall.
Each elastic collision transfers momentum of magnitude
to that wall. In the elementary model, the particle travels a perpendicular distance
between successive collisions with the same wall.
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The time-averaged force contributed by this particle is:

Adding the contributions of all particles and dividing by the wall area,
, gives:
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Random motion is isotropic, meaning that no direction is preferred:
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Using the density relation:
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We obtain:
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This elementary derivation models journeys between opposite walls. Collisions between particles redistribute velocities but do not change the equilibrium result.
Connecting kinetic theory to temperature
Multiply the pressure equation by volume and compare it with the molecular ideal gas equation:
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Therefore:
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Thus, average translational kinetic energy is proportional to absolute temperature.
At the same temperature, particles of different ideal gases have the same average translational kinetic energy. Lighter particles have greater root mean square speeds.
Internal energy — useful equivalent forms
For an ideal monatomic gas:
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For a fixed amount:
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At constant temperature, its internal energy remains unchanged.
During constant-pressure heating, the energy supplied is shared between increasing internal energy and work done by the expanding gas. It is therefore not generally equal to
alone.
B.3 Summary and key terms
Key ideas
- Gas pressure arises from particle collisions with container walls.
- The ideal gas model neglects particle volume and intermolecular forces between collisions.
- Real gases behave approximately ideally at sufficiently high temperature and low density.
- Boyle’s law, Charles’ law, and the pressure law describe relationships between pressure, volume, and temperature.
- The ideal gas equation can be written as
or
. - For a fixed amount of ideal monatomic gas, internal energy depends only on temperature.
Key terms
Pressure, absolute pressure, gauge pressure, mole, Avogadro constant, Boyle’s law, Charles’ law, pressure law, combined gas law, ideal gas equation, Boltzmann constant, internal energy, elastic collision, momentum, mean square speed, and root mean square speed.
IB exam tips
- Always use kelvin in gas-law calculations.
- Use absolute pressure rather than gauge pressure.
- For explanation questions, link macroscopic changes to collision frequency and momentum transfer.
- Use SI units when applying
: pressure in pascals and volume in cubic metres. - In two-state ratio calculations, consistent pressure and volume units can cancel.
- State what remains constant when selecting a gas law.
- Use
for an ideal monatomic gas. - For a fixed amount of ideal monatomic gas, use
. - Do not confuse mean square speed with root mean square speed.
Essential equations — B.3
Use absolute pressure and kelvin. The two-state gas laws require a fixed amount of gas.
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Constants:
,
, and
.
Practice — check your understanding
1. Boyle’s law
A fixed amount of gas occupies
at an absolute pressure of
. It is slowly compressed at constant temperature to
. Calculate its new pressure.
Show answer and working
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The volume units cancel because the same units are used on both sides.
Answer:
.
2. Charles’ law
A fixed amount of gas occupies
at
. It is heated at constant pressure to
. Calculate the final volume.
Show answer and working
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Answer:
.
Using Celsius directly would give an incorrect temperature ratio.
3. Combined gas law
A sealed sample initially has absolute pressure
, volume
, and temperature
. Its final volume is
and final temperature is
. Calculate the final pressure.
Show answer and working
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Answer:
. Both the smaller volume and higher temperature increase the pressure.
4. Root mean square speed
A gas has density
and absolute pressure
. Calculate its root mean square speed.
Show answer and working
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Answer:
to three significant figures.
5. Change in internal energy
A sealed sample contains
of an ideal monatomic gas. Its temperature rises from
to
. Calculate the change in internal energy.
Show answer and working
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The change is positive because the gas warms. This does not by itself determine the heat supplied unless the work done is also known.
6. Lab-style question — testing Boyle’s law
A student traps air in a gas syringe connected to a pressure sensor. Describe how to test Boyle’s law, identify the quantities to control, state a suitable graph, and explain two sources of error.
Show a suggested answer
Method: change the gas volume gradually, wait for temperature and pressure to stabilise, and record several pairs of absolute pressure and volume. Repeat measurements where practical.
Controls: keep the amount of gas fixed by preventing leaks. Keep temperature constant by compressing slowly and allowing thermal equilibrium with the surroundings.
Graph: plot
against
. A straight line through the origin supports
, within experimental uncertainty. Its gradient is
.
Error 1: rapid compression temporarily heats the gas, producing a pressure above the isothermal prediction.
Error 2: uncounted gas volume in the tubing and sensor causes a systematic error. Use the total enclosed volume, not just the syringe reading.
Check whether the sensor measures absolute or gauge pressure. Convert gauge values before testing the proportionality.
Faith connection — trusting God under pressure
“God is our refuge and strength, a very present help in trouble.”
Psalm 46:1 — KJV
Creation — order in the smallest details
Gas laws connect the motion of countless tiny particles with measurable pressure, volume, and temperature. As Christians, we appreciate the order of creation and the opportunity to understand it through careful study. The air around us supports life, and learning its properties can deepen our gratitude and guide us to use scientific knowledge responsibly.
The Fall — responding to life’s pressures
In a world affected by sin and brokenness, we experience pressures such as fear, conflict, illness, and disappointment. Psalm 46 invites us to find refuge in God. We can bring our concerns to Him in prayer and respond to others’ struggles with patience, honesty, and compassion.
Redemption — peace through Jesus Christ
Jesus Christ came to reconcile us to God through His death and resurrection. Turn to Him in repentance and faith, receive Him as your personal God, Saviour, and Lord, and trust His grace. His invitation in Matthew 11:28 encourages those who are weary and burdened to come to Him for rest.
Restoration — living with hope
God promises a renewed creation where suffering and death will end, as described in Revelation 21:4. That hope helps us live faithfully today: persevere in learning, use knowledge to serve others, and seek God’s guidance in difficult situations. Connect with other believers, read and study the Bible, and spend time in prayer so that your faith and knowledge of God continue to grow.
Reflect and pray
Reflect: What pressure are you facing today? What would it look like to bring that concern to God and seek support from someone you trust?
Prayer: Lord Jesus, thank You for the order of Your creation and for being my refuge and strength. Forgive my sins and help me trust You as my God, Saviour, and Lord. Give me patience in learning, peace in difficulty, and a willing heart to serve others. Help me grow through Your Word, prayer, and fellowship. Amen.