Pressure in gases

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Lesson notes

Brownian Motion

**Brownian motion** is the random movement of particles in a fluid (gas or liquid).
It was observed by **Robert Brown** in 1827 when he saw pollen grains moving randomly in water.
Smoke particles in air also show Brownian motion due to collisions with fast-moving air particles.
Gas particles move in **random directions** at a **range of speeds**.
Collisions with invisible gas particles cause larger visible particles to jitter.

Gas Pressure from Particle Collisions

Gas particles exert **pressure** when they collide with a surface.
Each collision exerts a tiny **force** on the surface; many collisions produce measurable pressure.
Pressure = force per unit area on the walls of the container.

Effect of Adding More Gas

Adding more gas particles (e.g., pumping air into a balloon) increases the **number of particles**.
More particles lead to **more frequent collisions** with the walls.
More collisions increase the **force** on the walls, raising the **pressure**.
Higher pressure causes the balloon to **expand** until internal and external pressures balance.

Effect of Decreasing Volume

Reducing the volume of a container (e.g., squeezing a gas) forces particles into a smaller space.
Particles collide with the walls **more often** because they have less distance to travel.
More frequent collisions increase the **force** and therefore the **pressure**.

Effect of Increasing Temperature

Heating a gas increases the **kinetic energy** and **speed** of its particles.
Faster particles collide with the walls **more often** and with **greater force**.
This raises the **pressure** inside the container.
If the container is flexible (e.g., a balloon), it **expands** as pressure increases.

Effect of Decreasing Temperature

Cooling a gas reduces the **kinetic energy** and **speed** of particles.
Slower particles collide **less often** and with **less force**.
Pressure inside the container **decreases**.
A flexible container (e.g., a balloon) will **shrink** as internal pressure drops.

Particle model of a gas: particles are far apart, moving randomly at high speed.

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Practice questions

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1.What is the name of the process where smoke particles move in a random way due to collisions with air particles?
Easy
- ABrownian motion
- BDiffusion
- COsmosis
- DEvaporation
2.What happens to the number of collisions between air particles and the walls of a balloon when more air is pumped into it?
Easy
- AIncreases
- BDecreases
- CStays the same
- DBecomes zero
3.When the volume of a container is reduced, what happens to the pressure of the gas inside?
Easy
- AIncreases
- BDecreases
- CStays the same
- DBecomes zero
4.Two containers have the same amount of gas particles. Container A is smaller than container B. Which container has the higher pressure?
Medium
- AContainer A
- BContainer B
- CBoth have the same pressure
- DCannot be determined
5.Two containers have the same amount of gas particles. Container A is at a lower temperature than container B. Which container has the higher pressure?
Medium
- AContainer B
- BContainer A
- CBoth have the same pressure
- DCannot be determined
6.What happens to the average speed of gas particles when the temperature is increased?
Medium
- AIncreases
- BDecreases
- CStays the same
- DBecomes zero
7.A balloon is cooled. What happens to its size?
Hard
- AIt shrinks
- BIt expands
- CIt stays the same
- DIt bursts
8.What is the cause of pressure exerted by a gas on the walls of its container?
Easy
- ACollisions of gas particles with the walls
- BThe weight of the gas particles
- CThe attraction between gas particles
- DThe repulsion between gas particles

