Gas pressure is all around us. The air inside a bicycle tyre pushes against its walls, compressed air operates tools and brakes, and Earth's atmosphere constantly pushes against everything on the planet.

Unlike solids and liquids, gases are easily compressed because their particles are relatively far apart. To understand gas pressure, we therefore need to think about what individual gas particles are doing.

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The Particle Model of a Gas

A gas consists of enormous numbers of tiny particles.

According to the particle model, gas particles:

• are relatively far apart
• move constantly
• move rapidly and randomly
• travel in different directions
• collide with one another
• collide with the walls of their container

Compared with particles in solids and liquids, gas particles have much more freedom to move throughout the available space.

You can explore how the particle arrangement of a gas differs from solids and liquids here:

Because the particles move freely, a gas spreads out to fill its container.

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How Do Gas Particles Create Pressure?

Imagine air trapped inside a sealed container.

The air particles are moving constantly in random directions.

When a particle reaches the wall of the container, it collides with it and changes direction.

During the collision, the particle exerts a tiny force on the wall.

One particle produces an extremely small force. However, a container contains an enormous number of particles, producing an enormous number of collisions every second.

Together, these collisions produce a measurable force on the walls.

Because pressure is force acting per unit area:

particle collisions → force on container walls → gas pressure

This is the central idea behind gas pressure:

Gas pressure is caused by gas particles colliding with surfaces.

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Pressure Acts in All Directions

Gas particles move randomly in every direction.

They therefore collide with every surface around them.

As a result, gas pressure acts in all directions.

Air inside a balloon, for example, pushes:

• upward
• downward
• sideways
• against every part of the balloon's inner surface

This is why a balloon expands in all directions when it is inflated.

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What Determines Gas Pressure?

Gas pressure depends on how often particles collide with the container walls and how strongly they collide.

Three important factors are:

1. Number of Gas Particles

More particles generally means more collisions.

If the container size and temperature remain constant:

more particles → more collisions → greater pressure

This is what happens when you pump additional air into a bicycle tyre.

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2. Volume

If the same gas is squeezed into a smaller space, the particles have less distance to travel before hitting the walls.

Therefore:

smaller volume → more frequent collisions → greater pressure

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3. Temperature

Heating a gas increases the average kinetic energy of its particles.

The particles move faster and collide with the walls more frequently and more forcefully.

If the gas is trapped in a rigid container:

higher temperature → faster particles → greater pressure

We will concentrate mainly on the effect of volume in this topic.

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Gas Pressure and Volume

Imagine gas trapped inside a cylinder with a movable piston.

If the piston is pushed downward, the gas is compressed into a smaller volume.

The number of gas particles has not changed.

But the particles now have less space in which to move.

They reach the container walls more frequently.

Therefore:

volume decreases → collision frequency increases → pressure increases

If the piston is pulled outward:

volume increases → collision frequency decreases → pressure decreases

This relationship applies when the amount of gas and its temperature remain constant.

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Boyle's Law

For a fixed amount of gas at constant temperature:

pressure is inversely proportional to volume.

This relationship is known as Boyle's Law.

It can be written:

P ∝ 1/V

or:

P₁V₁ = P₂V₂

where:

• P₁ = initial pressure
• V₁ = initial volume
• P₂ = final pressure
• V₂ = final volume

This means that if volume decreases, pressure increases.

If volume increases, pressure decreases.

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A Simple Example

Suppose a gas has:

Pressure = 100 kPa

Volume = 4 L

If the volume is reduced to 2 L while temperature remains constant, the volume has been halved.

The pressure therefore doubles:

New pressure = 200 kPa

So:

Notice the pattern:

½ the volume → 2× the pressure

¼ the volume → 4× the pressure

This is an inverse relationship.

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Worked Example 1: Boyle's Law

A gas occupies 6.0 L at a pressure of 100 kPa.

It is compressed to 3.0 L.

Calculate the new pressure.

Start with:

P₁V₁ = P₂V₂

Substitute:

100 × 6.0 = P₂ × 3.0

Rearrange:

P₂ = (100 × 6.0) ÷ 3.0

P₂ = 200 kPa

The pressure doubles because the volume has been halved.

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Worked Example 2: Expanding a Gas

A gas occupies 2.0 L at 300 kPa.

It expands to 6.0 L.

Calculate its new pressure.

P₁V₁ = P₂V₂

300 × 2.0 = P₂ × 6.0

P₂ = 600 ÷ 6.0

P₂ = 100 kPa

The volume became three times larger, so the pressure became three times smaller.

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A Syringe Demonstration

You can experience this relationship using a syringe with its opening blocked.

Pull the plunger outward and trap some air inside.

Then push the plunger inward.

As you decrease the volume, you feel increasing resistance.

