The concept of sublimation – iodine as an example

Abstract

Sublimation is a process that is defined unequally in different textbooks and in various chemistry
sources. Inexactness in defining basic concepts in chemistry can lead to alternative meanings for
different people. Inconsistent explanations, then, can serve as a basis for developing misconceptions
and preconceptions in latter students’ education. Thus, the notion that upon heating iodine only
sublimes, but does not melt is present in many chemistry textbooks, teachers lectures and,
therefore, in students minds and may be considered as one of the widespread misconceptions in
chemistry teaching. In this paper we offer a lecture demonstration showing the existence of all
three states of iodine, supported by a short video-clip, hoping to give a contribution to the
correction of misbelieves about the process of sublimation and the examples of subliming
substances.

Full text

marzo de 2012 • educación química 171emergent topics on chemistry education [experimental teaching]
emergent topics on chemistry education
[experimental teaching]
Introduction
(1) Misbelieves and misconceptions
There are several terms that refer to students’ misbelieves.
Some authors use the word “misconception” to define errone-
ous notions and others use “preconceptions” that are related
to previous knowledge or arise during the course of instruc-
tions. The expression “alternative conceptions” is considered
by some authors as some kind of compromise or agreement
that incorporates students’ faulty views during science teach-
ing (Horton, 2004, p. 5).
The misconceptions (incorrect notions) are powerful, ex-
tremely persistent and hard to change, creating obstacles to
further learning (Pabuçcu & Geban, 2006). The process of
previous learning plays an important role in students’ unders-
tanding and the quality of the subsequently learned concepts
(Roschelle, 1995). A large number of students (and some tea-
chers, too) believe that their established concepts are correct
because they make sense, meaning that they correspond to
their understanding of the phenomenon in question. Conse-
quently, when students face new information which, unlike
their alternative conceptions, does not fit their previously es-
tablished mental framework, they may ignore it or reject it
because it seems wrong (Horton, 2004, p. 1). They attempt to
solve problems in chemistry courses without real understan-
ding of a process or a phenomenon connecting them with
their previous information and concepts, which, however,
may not be scientifically correct. Students can be very suc-
cessful and intelligent; they may have high grades, but still
retain certain misconceptions. Identifying the weaknesses in
the concept-building is especially important during the stu-
dents’ first exposure to chemistry. The misconceptions they
build in the early stages of their development are the most
resistant to change during the subsequent instruction, the
students constructing the new knowledge on a faulty basis
and rearranging the new information and ideas to fit the fra-
mework of ideas they believe are correct. Thus, it is of utmost
importance to identify, confront and correct different miscon-
ceptions that students have. The knowledge of students’ mis-
conceptions is helpful in deciding where to start and how to
continue teaching.
(2) Subliming substances
It is interesting (but also disturbing) that some of the basic
concepts and terms used in the chemistry education from the
earliest stages up to the university level are not properly, pre-
cisely and unequivocally defined and seem to have different
meanings for different people. Rather surprisingly, the con-
cepts of sublimation and subliming substance seem to fall into
this category.
The IUPAC terminology compendium (McNaught &
Wilkinson, 1997) defines sublimation as “the direct transition
of a solid to a vapor without passing through a liquid phase.
Example: The transition of solid CO2 to CO2 vapor.” If this is
the complete definition of it and has no limitations, its micro-
scopic meaning would simply be passing of molecules from a
solid substance to the gaseous state of that substance. Thus
it would be completely analogous to evaporation – passing of
molecules from the liquid state/phase of the substance to its
The concept of sublimation –
iodine as an example
Marina Stojanovska,* Vladimir M. Petruševski,* Bojan Šoptrajanov**
ABSTRACT
Sublimation is a process that is defined unequally in different textbooks and in various chemistry
sources. Inexactness in defining basic concepts in chemistry can lead to alternative meanings for
different people. Inconsistent explanations, then, can serve as a basis for developing misconceptions
and preconceptions in latter students’ education. Thus, the notion that upon heating iodine only
sublimes, but does not melt is present in many chemistry textbooks, teachers lectures and,
therefore, in students minds and may be considered as one of the widespread misconceptions in
chemistry teaching. In this paper we offer a lecture demonstration showing the existence of all
three states of iodine, supported by a short video-clip, hoping to give a contribution to the
correction of misbelieves about the process of sublimation and the examples of subliming
substances.
KEYWORDS: sublimation, misconceptions, textbooks, experiments, iodine, chemistry teaching
* Institute of Chemistry, Faculty of Natural Sciences and Mathe-
matics, Ss Cyril & Methodius University, Skopje, Republic of Mace-
donia.
** Macedonian Academy of Sciences and Arts, Skopje, Republic of
Macedonia.
E-mail: marinam@pmf.ukim.mk
Educ. quím., 23(núm. extraord. 1), 171-3ª de forros, 2012.
© Universidad Nacional Autónoma de México, ISSN 0187-893-X
Publicado en línea el 24 de enero de 2012, ISSNE 1870-8404

