Gas Laws: Pressure, Volume & Temperature
Syllabus 2.1.2 (Pressure) & 2.1.3 (Gases) · Extended Tier
🎯 The Hook: Why Do Aerosol Cans Explode?
Ever seen the warning "Do not expose to heat" on a deodorant can? That's not decoration. Heat a sealed gas container and it can violently explode. But why? The answer lies in how billions of invisible gas particles behave when you change their environment.
📖 Baby Steps: What IS Gas Pressure?
A gas is made of billions of tiny particles (atoms or molecules) moving randomly at high speed. They're not sitting still—they're constantly zooming around.
When a gas particle hits the wall of its container, it pushes on that wall for a split second. That's a tiny force.
Now imagine billions of particles hitting the wall every second. All those tiny forces add up. The total force per unit area is what we call pressure.
Pressure = Force ÷ Area
Units: Pascals (Pa) or N/m²
🔬 Visual Clarification: The Particle Model
Each blue particle represents billions of real gas molecules. Notice how they constantly move and hit the walls. Higher speed = more forceful collisions = higher pressure.
🧠 Deep Theory: How Pressure Changes
Change 1: Increase Temperature (at constant volume)
What Happens: Particles move faster.
Why: Faster particles hit the walls harder and more often.
Result: Pressure increases.
⬆️ Temperature → ⬆️ Particle Speed → ⬆️ Pressure
Change 2: Decrease Volume (at constant temperature)
What Happens: Particles are squashed into a smaller space.
Why: Same number of particles hitting a smaller wall area means more collisions per second.
Result: Pressure increases.
⬇️ Volume → ⬆️ Collision Frequency → ⬆️ Pressure
Core vs Extended Content
✓ CORE (ALL STUDENTS)
- • Pressure as force per unit area
- • Qualitative changes (temp ↑ → pressure ↑)
- • Particle model explanation
★ EXTENDED ONLY
- • Kelvin temperature scale
- • Boyle's Law (p₁V₁ = p₂V₂)
- • Quantitative calculations
🌡️ Extended: The Kelvin Scale
The Celsius scale is convenient (0°C = water freezes), but it's not ideal for gas calculations. Scientists use the Kelvin scale where 0 K is absolute zero—the coldest possible temperature where particles stop moving entirely.
T (K) = θ (°C) + 273
Water Freezes
0°C = 273 K
Room Temp
25°C = 298 K
Water Boils
100°C = 373 K
🧮 Kelvin Converter
298 K
Kelvin Temperature
🧪 Virtual Lab: The Piston Experiment
A sealed syringe contains gas. When you push the piston down (reducing volume), watch what happens to the pressure. This demonstrates Boyle's Law.
Piston Position
Drag to compress the gas
130 kPa
Pressure
← Pull piston up (increase volume) | Push piston down (decrease volume) →
Observation: As volume decreases, pressure increases. The gas particles have less space to move, so they hit the walls more frequently.
📐 Boyle's Law (Extended Only)
For a fixed mass of gas at constant temperature, the relationship between pressure and volume is:
p × V = constant
or equivalently:
p₁V₁ = p₂V₂
Where:
- p₁ = initial pressure
- V₁ = initial volume
Where:
- p₂ = final pressure
- V₂ = final volume
🧮 Boyle's Law Calculator
Calculate final pressure (p₂):
200 kPa
Final Pressure (p₂)
p₁V₁ = p₂V₂
(100)(50) = (p₂)(25)
p₂ = (100 × 50) ÷ 25 = 200 kPa
⚠️ Examiner's Traps
TRAP 1: Temperature Units
Wrong: Using Celsius in gas law calculations.
Correct: ALWAYS convert to Kelvin first (T = θ + 273).
TRAP 2: Constant Conditions
Wrong: Applying Boyle's Law when temperature changes.
Correct: Boyle's Law ONLY works at constant temperature. Read the question carefully.
TRAP 3: Particle Explanation
Wrong: "Particles expand when heated."
Correct: Particles move faster (increased kinetic energy), not bigger. The gas expands because particles spread out more.
TRAP 4: Inverse Relationship
Wrong: "Pressure and volume are directly proportional."
Correct: They are INVERSELY proportional. When one goes up, the other goes down (p ∝ 1/V).
📋 Syllabus Checklist
Make sure you can do ALL of these before moving on:
2.1.2.1: Relate pressure to force and area (p = F/A)
2.1.3.1: Describe qualitatively the molecular model of matter (particles in motion)
2.1.3.2: Relate the properties of solids, liquids, and gases to particle motion
2.1.3.3: Explain pressure changes in terms of particle collisions with container walls
2.1.3.4 (Extended): Recall and use the equation T (K) = θ (°C) + 273
2.1.3.5 (Extended): Recall and use the relationship pV = constant for a fixed mass of gas at constant temperature
🎯 Summary: The Big Picture
1. Gas pressure comes from billions of particle collisions with container walls.
2. Increase temperature → particles move faster → more forceful collisions → higher pressure.
3. Decrease volume → particles hit walls more frequently → higher pressure.
4. (Extended) Use Kelvin scale for gas calculations: T(K) = θ(°C) + 273.
5. (Extended) Boyle's Law: p₁V₁ = p₂V₂ (at constant temperature).
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Physics.
Full Access
Unlock all 106 lessons