Why?

The trapped air particles are being compressed into a smaller space.

They collide with the plunger more frequently, producing greater pressure.

If you release the plunger, the compressed gas can push it outward again.

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Pressure-Volume Graphs

The relationship between gas pressure and volume is different from the straight-line pressure-depth relationship we saw for liquids.

For a fixed amount of gas at constant temperature:

P ∝ 1/V

Therefore, a graph of pressure against volume produces a curve.

At small volumes, pressure is high.

As volume increases, pressure decreases.

The graph becomes less steep as the volume becomes larger.

This shape is characteristic of an inverse relationship.

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Atmospheric Pressure

Earth is surrounded by a layer of gases called the atmosphere.

Although air may seem almost weightless, it has mass.

Gravity pulls the atmosphere toward Earth.

As a result, the atmosphere exerts pressure on Earth's surface.

This is called atmospheric pressure.

At sea level, average atmospheric pressure is approximately:

101 000 Pa

or:

101 kPa

This is also approximately:

1 atmosphere (1 atm)

So even while you are sitting still, the atmosphere is exerting a considerable pressure on your body.

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Why Doesn't Atmospheric Pressure Crush Us?

Atmospheric pressure acts on our bodies from all directions.

However, fluids and gases inside our bodies also exert outward pressure.

Under normal conditions, these pressures are largely balanced.

This is why we do not normally notice atmospheric pressure.

We tend to notice it when there is a difference in pressure, such as during:

• aircraft takeoff and landing
• mountain climbing
• diving
• rapid changes in altitude

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Atmospheric Pressure and Altitude

Atmospheric pressure decreases as altitude increases.

Why?

At low altitude, there is a large amount of atmosphere above you.

At high altitude, there is less atmosphere above you.

Therefore:

greater altitude → less air above → lower atmospheric pressure

Atmospheric pressure is therefore greatest near sea level and decreases as you travel upward through the atmosphere.

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Particle Explanation of Altitude

The atmosphere also becomes less dense as altitude increases.

Near Earth's surface, gravity causes more air particles to be concentrated in the lower atmosphere.

Higher in the atmosphere:

• particles are farther apart
• there are fewer particles in a given volume
• collisions occur less frequently
• pressure is lower

This connects atmospheric pressure directly to the particle model.

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Why Your Ears Pop

Your middle ear contains air.

Normally, the pressure inside your ear is close to the pressure of the surrounding atmosphere.

During rapid altitude changes, the external pressure may change faster than the pressure inside your ear.

This creates a pressure difference across the eardrum.

The Eustachian tube helps equalize the pressure.

Swallowing or yawning can help open this tube.

When the pressures equalize, you may feel or hear a small pop.

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Gas Pressure in Bicycle and Car Tyres

Tyres depend on compressed gas.

When air is pumped into a tyre, more gas particles are added.

The particles collide with the inside walls of the tyre, producing pressure.

The pressure helps the tyre:

• maintain its shape
• support the vehicle
• absorb bumps
• maintain suitable contact with the road

Too little pressure can cause excessive deformation.

Too much pressure can also cause problems.

This is why tyre pressure is regularly checked.

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Gas Pressure in Sports Balls

Footballs, basketballs, volleyballs, and many other sports balls contain compressed air.

The air particles push outward against the inside surface.

This pressure keeps the ball inflated.

If gas escapes:

number of particles decreases → collision rate decreases → internal pressure decreases

The ball becomes softer.

Adding air increases the number of particles and raises the pressure again.

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Aerosol Cans and Gas Cylinders

Many products store gases under pressure.

Examples include:

• aerosol cans
• fire extinguishers
• compressed-air cylinders
• diving cylinders
• medical oxygen cylinders

A large number of gas particles may be compressed into a relatively small volume.

This creates high pressure.

The containers must therefore be strong enough to withstand the force produced by the gas.

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Heating a Sealed Gas

Suppose gas is trapped inside a rigid container.

The volume cannot change.

Now heat the gas.

The particles gain kinetic energy and move faster.

As a result:

• collisions with the walls happen more frequently
• each collision tends to involve a greater change in momentum
• the pressure increases

Therefore:

temperature increases → particle speed increases → pressure increases

when volume remains constant.

This is why pressurized containers should not be exposed to excessive heat.

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Everyday Applications of Gas Pressure

Gas pressure appears in many technologies and everyday situations.

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Drinking Through a Straw

It is common to say that you "suck the drink upward."

A more accurate explanation involves pressure differences.

When you draw air from the straw, the pressure inside the straw decreases.

Atmospheric pressure pushing down on the surface of the drink is then greater than the pressure inside the straw.

The pressure difference pushes the liquid upward.

This is an important principle:

Fluids tend to move from regions of higher pressure toward regions of lower pressure.