educación química • marzo de 2012
172 emergent topics on chemistry education [experimental teaching]
gaseous state. It would be applicable to any solid, at any pres-
sure or any temperature above 0 K, the possible differences
being only quantitative and dependent on the vapor pressure
of the solid in question. Indeed, such a broad (and loose) def-
inition of sublimation is widely found in textbooks and other
sources of chemical information. For example, the definition
in a standard science textbook (Trefil & Hazen, 2000) is that
“some solids may transform directly to the gaseous state by
sublimation”, the term “solid” clearly implying a substance and
this makes the things to become more complicated.1
When examples of subliming substances are considered,
the most usually quoted ones are dry ice (solid carbon diox-
ide), iodine and naphthalene. Thus, in a classical chemistry
textbook (Choppin & Jaffe, 1965) it is stated that: “The tran-
sition directly from solid to gas is known as sublimation. Car-
bon dioxide is an example of a substance that sublimes (and)
… iodine is another example.” In the Chang’s book Chemistry
(Chang, 1990) naphthalene and iodine are given as examples
of volatile solids which may be in equilibrium with their va-
pors and, by implication, can be considered as subliming sub-
stances.
On the other hand, many articles can be found (Wisconsin
State Journal, 2010; Habby, 2011; Wikipedia, 2011; Silber-
berg, 2006), about the process of sublimation of snow and ice
which sublime, albeit slowly, below the melting-point tem-
perature. This phenomenon is operative for example when
linen are hung wet outdoors in freezing weather to be re-
trieved dry at a later time. The loss of snow from a snowfield
during a cold spell is often caused by sunshine acting directly
on the outer layers of the snow. Ablation is a process which
includes sublimation and erosive wear of glacier ice. The
snow sublimes through a process that is similar to evapora-
tion. In fact, whenever there is an interface of air and water
(either liquid or solid), the H2O molecules will have some
tendency, more or less pronounced, to leave the condensed
phase and the processes of water evaporation and sublima-
tion are observable at any temperature. Clearly, this is noth-
ing new or spectacular but we do not think of water as a
typical example of a subliming substance since ordinarily ice
first melts and then vaporizes.
In fact, depending on the properties of a given solid in
question, only a few substances will readily sublime under
ordinary laboratory conditions without ever passing through
the liquid state. Solid carbon dioxide (dry ice), with its triple
point in the phase diagram lying above 1 bar, is the typical
example of such a behavior. At ordinary atmospheric pressure
(i.e. at atmospheric pressure close to 1 bar) dry ice can not be
melted. Another (albeit somewhat exotic, radioactive and
very poisonous) substance with analogous properties is ura-
nium hexafluoride with its triple point being ≈ 337 K and
1.5 bar.
Other solid substances, especially if they are highly vola-
tile (characterized by their high vapor pressure), may sublime
at room temperature but if the temperature is carefully in-
creased, it is possible to melt them. Iodine, for example, at
ordinary pressures can exist in the liquid state at tempera-
tures in the interval from 113.6 to 184.4 °C (Petrucci, 2001).
Our relatively simple experiment (described below) provides
an impressive demonstration for this. It should be noted that
the triple point of iodine is found below 1 bar (113.5 ºC;
12.07 kPa) and such is also the case with naphthalene
(80.25 ºC; 1.0 kPa) or camphor (180.1 ºC; 51.44 kPa), the
latter compound being sometimes quoted, together with car-
bon dioxide, iodine and naphthalene, as a typical substance
that sublimes.
The examples given above lead to the necessity to set up a
more restrictive meaning of the concept of sublimation with
a view to the definition of a subliming substance.2 In our view,
sublimation (in the restrictive mining of the term) would be a
process where a solid substance on heating, at ordinary atmo-
spheric pressure, undergoes a solid gas transition directly,
without first melting, i.e. without the appearance of a liquid
phase. The typical example obeying such a restrictive defini-
tion would be solid CO2 but not iodine, naphthalene or cam-
phor. We believe that at the high-school level only this
restrictive definition is suitable (perhaps sometimes accom-
panied by a warning that a more precise definition exists). It
is the latter definition that is dealt with in the present paper
and this (in our view, as already pointed out), should be used
in the general pedagogical practice.
Unfortunately, the broad rather than the restrictive defini-
tion is firmly entrenched in the minds of students, teachers,
textbook authors and practicing chemists. Thus, if asked to
name a subliming substance, iodine is very likely to appear as
one of the preferred examples.
The problem: a lasting misconception
One of the widely spread misconceptions is that about the
sublimation of iodine. There are too many people (Chemical
forums, 2005; Trach, 2003) believing that, even at standard
pressure, iodine can only sublime and not be melted, and such
notions are indeed found in many books, including several
textbooks that are in use in Macedonia. Thus, at two instanc-
es (Aleksovska & Stojanovski, 2005; Doneva-Atanasoska,
Aleksovska & Malinkova, 2002) the authors say that upon
heating iodine transforms directly from a solid to a gaseous
state (“without being liquefied”), while in a textbook for the
1st year of reformed gymnasium (Cvetkovi
ć
, 2002) the defini-
1 The definition of evaporation (McNaught & Wilkinson, 1997) is
“The physical process by which a liquid substance is converted to
a gas or vapour” where there is an explicit mention of “a … sub-
stance”.
2 An alternative would be to coin a new term for the special type
of sublimation that is analogous to boiling rather than to evapo-
ration. This will be discussed in one of our forthcoming contribu-
tions.

