For years I thought that solid iodine, I2, only underwent sublimation as soon as heated at atmospheric push. I was so wedded to this concept that I percreated the demonstration for years. A small beaker containing a couple of grams of iodine, when heated in a fume hood, appeared to produce plenty of thick, purple vapour, which obscured the (purportedly) solid iodine at the bottom.

You are watching: Explain why iodine spontaneously sublimes in open air at 25.0 ∘c.

My faithful students listened to my schtick, provided the observation and grouped solid iodine with significant sublimators, such as solid CO2 — dry ice.

I was happy to proceed in this vein until I taken place across the phase diagram for iodine as soon as preparing a PowerPoint leskid. Either the diagram was wrong, or iodine does not undergo sublimation at traditional push. (A phase diagram of iodine is displayed below).

Given that empirical evidence is the trump card, I tried the demonstration after institution. This time, after heating iodine crystals over a warm plate for a few minutes and also observing the purple fumes, I picked up the beaker and also easily poured the contents onto a sheet of white paper.

Surpincrease, surprise! The iodine developed a splast pattern, indicative of the presence of a liquid. The phase diagram was correct — and I was wrong.

For years I led my students to think my misconception. A quick search — YouTube included — revealed that I am not the initially person to realize this misconception.1

Aside from wanting to send an apology letter to over a thousand also people, what I learned around the non-sublimation of iodine reflects me the need to constantly re-evaluate, through a crucial eye, what I do and also exactly how I do it.

I am not perfect — and never before will be.2

Notes and also references

Marina Stojanovska, Vladimir M Petrusevski, Bojan SoptrajanovThe idea of sublimation — Iodine as an instance, Emergent Topics in Chemisattempt Education, March 2012.Addressing Misconceptions about the Particulate Nature of Matter among Secondary-School and High-School Students in the Republic of Macedonia, Creative Education, 2012. Vol.3, No.5, pages 619-631.PDFs of both articles have the right to be uncovered through a Google search. Article (a) has a brief video-clip worth watching.If you doubt this, let me put you in call with my spouse.

Michael P Jansen mjansen>
Crescent School, Toronto ON

Phase diagram of iodine

Chem 13 News asked Lew Brubacher, past editor, to discuss Mike’s discovery. Lew prepared a phase diagram listed below (Fig. 1) in addition to comments on the complying with web page. Because this diagram is pertinent to both write-ups, we have actually contained it below. Before you read Lew’s comments on the following web page, take a while to research the phase diagram and consider what it reveals.

When I was asked to talk about Mike’s post (above), I sought a phase diagram of iodine. Amongst the few I discovered, 2 had significant errors. So I all set the one presented in Mike’s article at left. As Mike notes, iodine can indeed be a liquid at atmospheric push in between 113.7 oC and also 184.3 oC.

But as I assumed about phase diagrams and Mike’s post — and also having not operated through phase diagrams for more than 20 years(!) — a brand-new, admittedly naïve, question developed. In Mike’s experiment, all three phases are existing concurrently, however how can this be once the phase diagram mirrors that this deserve to occur only at the triple point? It progressively came ago to me that the conditions for which a phase diagram uses are not the problems of Mike’s experiment.


Phase diagram of iodine.

A phase diagram describes the case in which one is observing:

a solitary chemical substance,in a closed mechanism,under equilibrium conditions.

Mike’s experiment

involves not a single substance (it likewise has air),is an open up system, andis not at unicreate (equilibrium) temperature.

To elaborate: When Mike puts the flask containing solid iodine on a hot plate (at one environment pressure), the temperature will certainly not be unicreate throughout the flask. At the bottom, if the temperature is 113.7 oC or greater, there will be liquid iodine, though, as Mike notes, it might be difficult to view it with the vigorously coloured iodine vapour. In any type of event, it is not proper to try to use the phase diagram, in information, to the behaviour of the iodine in this flask. Here, we can have actually solid, liquid and also vapour current at the same time, also though the system is not at the triple allude — bereason the system is not a single component, it’s not at equilibrium, nor at uniform temperature and push throughout.

But having actually disposed of that red herring, we are left with an excusage to recall what a phase diagram does tell us. First, a word about constructing the phase diagram in Fig. 1 at left. A search verified that tright here is some disagreement on the standard information.

I made a decision to usage these information from Wikipedia:

triple point: 113.5 oC at 12.1 kPa (point 1 on phase diagram)melting point: 113.7 oC at 101.3 kPa (allude 2)boiling point: 184.3 oC at 101.3 kPa (point 3)crucial point: 546 oC at 11,700 kPa

Also, from the old Internationwide Critical Tables, I uncovered these densities — and also the matching molar volumes, Vm:

solid: 4.93 g/cm3 at 20 oC (Vm = 51.5 cm3/mol), and 4.92 g/cm3 near the melting point (Vm = 51.6 cm3/mol).Liquid: 4.00 g/cm3 close to the melting allude (Vm = 63.5 cm3/mol).

Of interemainder also is the vapour push of solid iodine (Table 1).

Table 1. Vapour push of solid iodine
From Internationwide Critical Tables, Vol 3, web page 201This is the melting point, according to this source

A phase diagram of a single substance explains the behaviour of that substance as the pressure and also temperature are changed. Imagine a sample of solid iodine in an idealized syringe that deserve to withstand also any temperature and also press, and is transparent, so you have the right to view what is happening inside. No other substance is existing — no air, water, etc. The device need to be at equilibrium, via a unidevelop temperature and press throughout. Suppose that you begin with the syringe at 101.3 kPa (1 atm) and also 20 oC, which is allude 4 in Fig. 1. All of the iodine is in the solid state. Now slowly warmth the sample, including energy at a continuous price, while the press remains constant. When the temperature gets to 113.7 oC (suggest 2), the temperature stops rising as the added warmth is offered to melt solid iodine. When melting is finish the temperature resumes its increase till it gets to 184.3 oC (allude 3), and the liquid begins to evapoprice. When vaporization is complete, the temperature aacquire begins rising. In this system, we watch iodine vapour just once the temperature is at 184.3 oC or greater.

In contrast, as soon as we observe a sample of solid iodine in the bottom of an unsealed glass flask, at 20 oC and 1 atm pressure, the device is various from the idealized mechanism simply described. There is air in the flask, and we view a faint colour of iodine vapour. Although the vapour push of iodine is exceptionally low (0.027 kPa, or 0.20 mm Hg), the vapour is coloured strongly sufficient that we can view it. Many of the 1 atm pressure is because of the air — around 79 kPa N2, 21 kPa O2, 1 kPa Ar, 0-2 kPa H2O(g), 0.04 kPa CO2 and also 0.027 kPa I2. If the flask is unsealed, the iodine vapour can escape. At some point, the iodine sublimes ameans, simply prefer an ice cube in a freezer.

See more: Pair It Down Or Pare It Down, Pare ​Definitions And Synonyms

In an additional application of the diagram, imagine beginning slightly to the best of allude 2, and increasing the push, on a line rising vertically. Originally, the iodine is liquid, yet will certainly change to solid at some allude. That’s to be meant, by Le Châtelier’s principle, bereason the molar volume of solid iodine is smaller than that of the liquid. In the instance of water, in contrast, the molar volume of the solid (ice) is better than of the liquid; hence, in water’s phase diagram the solid/liquid line has an adverse slope.