91Ó°ÊÓ

Critical temperature (T_c) is a key concept for understanding when a gas can be liquefied. This is the highest temperature at which a substance can exist as a liquid. Above this temperature, no amount of pressure can cause the gas to liquefy.
For a gas to be liquefied, it must be cooled below its critical temperature.
The critical temperature is unique to each substance.
Some important points to remember about critical temperature include:
- It acts as a boundary - above it, substances remain gaseous regardless of the pressure applied.
- It's a reflection of the strength of intermolecular forces - stronger forces result in a higher critical temperature.
Make sure to understand that exceeding the critical temperature of a gas makes liquefaction impossible, no matter the pressure.

Critical pressure (P_c) is another essential concept. This is the pressure required to liquefy a gas at its critical temperature.
Simply put, it's the minimal pressure needed to change a gas to a liquid at the critical temperature.
A few key points to remember about critical pressure:
- Below the critical temperature, increasing pressure sufficiently will cause liquefaction.
- At the critical temperature, if the pressure is not at least the critical pressure, the gas will not liquefy.
Just as with temperature, each substance has its own critical pressure.
Understanding the relationship between temperature and pressure around these critical points can be tricky but is crucial for predicting when liquefaction can occur.

The concept of an ideal gas is a foundational topic in chemistry and physics. An ideal gas is a theoretical gas that perfectly follows the Ideal Gas Law, which is expressed as:
PV = nRT
Here:
- P is pressure
- V is volume
- n is the number of moles
- R is the gas constant
- T is temperature
Remember, an ideal gas assumes completely elastic collisions and negligible intermolecular forces.
This model simplifies the study of gases and provides a good approximation in many situations.
However, real gases show deviations from this behavior, especially under conditions of high pressure and low temperature.
Understanding the behavior of real gases near their critical points involves recognizing these deviations and using more complex models.
This understanding is essential when dealing with liquefaction, as real gases need consideration of their critical temperature and pressure.