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P6.6 Gas Pressure and Temperature

GCSE Combined Science · AQA · Chapter P6: Molecules and Matter

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the particle model of a gas, pressure from molecular impacts, how temperature changes the pressure of a gas at constant volume. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain gas pressure in terms of molecules colliding with surfaces, and why it increases when a sealed gas is heated
  • Link the temperature of a gas to the average kinetic energy of its molecules, and explain random (Brownian) motion of smoke particles
  • Describe and interpret a pressure–temperature experiment for a gas at constant volume
  • Use the particle model to explain everyday effects such as tyre pressures and a tin lid blowing off, and say what makes a good scientific model

Key words

  • Gas pressure: The total force per unit area exerted by gas molecules hitting a surface. It acts at right angles to the surface.
  • Average kinetic energy: The mean kinetic energy of the gas molecules. The higher the temperature, the greater the average kinetic energy.
  • Random motion: Unpredictable motion: speed and direction keep changing because of collisions.
  • Brownian motion: The jerky, random movement of small visible particles (e.g. smoke) caused by collisions with fast-moving molecules.
  • Fixed mass of gas: A sealed sample, so the number of molecules does not change.
  • Particle (kinetic) model: A scientific model: gas particles in constant random motion, colliding with each other and the walls; their average kinetic energy sets the temperature.

Gas pressure and temperature

Gas molecules move at high speed in random directions. Each time one hits a wall and bounces off, it exerts a tiny force on the wall. Billions of impacts every second add up to a steady force. The force per square metre is the pressure, and its total effect is a net force at right angles to each wall.

The temperature of a gas is linked to the average kinetic energy of its molecules. If a gas in a sealed container (fixed volume) is heated:

  • its molecules gain kinetic energy and move faster on average
  • they hit the walls more often (more impacts per second)
  • each impact is harder (bigger force)
  • so the pressure increases.

Safety: Never heat a sealed can or aerosol. As the temperature rises, the gas pressure inside can rise enough to burst it.

Pressure–temperature experiment

A flask of dry air is connected to a pressure gauge and placed in a water bath. The water is heated in stages; at each stage it is stirred so the water and air are at the same, even temperature, then the temperature and pressure are read. (This is done by a teacher behind a safety screen.)

The graph is a straight line: the pressure rises by the same amount for each equal rise in temperature. It does not go through the origin, because the air still has a pressure (about atmospheric pressure, 100 kPa) at 0 °C – the molecules are still moving.

Evidence for moving molecules: Brownian motion

Air molecules are far too small to see. But if smoke is put into a small glass cell, lit from the side and viewed through a microscope, the smoke particles are seen as tiny specks of light that jiggle about at random, constantly changing direction.

The air molecules are moving fast in random directions and hit each smoke particle unevenly, giving it a push in a random direction each time. This is evidence that gas molecules are in constant, fast, random motion. If the air is warmer, the molecules move faster and the smoke particles jiggle more.

Using the particle model

A demonstration (behind a safety screen, with eye protection): a little water is put in a tin with a tight lid and the tin is heated. Some water evaporates, so there are more gas molecules inside, and all of them move faster as the temperature rises. They hit the lid more often and harder, until the force from the gas inside is big enough to push the lid off.

Worked example: tyre pressure after a long drive

A car tyre is at 230 kPa when cold. After a long motorway drive the air in the tyre is warmer and the pressure is 250 kPa.
The volume of the tyre hardly changes, so the air molecules move faster, hit the inside of the tyre more often and harder, and the pressure rises.
Increase = 250 − 230 = 20 kPa

Answer: the pressure rises by 20 kPa – drivers should check tyre pressures when the tyres are cold

Key fact: The particle model is a scientific model: an idea that explains observations, makes predictions that can be tested, and fits with other ideas in science. Models are kept while their predictions agree with experiments and are changed when new evidence does not fit.

Common mistakes

  • Saying the molecules “get bigger” or “expand” when a gas is heated. The molecules move faster; they do not change size.
  • Explaining pressure only as “more collisions”. Say collisions with the walls, more often and (for heating) with more force.
  • Saying the molecules hit “each other” harder to explain pressure. Pressure is caused by impacts on the walls of the container.
  • Thinking the pressure is zero at 0 °C. At 0 °C the molecules are still moving fast and still hit the walls.

Revision questions

Try these, then check your answers below each one.

Question 1. What causes the pressure of a gas on the walls of its container? (Choose from: Gas molecules colliding with the walls, Gas molecules colliding with each other, The weight of the molecules pressing down, The molecules expanding when heated.)

Answer: Gas molecules colliding with the walls

Working: Molecules hit the walls and bounce off. Each impact exerts a force; the total force per unit area is the pressure.

Question 2. The temperature of a gas is linked to which property of its molecules? (Choose from: Their average kinetic energy, Their size, Their total mass, The number of molecules.)

Answer: Their average kinetic energy

Working: The higher the temperature, the greater the average kinetic energy of the molecules.

Question 3. True or false? Smoke particles in air move about at random because they are hit unevenly by fast-moving air molecules.

Answer: True

Working: True. The random motion of smoke particles is evidence that air molecules are moving fast at random.

Related lessons

Continue in the full course

This lesson is section P6.6 (AGCSC3505) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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