Charles's Law
At constant pressure, V is proportional to Kelvin temperature.
About this topic
Charles's law states that at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature: V1/T1 = V2/T2, with T in kelvin. Heating a gas makes it expand; cooling it makes it contract.
A graph of volume against temperature in Celsius is a straight line that, if extended, hits zero volume at about minus 273.15 degrees, which led to the idea of absolute zero and the Kelvin scale. In reality gases liquefy before then. Particles move faster when heated and push harder on the walls, so the volume must increase to keep the pressure steady. Jacques Charles first studied this in the 1780s while working with hydrogen balloons.
Worked example: a balloon of 1.00 L at 300 K is placed in a freezer at 250 K. V2 = 1.00 x 250 / 300 = 0.833 L. This explains why a basketball seems flat on a cold day, why hot air balloons rise (the heated air is less dense), and why a dented ping-pong ball regains its shape in hot water.
The big mistake is using degrees Celsius, which can give zero or negative volumes. Convert by adding 273.15. Also, doubling the Celsius temperature does not double the volume; doubling the kelvin temperature does.
Key ideas
- V₁/T₁ = V₂/T₂
- Use kelvin
- Hot-air balloons
Quick check
Use cases: where Charles's Law helps
- Homework: review the key idea behind Charles's Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Charles's Law without notes.
- Visual learning: play with the interactive lab to see how Charles's Law changes when you adjust its inputs.
- Teaching a friend: explain Charles's Law in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Charles's Law?
At constant pressure, V is proportional to Kelvin temperature.
What will I learn in Charles's Law?
V₁/T₁ = V₂/T₂. Use kelvin. Hot-air balloons.
Is Charles's Law part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Charles's Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Charles's Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.