Lesson overview
Gas pressure and temperature: Why can a sealed can burst when it gets hot?
Trap some air in a syringe joined to a pressure gauge and squeeze it to smaller and smaller volumes, with a magnifier showing a model of its particles. Then lock the plunger, cool the air with ice and warm it on a hotplate, and extend the graph to find where the pressure would reach zero.
Suggested sequence
- Lesson 1 · Squeezing a gas. Gas pressure from particles hitting the walls, as a force per unit area; squeezing a gas at a steady temperature, and pV = constant with its graphs. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Heating a gas at a fixed volume. Heating a gas at a fixed volume, in particle terms; the kelvin scale and T = θ + 273. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Thermal expansion. Thermal expansion of solids, liquids and gases, why gases expand most, and where expansion is useful or a problem. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 3 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Squeezing a gas
- Describe the pressure of a gas, and changes in it, in terms of the random motion of its particles and their collisions with a surface (2.1.2.3)
- Explain gas pressure as the force of the particle collisions on each unit of area of a surface (2.1.2.7)
- Describe and explain, in terms of particles, how the pressure of a fixed mass of gas at a constant temperature changes when its volume changes (2.1.3.1)
- Use pV = constant for a fixed mass of gas at a constant temperature, including graphs of p against V and p against 1/V (2.1.3.3)
Part 2: Heating a gas at a fixed volume
- Describe and explain, in terms of particles, how the pressure of a fixed mass of gas at a constant volume changes when its temperature changes (2.1.3.1)
- Convert temperatures between the Celsius and Kelvin scales using T (in K) = θ (in °C) + 273 (2.1.3.2)
Part 3: Thermal expansion
- Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure (2.2.1.1)
- Describe some of the everyday applications and consequences of thermal expansion (2.2.1.2)
- Explain, in terms of the motion and arrangement of particles, the relative order of magnitude of the expansion of solids, liquids and gases as their temperatures rise (2.2.1.3)
Misconceptions to listen for
- “The weight of the air presses down on the bottom of the syringe.” — The weight of 50 cm³ of air is tiny: from the top of the syringe to the bottom it changes the pressure by only about a thousandth of a kilopascal. The gauge reads 100 kPa whichever way up the syringe is held, and the air pushes just as hard on the side walls. The push comes from moving particles hitting in all directions.
- “The particles are packed tightly and push outwards on the walls.” — In a gas the particles are far apart, with empty space between them. They push on a wall only when they hit it and bounce off.
- “(3) volume” — Pressure is the force on each unit of area of a surface, such as each square centimetre of a wall, not on each unit of volume of the gas.
- “(4) slowly” — Slower particles hit the walls less often and less hard, so the pressure would fall, not rise.
- “The pressure on the lid is bigger, because it is bigger.” — The lid gets a bigger force, because it has more area. Pressure is the force on each unit of area, and each square centimetre of the lid is hit just as often as each square centimetre of the window.
- “The pressure on the window is bigger, because the force is concentrated on a small area.” — The small window gets fewer hits, so a smaller force, on a smaller area. The force on each square centimetre is the same.
- “The particles get smaller when the gas is squeezed.” — The particles keep their size. Only the space between them gets smaller.
- “The particles move faster because they are squeezed, so they hit harder.” — At a constant temperature the particles keep the same speed, so each hit is just as hard. What changes is how often they hit.
- “p₂ = 100 × 20 ÷ 60 = 33 kPa.” — A smaller volume must give a bigger pressure. Keep p × V the same: 6000 ÷ 20.
- “The volume fell by 40 cm³, so the pressure rises by 40 kPa to 140 kPa.” — The product p × V stays the same, not the sum. Equal squeezes do not add equal amounts of pressure.
Scope
Gas pressure and changes in it from the motion of particles and their collisions with a surface (2.1.2.3), and pressure as the force of the collisions per unit area (2.1.2.7); the effect on the pressure of a fixed mass of gas of a change of temperature at constant volume and of a change of volume at constant temperature, in particle terms (2.1.3.1); converting between kelvin and degrees Celsius, T (in K) = θ (in °C) + 273 (2.1.3.2); pV = constant at constant temperature, with its graphs (2.1.3.3); thermal expansion of solids, liquids and gases at constant pressure, its uses and consequences, and why solids expand least and gases most (2.2.1.1–2.2.1.3). Absolute zero as the temperature of least particle motion and the particle model of matter are in PART01; pressure as force ÷ area and pressure in liquids are in PRES01.