Balloon in the Sun: Charles' Law
Volume rises in step with kelvin temperature at constant pressure
What this lesson covers
Why it matters
Leave an inflated balloon in the bright sun and it swells until it may even burst; put it in the fridge and it sags and wrinkles. Same balloon, same air — only the temperature has changed, and the volume has followed it faithfully.
Predict first
A balloon holds 1 litre of air at 300 K. You warm it gently to 600 K at the same pressure. Its new volume is…
Charles' law: at constant pressure, the volume of a fixed mass of dry gas is directly proportional to its absolute (kelvin) temperature. Double 300 K to 600 K and 1 L becomes 2 L, because V₁/T₁ = V₂/T₂.
- 2 litres — doubling the kelvin temperature doubles the volume — correct
- Still 1 litre — a balloon only grows if you blow more air in
- 600 litres — the volume equals the temperature number
What you do
Warm the gas gently: slide the temperature to each dashed target (in kelvin) and record the point. The volume climbs in step — V ÷ T stays fixed, and your straight line points directly at absolute zero.
Check yourself
Charles' law says that at constant pressure, the volume of a fixed mass of dry gas is…
At constant pressure, V ∝ T (kelvin), so V₁/T₁ = V₂/T₂. The book's original wording says the gas expands or contracts by 1/273 of its volume at 0°C for each 1°C change — both statements describe the same law.
20 mL of hydrogen at −13°C is heated to 117°C at constant pressure. What is the new volume?
Convert first: T₁ = −13 + 273 = 260 K, T₂ = 117 + 273 = 390 K. Then V₂ = V₁ × T₂/T₁ = 20 × 390 ÷ 260 = 30 mL. Using Celsius values gives a meaningless negative volume.
Why must temperatures be converted to kelvin before using V₁/T₁ = V₂/T₂?
The zero on the Celsius scale is arbitrary, so its values can be negative and their ratios mean nothing physically. The kelvin scale begins at absolute zero (−273°C), where molecular motion would cease, so doubling the kelvin temperature really does double the volume.
- directly proportional to its absolute (kelvin) temperature — correct
- inversely proportional to its pressure
- directly proportional to its Celsius temperature
- 30 mL — correct
- −180 mL
- 20 mL
- Only the kelvin scale starts at absolute zero, so volume is truly proportional to it — correct
- A kelvin degree is bigger than a Celsius degree
- Thermometers cannot read gas temperatures in Celsius