1
$\begingroup$

Certain objects such as metals, glass, plastic, sugar etc. melt when heated to a certain temperature but other materials such as a piece of wood, paper, a piece of rock etc. do not. Why is that?

$\endgroup$
  • 3
    $\begingroup$ Half answer from a non-chemist: burning certain things will cause a reaction with oxygen that produces other substances, many of which will also be solids and gases. Some substances sublimate when heated. Although your example of rock not melting is incorrect: a large portion of our planet is moltern rock! $\endgroup$ – Polynomial Nov 12 '16 at 11:32
  • 1
    $\begingroup$ chemistry.stackexchange.com/questions/31050/can-you-melt-wood $\endgroup$ – Mithoron Nov 12 '16 at 11:35
  • $\begingroup$ And you can melt rock. See "Volcano" $\endgroup$ – DrMoishe Pippik Nov 13 '16 at 1:40
  • $\begingroup$ @Polynomial actually, a large portion of our planet is no molten rock. A large portion of our planet is molten metal (mostly iron) residing in the outer core. The rocky part of our planet (mantle are crust) is overwhelmingly solid and is molten only in isolated cases, that in a minority of them we see their surface expression as volcanoes. $\endgroup$ – Gimelist Jan 7 '17 at 21:20
  • $\begingroup$ You can melt rock. How do you think we get pure metal and minerals (from ore)? $\endgroup$ – Aryan Beezadhur Mar 11 at 16:59
3
$\begingroup$

The problem with melting most substances is that you would be doing it in a non-innocent atmosphere and that sometimes even the functional groups inside a compound are non-innocent.

In a perfectly innocent world, everything could be molten given sufficient temperature. Given real world paper, remember that reaction $(1)$ is exothermic and the only reason why it does not spontaneously occur is the lack of activiation energy.

$$\ce{C_nH_mO_l + \frac{$2n + m - l$}{2} O2 -> n CO2 + \frac{$m$}{2} H2O}\tag{1}$$

Once you supply sufficient heat, the activation energy will be overcome and the reaction suddenly becomes spontaneous; for paper, I am told that this happens at around $451~\mathrm{^\circ F}$, which is $233~\mathrm{^\circ C}$ in proper units.

Now you could suggest melting things in an inert argon atmosphere — or maybe even take helium for additional inertness. But then still you have potential reactions that can occur with organic materials. What then will happen might be called smouldering. Technically, this is no more than burning without oxygen.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Many compounds will decompose when heated even in an inert atmosphere but I wouldn't describe that as smouldering. That would be burning slowly (perhaps because there isn't enough oxygen to create a vigorous flame). $\endgroup$ – matt_black Nov 14 '16 at 18:08
1
$\begingroup$

There are multiple reasons why things don't melt

The biggest chemical explanation of why some substances don't melt is that many decompose instead. This is true for many complex organics like paper. IF you add enough energy to (theoretically) melt them you have already added enough to shake the component molecules apart. This is true whether or not they are in an inert atmosphere which they can't react with (in air, paper burns before you have added enough energy to decompose it). Some substance are so unstable they fall apart pretty quickly with any added energy (nitrogen tri iodide, for example, which explodes violently with small amounts of heat or, more commonly, just touching it).

Another reason is physical. Some compounds don't have a liquid state at ambient pressure. Carbon dioxide goes directly from solid to vapour at any pressure below about 5 atmospheres. This is a basic physical property.

Mostly, though, what happens to many substances is a reaction with the environment that happens before you have added enough heat to liquefy it. But many substances not normally seen melting will melt with enough heat. Many rocks melt (some decompose). All metals melt, even tungsten, if you heat them enough, though many require special techniques and inert atmospheres for this to be seen.

| improve this answer | |
$\endgroup$
-1
$\begingroup$

Everything has a liquid state, so your initial assumption is technically incorrect. But your question is a good one, so let me list the things needed to clarify.

Why does some compounds/solids not turn into liquids at atmospheric pressure? You have to factor in to your question that the states of matter are governed by both temperature AND pressure. Some compounds will not become liquids at room temperature no matter what you do to the pressure, and some compounds will not become liquids at 1 atmosphere, no matter what you do to the temperature. (They will certainly become "fluids" at some point though, but that is an unnecessary complication). If you plot the temperature and the pressure on y and x scale of a chart, the graph explaining the behaviour of a material will look like a slightly right leaning lowercase "y" for most compunds. Check that out @ Wikipedia

Now, some of the things you have listed do become liquids, but at higher temperatures than what people generally see. Stones for instance, melt.

Also, as others have pointed out, some states are more reactive than others. In general reaction kinetics make reactions speed up from solids->liquids->gases so something that is idle and stable as a solid can vanish as a liquid, due to reaction. Solid states of matter, for instance, generally react so slowly that you can safely assume that solids do not burn*). Which begs alot of questions, with interesting answers. =)

*) Naturally excepting some of the more interesting reactions revolving around Cl3F and the likes

| improve this answer | |
$\endgroup$
  • $\begingroup$ Your first sentence seems quite inaccurate. There are materials that react instead of melting. It is not about Oxygen presence, pyrolysis is a common, real and applied example. $\endgroup$ – jlandercy Nov 14 '16 at 15:01
  • $\begingroup$ We have to disagree on it then. Everything has a liquid state. Certainly, you can engineer temperature and pressure combinations that thermodynamically hinder liquid formation, but that does not at all prove that the liquid does not exist, only that it does not form for that certain p-T combination. That is like saying liquid argon does not exist, because the world is hotter than its boiling point. $\endgroup$ – Stian Yttervik Nov 14 '16 at 20:35
  • $\begingroup$ What about CaCO3, you will never get it melt, it will loose CO2 and then CaCO will melt, this fact is used in Thermo Gravimetry. How will you get wood melt, it is complex matrix essentially based on Cellulose and Lignin, you will get reactions before it reaches a liquid state provided such state it exists for such a material. It really seems your sentence is too general to be true in real applied Chemistry. $\endgroup$ – jlandercy Nov 15 '16 at 10:26
  • $\begingroup$ About you example on Argon, I never said such a thing. And by the way you are only speaking in term of pure substance. I gave you an example of pure substance that cannot melt before decomposing. $\endgroup$ – jlandercy Nov 15 '16 at 10:34
  • $\begingroup$ Of course CaCO3 can and will melt, if the temperature and the (partial) pressure (of CO2) is high enough. For every substance there is a temperature and a pressure where a liquid exists. This is starting to sound like a really nitpicking debate, which does not answer the OPs question at all, so either lets take it to chat or agree to disagree. $\endgroup$ – Stian Yttervik Nov 15 '16 at 12:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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