Iodine

Iodine can sublime upon heating i.e. it changes from solid state directly to gas state without any liquid phase appear.

Then why does it have a melting point?

Update:

That mean iodine does have a melting point as other substances at 1 atm

But the difference between the melting pt. and boiling pt. is so low that even at the melting point, many iodine molecules are turned into gas state instead of liquid state(by evaporation)?

3 Answers

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  • 9 years ago
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    You're right that iodine sublimes under room pressure.

    HOWEVER, iodine liquid CAN exist under room pressure; for iodine to liquefy, additional pressurization is NOT needed.

    First, do you know evaporation? Liquid not reaching boiling point can still turn into gas.

    Similarly, Solid can also turn into gas without reaching boiling point, and even haven't melted.

    A lot of solid can turn into gas (sublime) without turning into liquid; melting is not necessary for "evaporation" of substances.

    Can you smell mothballs? Or the smelly "camphor" in public toilets? Those solids turn into vapor without melting.

    Same for iodine. Iodine can vaporize under boiling point and even below melting point. But it doesn't mean that iodine liquid under room pressure doesn't exist.

    Look at the triple point of iodine. Above 12.1kPa liquid iodine can exist; and the atmospheric pressure is 101.3kPa. So liquid Iodine exists; you just haven't heat it up to its m.p. , 387K / 113*C.

    http://www.youtube.com/watch?v=P2CWh2CQu90

    Youtube thumbnail

    This video quite clearly shows that iodine liquid can exist under room pressure; you're just not heating it enough.

    Carbon dioxide, on the other hand, cannot liquefy under room pressure, as its pressure of triple point is 520kPa, higher than that of atm. pressure.

    You can see from Wikipedia that it gives -78*C at "melting point", noting (subl.) ;

    "boiling point" is -57*C, noting at 5.185bar = 518.5 kPa, higher than atm. pressure. Then liquid CO2 cannot exist as liquid under rm. conditions.

    2012-05-08 02:06:02 補充:

    One is easily confused by the word "sublimation".

    Iodine "sublime" when heated -- here "sublime" is just like evaporation, needs not reaching boiling point.

    2012-05-08 02:08:49 補充:

    Carbon dioxide "sublimes" when heated -- here "sublime" can mean either "evaporation of solid" or "boiling of solid", depending on temp. of the system.

    But that video should have made good demonstration.

    2012-05-08 02:11:13 補充:

    Pay attention to video especially at 0:31 .

    2012-05-10 01:44:24 補充:

    Billyionaire:

    you're right.

    In other words, iodine's vapor pressure is high even under m.p.

    According to a handbook, vapor pressure of I2 can be calculated as:

    2012-05-10 01:48:10 補充:

    log(P) = [1000A / T] + [B x log(T)]

    for pressure,P in atm, emp,T in K

    for solid, A = -3.518, B = -2.02

    for liquid, A = -3.225, B = -4.86

    for gas, A = -3.909, B = 0.25

    plug in the numbers into Excel and calculate vapor pressure at diff. temp.

    2012-05-10 01:58:42 補充:

    at m.p., vapor pressure is 0.117atm.

    Ref.:

    Knacke O., Kubaschewski O., Hesselmann K.. Thermochemical properties of inorganic substances (2nd ed.). 1991: Berlin: Springer-Verlag

  • 9 years ago

    Basically, is that the state of a subatance can be affected by TEMPERATURE and surrounding PRESSURE.

    iodine can sublime is atmospheric pressure. BUT it can also have its liquid state in other pressure.

  • 9 years ago

    碘只係係常壓下被加熱

    先由固體昇華做氣體。

    並唔代表佢冇液體狀態,只要加壓都可以以液態出現。

    詳細可以睇返iodine 既phase diagram

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