Ever wondered why a balloon shrinks when you leave it in a cold room? That’s gas law magic at work.
Gas laws tell us how pressure, volume, temperature and amount of gas are linked. Change one, and the others react – just like water in a hose changes speed when you squeeze the nozzle.
What are Gas Laws?
Gas laws are simple rules that describe how a gas behaves when you tweak its pressure, volume, temperature or amount.
Pressure is the force the gas particles push on the walls of their container, measured per unit area – think of it as how hard you press a balloon with your hand.
Volume is the space the gas occupies, like the size of the balloon.
Temperature tells how fast the particles are moving – hotter means they zip around faster.
Amount (or moles) is a count of how many particles you have, similar to counting the number of marbles in a jar.
Four Core Gas Laws you’ll see in ISC exams
Boyle’s Law – Pressure vs Volume
When temperature and amount stay the same, pressure and volume are inversely related. Double the pressure, and the volume halves.
Formula: P₁V₁ = P₂V₂
Charles’s Law – Volume vs Temperature
If pressure and amount are constant, volume grows as temperature rises. Warm a gas and it expands.
Formula: V₁/T₁ = V₂/T₂ (temperature in Kelvin)
Gay‑Lussac’s Law – Pressure vs Temperature
Keep volume and amount fixed, and pressure climbs with temperature.
Formula: P₁/T₁ = P₂/T₂
Avogadro’s Law – Volume vs Amount
At constant pressure and temperature, more moles mean more volume. Double the moles, double the volume.
Formula: V₁/n₁ = V₂/n₂
Quick Comparison Table
| Law | What stays constant? | Relationship | Key formula |
|---|---|---|---|
| Boyle’s | Temperature, amount | Pressure ↔ Volume (inverse) | P₁V₁ = P₂V₂ |
| Charles’s | Pressure, amount | Volume ↔ Temperature (direct) | V₁/T₁ = V₂/T₂ |
| Gay‑Lussac’s | Volume, amount | Pressure ↔ Temperature (direct) | P₁/T₁ = P₂/T₂ |
| Avogadro’s | Pressure, temperature | Volume ↔ Amount (direct) | V₁/n₁ = V₂/n₂ |
Worked Example – Using Boyle’s Law
Suppose a 2.0 L gas container is at 1.0 atm pressure. You compress it to 0.5 L. What is the new pressure?
- Write down what you know: P₁ = 1.0 atm, V₁ = 2.0 L, V₂ = 0.5 L.
- Plug into Boyle’s formula: P₁V₁ = P₂V₂ → 1.0 atm × 2.0 L = P₂ × 0.5 L.
- Solve for P₂: P₂ = (1.0 × 2.0) / 0.5 = 4.0 atm.
So the pressure jumps to 4 atm – four times bigger because the volume is a quarter of the original.
Tips to Remember the Laws
- Think of a syringe: pushing the plunger (decreasing volume) makes the gas harder to push out (higher pressure) – that’s Boyle’s.
- Imagine a hot air balloon: heat the air (raise temperature) and the balloon swells – that’s Charles’s.
- Picture a sealed canister in a freezer: temperature drops, pressure drops – that’s Gay‑Lussac’s.
- When you add more balloons to a room with the same pressure, the room needs more space – that’s Avogadro’s.
📝 Likely Exam Questions
- State Boyle’s law and give its mathematical expression.
Answer: At constant temperature and amount, pressure and volume are inversely proportional. P₁V₁ = P₂V₂. - A gas occupies 8.0 L at 300 K. What volume will it have at 450 K if pressure remains unchanged?
Answer: Using Charles’s law, V₂ = V₁ × (T₂/T₁) = 8.0 L × (450/300) = 12.0 L. - How does the pressure of a fixed amount of gas change when its temperature is doubled, keeping volume constant?
Answer: According to Gay‑Lussac’s law, pressure is directly proportional to temperature, so pressure also doubles. - Explain why the volume of a gas increases when more moles are added at constant pressure and temperature.
Answer: Avogadro’s law states volume is directly proportional to amount of gas; more moles mean more particles, which need more space. - Calculate the final pressure when 0.5 mol of an ideal gas at 1 atm and 273 K is heated to 546 K in a rigid container.
Answer: Using P₁/T₁ = P₂/T₂, P₂ = P₁ × (T₂/T₁) = 1 atm × (546/273) = 2 atm.