Each gas law holds one quantity fixed. Identify the fixed quantity, pick the matching law, and always use kelvin for temperature.
This lesson is part of heat. It is independent of the heat-energy lessons, so you can study it before or after them.
Which law matches which condition?
Use this table whenever a gas question appears.
| Constant | Law | Equation |
|---|---|---|
| Temperature | Boyle | P₁V₁ = P₂V₂ |
| Pressure | Charles | V₁ ÷ T₁ = V₂ ÷ T₂ |
| Volume | Pressure law | P₁ ÷ T₁ = P₂ ÷ T₂ |
The mass of gas is fixed in all three. Write the constant quantity in the margin before choosing the equation.
Worked example: Boyle’s law
A gas at 100 kPa has a volume of 600 cm³. It is compressed slowly to 400 cm³ at constant temperature. Then P₂ = P₁V₁ ÷ V₂ = 100 × 600 ÷ 400 = 150 kPa.
Worked example: pressure law
A sealed container holds gas at 27°C and 200 kPa. It is heated to 87°C. The volume is fixed.
- Convert: T₁ = 27 + 273 = 300 K and T₂ = 87 + 273 = 360 K.
- P₂ = P₁ × T₂ ÷ T₁ = 200 × 360 ÷ 300 = 240 kPa.
Worked example: Charles’s law
2.0 dm³ of gas at 27°C is heated to 127°C at constant pressure. T₁ = 300 K and T₂ = 400 K. So V₂ = 2.0 × 400 ÷ 300 = 2.67 dm³.
The mistake that costs marks
The common slip is using °C. In the pressure law example it gives 200 × 87 ÷ 27 = 644 kPa, which is far too large. The other slip is choosing the wrong law because the question mentions heating.
| Slip | Effect | Fix |
|---|---|---|
| Using 87 and 27 instead of 360 and 300 | 644 kPa instead of 240 kPa | Convert to kelvin |
| Using Boyle’s law for a heated gas | Ignores temperature | Find the constant quantity first |
| Mixing kPa and atm | Unit mismatch | Use one unit for both pressures |
Do a sense check. Heating a gas at constant volume must raise its pressure, and compressing it at constant temperature must also raise its pressure. The units and significant figure checker helps you check conversions.
Check yourself
500 cm³ of gas at 7°C is heated to 77°C at constant pressure. Find the new volume.
Answer
Pressure is constant, so use Charles’s law. T₁ = 7 + 273 = 280 K and T₂ = 77 + 273 = 350 K.
V₂ = 500 × 350 ÷ 280 = 625 cm³.
The volume rose, as it should when a gas is heated at constant pressure.
What to study next
Put the gas ideas next to energy ideas with interpreting temperature-time graphs, or use the heat practice set.
To have a teacher go through gas law questions with you, see online one-to-one Physics tuition.