What actually happens at critical temperature, and why do CO2 and NH3 have such different critical temperatures?

I have been revising the liquid state portion of States of Matter and have a few doubts that I could not resolve from NCERT alone.

First, the critical temperature. NCERT defines $T_c$ as the highest temperature at which a gas can be liquefied, and says above $T_c$ no amount of pressure will liquefy it. I can recite this but I do not understand what physically stops the liquefaction. What changes at that point?

Second, the book gives a table: $CO_2$ about 304 K, $NH_3$ about 405 K, $H_2$ about 33 K, $He$ about 5 K. Is there a pattern linking critical temperature to the nature of the gas? I assume it is related to intermolecular forces, but I would like to confirm and see if there is a formula.

Third, a smaller question: for liquids, how do viscosity and surface tension change with temperature? I know that for gases viscosity increases with temperature, and I wanted to check whether liquids behave the same way.

I would appreciate answers in a form that I can include in my notes.

7 answers & replies

[+]Ishita GoyalDropperSample· 14d ago

Quick definitions first. $T_c$ : highest temperature at which a gas can be liquefied. $P_c$ : the pressure needed to liquefy at $T_c$ . $V_c$ : volume of one mole at $T_c$ and $P_c$ .

Above $T_c$ the molecules have so much kinetic energy that the attractive forces between them can never hold them together as a liquid, no matter how closely pressure pushes them. At the critical point the meniscus between liquid and vapour disappears and the two phases become one. Above it you get a supercritical fluid, which behaves neither fully like a gas nor like a liquid.

[+]Pooja Iyer12thSample· 14d ago

Pattern: stronger intermolecular forces give higher $T_c$ . $NH_3$ has hydrogen bonding, so ~405 K. $CO_2$ has bigger London forces than $H_2$ so 304 K. $H_2$ and $He$ have extremely weak forces, so very low $T_c$ , you need serious cooling.

In van der Waals terms: $T_c = \dfrac{8a}{27Rb}$ where $a$ measures the attraction between molecules. Larger $a$ gives larger $T_c$ . $NH_3$ has a big $a$ , $He$ has a very small one. That's the formula link you wanted.

[+][–]Meera Patel11thSample· 13d agoAuthor
Thank you, the van der Waals relation is exactly what I wanted for my notes.

[+]Dev Agarwal12thSample· 14d ago

For liquids both increase with temperature. Hotter means faster molecules, more collisions, so more resistance to flow, more viscosity. Like gases.

[+]Aman YadavDropperSample· 13d ago

Wrong. Liquids: viscosity decreases as temperature rises. Molecules gain enough energy to overcome the attraction to neighbours, so they flow easier. That's why honey pours faster when warm. Surface tension also decreases with temperature. Gases are the opposite: viscosity rises with temperature. Vapour pressure of liquids rises with temperature.

[+][–]Dev Agarwal12thSample· 13d ago
ohh i did the gas logic on liquids. yes honey, noted. my bad

[+]Tushar PawarDropperSample· 13d ago

Practical example for $T_c$ : a CO2 cylinder holds liquid CO2 at room temperature under high pressure, because room temp (~298 K) is below its $T_c$ of 304 K. But oxygen and nitrogen cylinders hold only compressed gas, since room temperature is far above their $T_c$ (around 155 K and 126 K). You can't liquefy them with pressure alone at room temperature, they have to be cooled first.

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