More questions

9.Which of the following describes the motion of particles in a gas?
Medium
- ARandom directions and a range of speeds
- BRandom directions but all the same speed
- COrderly directions and a range of speeds
- DOrderly directions and all the same speed
10.What happens to the force exerted by gas particles on the walls of a container when the volume is increased?
Medium
- ADecreases
- BIncreases
- CStays the same
- DBecomes zero
11.Brownian motion was first observed by Robert Brown in 1827. What particles was he observing?
Easy
- APollen particles in water
- BSmoke particles in air
- CDust particles in air
- DGas particles in a container
12.A sealed container of gas is heated. Which of the following statements is true?
Hard
- AThe pressure increases because particles move faster and collide more often
- BThe pressure decreases because particles move faster and collide less often
- CThe pressure stays the same because the number of particles is constant
- DThe pressure increases because particles become larger
13.A sealed container of gas is heated. Which of the following best explains why the pressure increases?
Medium
- AThe gas particles increase in size.
- BThe gas particles move faster and collide more frequently with the walls.
- CThe gas particles become more massive.
- DThe number of gas particles increases.
14.In an experiment, a gas is trapped in a syringe. The plunger is pushed in, reducing the volume. Which graph shows the relationship between pressure and volume for this gas at constant temperature?
Hard
- AA straight line with positive slope through the origin.
- BA curve that decreases as volume increases (inverse relationship).
- CA horizontal line.
- DA curve that increases as volume increases.
15.The diagram shows the particle arrangement in a gas. What causes the pressure exerted by the gas on the walls of its container?
Easy
- AThe particles sticking to the walls.
- BThe particles colliding with the walls.
- CThe particles vibrating in fixed positions.
- DThe particles expanding against the walls.
16.Two identical balloons are filled with the same number of gas particles. Balloon A is placed in a freezer, and Balloon B is left at room temperature. Which balloon has a smaller volume?
Medium
- ABalloon A because the particles move slower and collide less forcefully.
- BBalloon B because the particles move faster and expand the balloon.
- CBalloon A because the particles shrink in size.
- DBalloon B because the particles lose energy.
17.A gas occupies 50 cm³ at a pressure of 100 kPa. If the pressure is increased to 200 kPa at constant temperature, what is the new volume?
Hard
- A25 cm³
- B100 cm³
- C50 cm³
- D75 cm³
18.A student observes smoke particles under a microscope. The particles move in a random, jerky manner. What is the best explanation for this motion?
Medium
- AThe smoke particles are alive and moving on their own.
- BAir particles are colliding with the smoke particles.
- CThe smoke particles are being pushed by light.
- DThe smoke particles are repelling each other.
19.What happens to the pressure of a gas if the volume of its container is increased while temperature stays constant?
Easy
- APressure increases.
- BPressure decreases.
- CPressure stays the same.
- DPressure becomes zero.
20.A gas is at a pressure of 2 atm and volume 3 L. The volume is changed to 1 L at constant temperature. What is the new pressure?
Hard
- A6 atm
- B2 atm
- C1.5 atm
- D0.67 atm
21.In a sealed container, the temperature of a gas is doubled (in Kelvin). What happens to the average kinetic energy of the gas particles?
Medium
- AIt doubles.
- BIt halves.
- CIt stays the same.
- DIt quadruples.
22.A student has two syringes containing the same amount of gas. Syringe A has a volume of 20 mL, Syringe B has a volume of 40 mL. Which syringe has higher pressure?
Medium
- ASyringe A because it has a smaller volume.
- BSyringe B because it has a larger volume.
- CBoth have the same pressure.
- DSyringe A because it has more particles.
23.What is the name of the random motion of particles in a fluid caused by collisions with other particles?
Easy
- ABrownian motion
- BNewtonian motion
- CEinstein motion
- DRandom walk
24.A bicycle pump is used to inflate a tyre. As more air is pumped in, the tyre becomes harder. Which statement correctly explains this?
Medium
- AThe number of air particles inside the tyre decreases, reducing collisions.
- BThe number of air particles inside the tyre increases, increasing collisions and pressure.
- CThe air particles expand to fill the tyre, increasing pressure.
- DThe air particles slow down, increasing pressure.
25.In an experiment to demonstrate Brownian motion, a student observes smoke particles in air through a microscope. Which of the following best describes the motion of the smoke particles?
Medium
- AThey move in straight lines at constant speed.
- BThey move in a random, jerky manner.
- CThey move in circles around the air particles.
- DThey remain stationary.
26.A student sets up an experiment with a gas syringe to investigate the relationship between pressure and volume of a gas. The plunger is pushed in to reduce the volume. Which of the following correctly explains the change in pressure?
