Critical Temperature: What Makes Liquification Impossible?

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In summary, a critical temperature is the point at which a substance can no longer be liquified, regardless of pressure applied. This is due to the intermolecular interaction energy being larger than the molecular kinetic energy. At the critical point, the substance loses its liquid character and becomes a gas-like dispersion of tiny, liquid-like clusters. Increasing the temperature or decreasing the pressure beyond this point results in a true gas. This phenomenon is similar to spinodal points in superheated liquids and the spinodal point may depend on the heating rate.
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Pengwuino
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So as i understand it, a critical temperature is the temperature at which a substance must be under in order for it to be liquified. Any temperature above this means it is impossible to liquify no matter how much pressure you apply. So what exactly is happening that makes it impossible?
 
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The liquid phase is characterized by the intermolecular interaction (or potential) energy being larger than the molecular KE. While the latter quantity can be increased indefinitely by increasing the temperature, the PE has a maximum (negative) value at some characteristic intermolecular separation (see figure). So increasing the pressure can not increase the PE beyond this extremal value.

In the vicinity of the critical point, the PE and KE are roughly equal (~0.1 eV for water), and the phase loses its liquid character. Just beyond the CP, what usually happens is that the molecules tend to group up into tiny (~ a few nanometers across) clusters moving about at large velocities (or KE). However, the velocities of molecules within the cluster are somewhat smaller. So, in this regime, the substance consists of a gas-like dispersion of tiny, liquid-like clusters.

Increasing the temperature (or decreasing pressure) beyond this point results in a true gas.

Figure (see link) : 2-atom PE as a function of interatomic distance, for H-atoms.
http://www.raunvis.hi.is/~agust/F14_3.GIF
 
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Hi,Gokul, what happens at critical points is similar to (or the same as) that at spinodal points of superheated liquids? By the way, does spinodal point depend on heating rate? Thanks.
 

1. What is critical temperature?

Critical temperature is the temperature at which a substance can no longer exist in liquid form, regardless of the amount of pressure applied to it. It is the maximum temperature at which a substance can undergo a phase transition from liquid to gas.

2. Why is liquification impossible at critical temperature?

At critical temperature, the substance's vapor pressure becomes equal to the external pressure applied to it. This means that the substance can no longer be compressed into a liquid state as its vapor molecules are exerting the same amount of pressure as the external pressure, resulting in a balance and no phase change.

3. How does critical temperature affect substances?

Critical temperature is an important factor in determining the physical properties of substances. It affects the boiling point, vapor pressure, and compressibility of a substance. Substances with a higher critical temperature tend to have a lower boiling point and are more difficult to liquify.

4. Can critical temperature be changed?

No, critical temperature is a unique property of each substance and cannot be changed. However, it can be affected by external factors such as pressure and the presence of impurities in the substance.

5. What are some examples of substances with a critical temperature?

Water has a critical temperature of 374 degrees Celsius, while carbon dioxide has a critical temperature of 31 degrees Celsius. Other examples include nitrogen (-147 degrees Celsius), ethanol (243 degrees Celsius), and methane (-82 degrees Celsius).

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