Critical Temperature
Definition and meaning of Critical Temperature in chemistry.
Critical temperature is the highest temperature where a substance can become a liquid by adding pressure. Above this exact point, no amount of squeezing will turn the gas into a liquid. The substance will instead stay in a gas-like state no matter how hard you compress it.
In more detail
Every chemical has its own specific critical temperature, known as Tc. This specific value depends on how strongly the molecules attract one another. Stronger intermolecular forces lead to a much higher critical temperature.
As a substance heats up, its molecules dart around much faster. This rapid movement gives the molecules a very high kinetic energy. Eventually, this motion overpowers the attractive forces pulling the molecules together.
At the critical temperature, these pulling forces fail to maintain a liquid phase. The separate gas and liquid states merge into one single phase. The clear visible boundary between liquid and vapor completely vanishes.
The energy needed to boil the liquid falls all the way to zero. Pushing the pressure past the critical pressure creates something entirely new. The substance becomes a supercritical fluid.
This fluid moves like a gas but dissolves things like a liquid. A common student mistake is thinking the substance just boils here. Boiling only happens below the critical point when liquid turns to vapor. At the critical temperature, the very idea of a liquid ceases to exist.
Key facts
| Symbol | Tc |
|---|---|
| CO2 critical temperature | 31.1 °C (304.2 K) |
| Water critical temperature | 374 °C (647 K) |
| Associated quantity | Critical pressure (Pc) |
| Resulting state | Supercritical fluid |
| Field | Physical Chemistry |
Carbon dioxide has a critical temperature of 31.1 °C, or 304.2 K. Its critical pressure is 72.8 atm. If the room is below 31.1 °C, you can squeeze CO2 gas into liquid CO2. You just need to apply enough pressure to the gas. However, on a hot summer day above 31.1 °C, this fails entirely. You cannot make liquid CO2 no matter how hard you press. Instead, the carbon dioxide becomes a supercritical fluid. Coffee companies use this special fluid to remove caffeine from coffee beans. It works perfectly because it leaves no toxic chemicals behind.
Frequently asked questions
What happens to a substance above its critical temperature?
It becomes a supercritical fluid. This means it acts like a dense gas and a thin liquid all at once. Compression alone can never force it to condense.
Why does stronger molecular attraction raise the critical temperature?
Stronger attractive forces pull the molecules together very tightly. The substance needs much more heat energy to break these bonds. This pushes the critical temperature to a much higher value.
Can you boil a liquid above its critical temperature?
No, boiling is completely impossible above this exact temperature. The liquid phase and gas phase have merged into one phase. You cannot transition between two things that are now identical.