Medium
- AThe gas particles move faster, increasing the force on the walls.
- BThe gas particles collide with the walls more often, increasing the force on the walls.
- CThe gas particles become smaller, increasing the number of collisions.
- DThe gas particles expand, increasing the volume.
27.A student uses a pressure sensor to measure the pressure of a gas in a sealed container as the temperature is increased. Which graph best shows the relationship between pressure and temperature (in kelvin) for a fixed volume of gas?
Hard
- AA straight line through the origin with positive slope.
- BA curve that increases and then levels off.
- CA straight line with negative slope.
- DA horizontal line.
28.In a laboratory demonstration, a teacher uses a smoke cell and a microscope to show Brownian motion. Which of the following is the best reason why smoke particles are used instead of air molecules?
Medium
- ASmoke particles are visible under a microscope, while air molecules are too small to see.
- BSmoke particles are heavier and do not move.
- CSmoke particles are coloured, making them easier to track.
- DSmoke particles move in a straight line, which is easier to observe.
29.A student conducts an experiment to investigate the effect of temperature on the pressure of a gas at constant volume. The student heats a sealed container and records the pressure at different temperatures. Which of the following must be kept constant to ensure a fair test?
Hard
- AThe volume of the container and the number of gas particles.
- BThe temperature and the volume of the container.
- CThe pressure and the number of gas particles.
- DThe size of the container and the type of gas.
30.A gas is trapped in a syringe. The plunger is pulled out to increase the volume. Which of the following statements is correct?
Medium
- AThe pressure increases because the particles collide more often.
- BThe pressure decreases because the particles collide less often.
- CThe pressure stays the same because the number of particles is unchanged.
- DThe pressure increases because the particles move faster.
31.Which of the following is the correct definition of pressure exerted by a gas?
Easy
- AThe force exerted by gas particles on the walls of the container per unit area.
- BThe total kinetic energy of all gas particles in the container.
- CThe number of gas particles in the container.
- DThe speed of the fastest gas particle.
32.A student investigates the relationship between pressure and volume of a gas using a pressure sensor and a syringe. The student records the following data: at volume 40 cm³, pressure is 100 kPa; at volume 20 cm³, pressure is 200 kPa. Which of the following conclusions is supported by the data?
Hard
- APressure is directly proportional to volume.
- BPressure is inversely proportional to volume.
- CPressure is independent of volume.
- DPressure is directly proportional to the square of volume.
33.In a sealed container, the temperature of a gas is increased. Which of the following statements about the average kinetic energy of the gas particles is correct?
Medium
- AThe average kinetic energy decreases.
- BThe average kinetic energy increases.
- CThe average kinetic energy stays the same.
- DThe average kinetic energy becomes zero.
34.A student performs an experiment to demonstrate Brownian motion using smoke particles. The student notices that the motion becomes more vigorous when the air is heated. Which of the following explains this observation?
Hard
- AThe smoke particles expand when heated, making them move faster.
- BThe air particles move faster, causing more forceful collisions with the smoke particles.
- CThe smoke particles become lighter, so they move more easily.
- DThe air pressure decreases, allowing the smoke particles to move freely.
35.What happens to the pressure of a gas in a sealed container when the volume is decreased?
Easy
- AThe pressure increases.
- BThe pressure decreases.
- CThe pressure stays the same.
- DThe pressure becomes zero.
36.A student measures the pressure of a gas in a syringe at different volumes. The results are shown in the table. Which of the following is the most likely value for the pressure when the volume is 30 cm³? (Assume constant temperature.) Volume (cm³) | Pressure (kPa) 10 | 300 20 | 150 40 | 75
Medium
- A100 kPa
- B200 kPa
- C150 kPa
- D50 kPa
37.What is the name of the process where particles move randomly due to collisions with other particles in a fluid?
Easy
- ABrownian motion
- BDiffusion
- COsmosis
- DRandom motion
38.When air is pumped into a balloon, what happens to the number of collisions between air particles and the balloon walls?
Easy
- AIncreases
- BDecreases
- CStays the same
- DBecomes zero
39.A gas is in a sealed container. The volume of the container is reduced. What happens to the force exerted by the gas particles on the walls?
Medium
- AIncreases
- BDecreases
- CStays the same
- DBecomes zero
40.Two containers hold the same number of gas particles. Container X is smaller than container Y. Which has higher pressure?
Medium
- AContainer X
- BContainer Y
- CBoth have the same pressure
- DCannot be determined

