Charles's Law
Definition and meaning of Charles's Law in chemistry.
Charles's Law states that the volume of an ideal gas is directly proportional to its absolute temperature. This physical rule only works if the gas pressure and amount of gas remain completely constant. You can write this relationship as the mathematical equation V1/T1 = V2/T2.
In more detail
When you heat a confined gas, you add thermal energy to the system. This energy increases the kinetic energy of the individual gas molecules. The molecules start moving with much greater speed and velocity.
These faster molecules hit the container walls more frequently and with much more force. The gas must expand its volume to keep the pressure constant against these stronger collisions. You must measure the temperature using an absolute scale, like kelvin.
This absolute scale is required because the gas volume must mathematically reach zero at absolute zero. Absolute zero equals exactly 0 K or negative 273.15 degrees Celsius. Real gases only perfectly follow this law at high temperatures and low pressures.
Under these conditions, the physical size of the molecules and their attractive forces do not matter. However, the law breaks down when the gas gets very cold. Near the boiling point, attractive forces pull the slow molecules closer together.
The gas volume shrinks faster than the temperature drops. This law is a specific part of the broader ideal gas law (PV = nRT). The French physicist Jacques Charles discovered the principle in the 1780s using gas expansion experiments. Joseph Louis Gay-Lussac later published the formal math in 1802.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | V/T = k (constant P, n) |
| Also known as | Law of volumes |
| Discovered by | Jacques Charles (c. 1787); published by Gay-Lussac (1802) |
| Scale Requirement | Absolute temperature scale (kelvin) |
| Core Principle | Kinetic molecular theory and expanding collision volumes |
| Derived Equation | V1/T1 = V2/T2 |
Imagine a flexible balloon that holds exactly 2.0 L of air at 300 K. You slowly heat the balloon at a constant pressure until the temperature reaches 600 K. You can find the new volume using the equation V1/T1 = V2/T2. When you calculate (2.0 L)/(300 K) = V2/(600 K), you find the final volume doubles to 4.0 L.
Frequently asked questions
Why must temperature be in kelvin for Charles's Law?
The direct relationship requires an absolute scale where zero volume mathematically aligns with zero temperature. Using the Celsius scale introduces negative values and breaks the math.
How does Charles's Law relate to the ideal gas law?
Charles's Law is a special case of the ideal gas law (PV = nRT). It isolates volume and temperature by keeping pressure and moles perfectly constant.
Do real gases perfectly obey Charles's Law at all temperatures?
No, real gases only approximate this law at high temperatures and low pressures. Near boiling points, attractive forces cause the volume to shrink faster than predicted.