-1
$\begingroup$

We learned in school that state change is a matter of molecules moving faster and faster. And we learned "heat em up, speed em up, spread em out". Why then can some liquids have higher temperatures than some gases?

$\endgroup$
2
  • $\begingroup$ Note that "quick questions" without explicitly expressed solving effort are not very welcome. It may lead to hesitation to answer or even closing such a question. If you ask on Chemistry SE site, the site policy expects you to elaborate the question e.g. by searching in textbook+online resources, writing what you already know, what was found, understood or tried for the answer to be found. Be aware the network of Se sites aims for a collection of high quality answers AND questions. $\endgroup$ – Poutnik Oct 23 '20 at 5:46
  • $\begingroup$ You are also comparing different stuff. Besides the intrinsic motion of molecules, consider different interactions among them. It is the latter point, if not overcame, that dictates if you have a liquid or a gas. $\endgroup$ – Alchimista Oct 23 '20 at 6:28
2
$\begingroup$

Interactions between molecules determine whether something is a gas, solid or liquid not just temperature

You have missed an important component of why some things are gases and others are liquids. While, in general, the hotter things are the more the molecules are spread out and the faster they will move, the specific reason why some things are liquids and other are gases depends on the interactions among those molecules.

Water molecules, for example, are fairly 'sticky' to other molecules so have to have a lot of energy to make them a gas. Argon atoms are not at all 'sticky' and it takes far less energy to make them gaseous. So argon is a gas at room temperature and water is a liquid. Other molecules similar to water (like hydrogen sulfide) have weaker attractive forces ('stickiness') so are gaseous at the same temperature as liquid water.

Another factor is the mass of the molecules. Heavy molecules with a given kinetic energy (or temperature) move more slowly than small molecules. So may also need more energy to make them a gas. Methane is a gas at room temperature but pentane is a liquid.

So the average energy (or temperature) isn't everything. You also need to know how sticky a molecules it to know whether it will be a gas or liquid at a given temperature. Hence some things are liquids, some are solids and others ar gases despite having the same temperature.

$\endgroup$
3
$\begingroup$

Of course heating a gas increases the speed of the molecules. But the speed and the existence of the gas molecules do not depend exclusively on the temperature. The mass and the interatomic forces have a greater importance.

First, light molecules are easier to put in the gas state than heavy molecules. That is why :

  • the lightest molecule $\ce{H2}$ is a gas already at very very low temperatures $(-252°C = 21 K)$

  • the relatively light molecule $\ce{CH4}$ is more difficult to vaporize $(-161°C = 112 K)$,

  • the heavier molecule $\ce{n-C4H10}$ is still more ($-1°C = 272 K$)

  • the extremely heavy molecules containing more than $20$ Carbon atoms are much more difficult to vaporize.

But this effect of the mass is increased if there are attractions between the molecules. For example water molecule has a molar mass ($18 g/mol$) not very different from methane $\ce{CH4}$ ($16 g/mol$}. But its boiling point is much higher : $100°C = 373K$). On the absolute temperature scale, it is more than three times the boiling point of $\ce{CH4}$ ($112 K$). This is due to special interaction forces called hydrogen bonds, that maintains water molecules in liquid (or even solid) state at temperatures between about $-100°C$ and $+ 100°C$

Furthermore, ionic compounds like $\ce{NaCl}$ are extremely difficult to vaporise $(1413°C)$, because of the huge attractions between $\ce{Na+}$ and $\ce{Cl-}$ ions.

$\endgroup$
2
  • $\begingroup$ How often is water liquid at atmospheric pressure and -100 C? $\endgroup$ – matt_black Oct 23 '20 at 15:56
  • $\begingroup$ @Matt black. OK. I should have written "in condensed phase" instead of liquid. I'll edit this change. $\endgroup$ – Maurice Oct 23 '20 at 16:52

Not the answer you're looking for? Browse other questions tagged or ask your own question.