Revision notes

## Pressure in Gases - **Gas particle motion**: Particles move in random directions at a range of speeds (Brownian motion). - **Brownian motion**: Observed by Robert Brown (1827) – pollen grains move randomly due to collisions with fast-moving air particles. - **Pressure**: When gas particles collide with a surface, they exert pressure. More collisions → higher force → higher pressure. - **Adding particles**: More air particles → more collisions → higher pressure → balloon expands. - **Reducing volume**: Smaller volume → particles collide more often → higher force → higher pressure. - **Increasing temperature**: Higher temperature → particles gain kinetic energy and speed → more frequent collisions → higher pressure (balloon expands when heated). - **Key relationships**: - Pressure ∝ number of particles (at constant volume and temperature) - Pressure ∝ 1/volume (at constant number and temperature) - Pressure ∝ temperature (at constant number and volume)

Brownian motion: The random movement of particles (e.g., smoke or pollen) caused by collisions with fast-moving gas particles.
Pressure: The force exerted per unit area on a surface due to collisions of gas particles.

Flashcards

What is Brownian motion?

Brownian motion is the random movement of particles (e.g., smoke or pollen) due to collisions with fast-moving gas particles.

Who discovered Brownian motion?

Robert Brown discovered Brownian motion in 1827.

Why do smoke particles move randomly in air?

Smoke particles move randomly because they are hit by air particles moving in random directions at different speeds.

What happens when a gas particle collides with a surface?

When a gas particle collides with a surface, it exerts pressure on that surface.

Why does a balloon get bigger when you blow air into it?

Adding air increases the number of particles inside, leading to more collisions with the walls, higher force, higher pressure, and expansion.

What happens to pressure when the volume of a gas container is reduced?

Reducing volume causes more frequent collisions with the walls, increasing force and pressure.

What happens to gas particle speed when temperature increases?

The average speed of gas particles increases as temperature increases.

Why does a balloon expand when heated?

Heating increases particle speed and kinetic energy, causing more frequent collisions, higher force, higher pressure, and expansion.

What happens to the number of collisions when you pump more air into a balloon?

The number of collisions between air particles and the balloon walls increases.

What happens to force on the walls when more air is pumped into a balloon?

The force exerted by air particles on the walls increases.

What happens to pressure inside a balloon when more air is pumped in?

The pressure inside the balloon increases.

What happens to the size of a balloon when more air is pumped in?

The balloon expands (gets bigger).

What happens to force on the walls when the volume of a gas container is increased?

The force exerted by gas particles on the walls decreases.

What happens to pressure inside a container when its volume is increased?

The pressure inside the container decreases.

Two containers have the same number of gas particles. Container A is smaller than B. Which has higher pressure?

Container A has higher pressure because particles collide with the walls more often in a smaller volume.

Two containers have the same number of gas particles. Container A is at lower temperature than B. Which has higher pressure?

Container B has higher pressure because its particles move faster, causing more frequent and forceful collisions.

What happens to the size of a balloon when it is cooled?

The balloon shrinks because cooling reduces particle speed, leading to fewer collisions, lower pressure, and contraction.

How do gas particles move?

Gas particles move in random directions and at a range of different speeds.

What is the relationship between temperature and kinetic energy of gas particles?

Increasing temperature increases the kinetic energy of gas particles.

What causes pressure in a gas?

Pressure is caused by gas particles colliding with the walls of the container.

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