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Gas Pressure in Aircraft

Aircraft often travel at altitudes where atmospheric pressure is much lower than at Earth's surface.

Passenger cabins are therefore pressurized.

The cabin is maintained at a pressure higher than the outside atmosphere.

The aircraft's structure must therefore withstand a pressure difference between the inside and outside of the fuselage.

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Connecting Gas Pressure to Particle Motion

Whenever you encounter a gas-pressure problem, try to explain it using particles.

Ask:

How many particles are present?

How fast are they moving?

How much space do they have?

How frequently are they colliding with surfaces?

For example:

Compressing a gas

Less space → more frequent collisions → higher pressure.

Adding gas

More particles → more collisions → higher pressure.

Heating a gas

Faster particles → more frequent and stronger collisions → higher pressure.

Removing gas

Fewer particles → fewer collisions → lower pressure.

This particle explanation is much more useful than simply memorizing rules.

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Common Mistakes

Mistake 1: Thinking gas particles create pressure by pushing continuously

Gas particles are not continuously pushing against the walls.

They are moving freely and colliding with the walls.

These collisions produce the pressure.

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Mistake 2: Thinking smaller volume means fewer particles

Compressing a sealed gas does not remove particles.

The same particles simply occupy a smaller volume.

Same number of particles + smaller volume → more frequent collisions

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Mistake 3: Saying particles become larger when a gas is heated

Heating does not normally make the particles themselves significantly larger.

Instead, they gain kinetic energy and move faster.

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Mistake 4: Thinking atmospheric pressure increases with altitude

It is the opposite.

Altitude increases → atmospheric pressure decreases

There is less atmosphere above you at greater altitude.

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Mistake 5: Confusing pressure with particle speed

Faster particles can increase pressure, but pressure itself is not speed.

Pressure results from the combined effects of particle collisions with surfaces.

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Mistake 6: Saying there is "no pressure" at high altitude

Atmospheric pressure becomes lower with altitude, but it does not suddenly become zero.

The atmosphere gradually becomes thinner with increasing altitude.

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Mistake 7: Forgetting the conditions for Boyle's Law

The simple relationship:

P₁V₁ = P₂V₂

assumes:

• a fixed amount of gas
• constant temperature

If the gas is heated significantly while being compressed, the situation becomes more complicated.

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Check Your Understanding

1. Recall

What causes gas pressure inside a container?

2. Particle Explanation

Explain why a balloon pushes outward on its surface in all directions.

3. Volume

A sealed gas is compressed to a smaller volume while its temperature remains constant.

What happens to:

a. the number of gas particles?

b. the frequency of collisions?

c. the pressure?

4. Boyle's Law

A gas occupies 8 L at 100 kPa.

It is compressed to 4 L at constant temperature.

What is the new pressure?

5. Apply

A sealed syringe contains air. The opening is blocked and the plunger is pushed inward.

Explain why the plunger becomes increasingly difficult to push.

6. Atmospheric Pressure

Why is atmospheric pressure lower on top of a high mountain than at sea level?

7. Everyday Application

Explain why a basketball becomes softer when some of the air escapes.

8. Challenge

Two identical sealed containers contain the same amount of gas.

Container A is at 20°C.

Container B is heated to a much higher temperature.

Predict which container has the greater pressure and explain your answer using the motion and collisions of gas particles.

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Key Terms

• Gas pressure – pressure produced by gas particles colliding with surfaces
• Particle model – model describing matter as particles in constant motion
• Collision – interaction in which moving particles strike another particle or surface
• Volume – amount of space occupied by a substance
• Compression – reduction in the volume of a gas
• Atmospheric pressure – pressure produced by Earth's atmosphere
• Altitude – height above a reference level, usually sea level
• Boyle's Law – relationship between pressure and volume for a fixed amount of gas at constant temperature
• Kinetic energy – energy an object or particle has because it is moving
• Pneumatic system – system that uses compressed gas to transfer force or energy

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Key Takeaways

• Gas particles are in constant random motion.
• Gas pressure results from particles colliding with surfaces.
• Gas pressure acts in all directions.
• Compressing a gas into a smaller volume causes particles to collide with the walls more frequently, increasing pressure.
• For a fixed amount of gas at constant temperature, pressure and volume have an inverse relationship.
• Boyle's Law can be written as P₁V₁ = P₂V₂.
• Heating a gas in a rigid container makes its particles move faster and generally increases pressure.
• Atmospheric pressure is caused by Earth's atmosphere and decreases with increasing altitude.
• Gas pressure is used in tyres, sports balls, aerosol cans, aircraft, syringes, pneumatic systems, and compressed-gas cylinders.
• The best way to explain changes in gas pressure is to consider particle motion, collision frequency, and collision effects.