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A table does not sublimate, and nor does a spoon. Ice does, however. What is the fundamental difference?

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At high enough temperature and low enough pressure, a metal spoon will definitely sublimate! A wooden table is a more complicated substance, wood will tend to decompose (or burn if oxygen is present) before it sublimates. –  steveverrill May 28 '14 at 13:11

3 Answers 3

up vote 18 down vote accepted

This relates to the difference between how ice, iron spoons and wooden tables stick together.

In wood, most or all of the bonds between the individual atoms and fibre units are covalent; making them very strong. Similarly, a spoon is an alloy of iron, nickel, chrome and carbon, also a very coherent structure.

In ice, on the other hand, the units of the crystal is held together with comparatively weak hydrogen bonds, meaning that not a lot of energy is required for a surface molecule of water to escape, in a process called sublimation.

Usually, once a water molecule escapes the ice, it is, given a relatively constant temperature, very unlikely to rejoin the body of ice; meaning that especially in open air and windy conditions, ice will sublimate at sub-zero temperatures, although as the temperature of the ice drops, this effect becomes steadily less noticeable.

Edit: It is worth noting that given enough time, a table or a spoon can indeed sublimate as well, at any temperature, but the odds against it are utterly staggering.

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I'm not sure whether "odds against it" is a good description here (but I'm not a native English speaker): odds against it to me sounds like a small chance that the spoon evaporates - whereas I'd describe it as an extremely slow process, which however wouldn't be subject to a chance whether it happens or not: I'd say it happens, just far to slow to play any practical role. –  cbeleites May 27 '14 at 19:01
@cbeleites: Fair point; but given enough time the spoon will evaporate, but odds are, before that happens, it'll be destroyed by some other means; like the sun exploding. –  Williham Totland May 27 '14 at 20:40
@WillihamTotland I find this fascinating. You note that as the temperature of the ice drops, the sublimation effect becomes less noticeable. Presumably the effect would be greatest in a block of ice just below freezing. What about the iron spoon? If we heated it up to just below its melting point, would we able to observe the effect? –  emory May 28 '14 at 0:23
You might want to read the prohibition on cadmium plating in space by NASA: "Cadmium is known to sublimate in a hard vacuum environment (especially at temperatures above 75°C). The sublimation products, which are conductive, can redeposit resulting in short circuits. The sublimation products may also interfere with sensitive optics." –  MichaelT May 28 '14 at 1:11
@emory: The conditions under which steel (or iron) sublimates are fairly peculiar. Even in the extremely harsh conditions of interstellar space ablation resulting from impacts with stray hydrogen nuclei is likely to be a far bigger factor in mass loss than sublimation. –  Williham Totland May 28 '14 at 8:20

One term I'm missing so far in the discussion is the vapor pressure.

You may say that the water vapor pressure above ice is far higher than the iron vapor pressure above iron (at "normal" temperatures), or the vapor pressure of the various substances forming the table over that table.

  • See here for the water vapor pressure. I take from the diagram that at -20 °C, the water vapor pressure above ice is approximately 1 mbar $\approx$ 1 hPa, and at slightly below 0°C, about 7 mbar.

  • According to this table in Wikipedia, you need to heat iron well above the melting point to reach such iron vapor pressures. They also cite a formula for the iron vapor pressure above solid iron:

    $\log (P/\mathrm{Pa}) = 12.106 - 21723 / (T/\mathrm{K}) + 0.4536~\log (T/\mathrm{K}) - 0.5846 (T/\mathrm{K})^{-3}$ with the specification that this could be used from 298 K upwards.

    Plugging in 298 K, I get $p_{Fe} \approx 10^{-60}$ Pa.

    This is so low that lots of other processes will lead to more substantial loss of iron from the spoon via the gas phase (e.g. according to this $\ce{FeCl3}$ has a vapor pressure of 1 mmHg $\approx$ 133 hPa at 194 °C).

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<< This answer was posted before the question was completely rephrased and originally asked "why does a table not evaporate" >>

Who said a spoon doesn't evaporate?

Ice is a solid below its melting point ($0~^\circ\mathrm{C}$) but above its melting point it goes through a phase transition and melts to become a liquid (so it's liquid at room temp). Then when it's heated above room temperature to its boiling point ($100~^\circ\mathrm{C}$), it goes through another phase transition and it evaporates to become gas.

Iron (assuming an iron spoon here to keep it simple) is a solid (at room temperature) then when it's heated above room temperature to its melting point ($1538~^\circ\mathrm{C}$) it becomes a liquid. Then when it's heated above its melting point to its boiling point ($\approx2861~^\circ\mathrm{C}$), it evaporates.

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But wait, maybe I was unclear, but doesn't ice evaporate even in a fridge?.. Of course everything can evaporate given a high temperature. –  roman.brodetski May 27 '14 at 8:23
Hm, maybe I use a wrong english word?.. Maybe I meen sublimation here? –  roman.brodetski May 27 '14 at 8:23
@roman.brodetski: not sure about the English terms, but yes, there is a water vapor pressure above ice as well, in fridge and freezer –  cbeleites May 27 '14 at 18:58
This answer increases the confusion created by the language issue in the question. It would probably be best to remove it. The question is not about evaporation of a material once it reaches that material's boiling point, but evaporation that happens during a material's normally solid state. It should be clear that he wasn't talking about boiling water when he said that ice "evaporates". He is talking about submlimation, where a solid goes straight to the gas phase. A detailed explanation of what he wasn't talking about is not helpful. –  msouth May 27 '14 at 22:00

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