marzo de 2012 • educación química 173emergent topics on chemistry education [experimental teaching]
tion and the examples given are similar, but heating is not
mentioned explicitly.
Con sequentl y, many teachers honestly believe that iodine
is a typical example of a substance that, irrespective of the
experimental conditions, sublimes without being melted, be-
ing ignorant, consciously or subconsciously, of the incom-
pleteness in their understanding of the meaning of the term.
Thus, in their lectures instructors loosely use the term “subli-
mation” and the imprecise definition of sublimation is in-
stilled in the student’s minds as a truism. The notion is
strengthened by the fact that students could have seen the
demonstration in which iodine crystals are heated to release
violet vapor and it has been explained in terms of sublimation
(Kotz, Treichel & Townsend, 2009). In such a case, they have
a false impression that a liquid is not produced since the deep
color of the iodine vapor that is quickly released often masks
the appearance of the liquid phase (Yahoo answers, 2009). In
fact, iodine vapor can be seen even without heating. If, name-
ly, iodine crystals are put in a test tube (or better, sealed into
a larger vessel), not very intense violet vapor can be observed
inside the ampoule almost instantaneously (Figure 1) this be-
ing indeed associated with the sublimation of solid iodine due
to its relatively high vapour pressure.
Unfortunately, the combined effect of the instructor’s
teaching and the student’s personal experience (imprecise
and incomplete as it turns out to be) forms a basis for a mis-
conception that is readily accepted by the students. They
“know” that solid iodine can only sublime and can not be first
melted and in their mind this is final.
However, as mentioned above, it is a known fact that at
atmospheric pressure iodine is liquid in the interval from
113.5 to 184.4 °C meaning that iodine first melts and then
vaporizes rather than “skipping” the liquid phase (Wikipedia,
2011; Heilman, 2004). Indeed, as discussed below, it is pos-
sible to obtain liquid iodine at atmospheric pressure by con-
trolling the temperature at just above the melting point of
iodine and see the melt. This is not new at all. There are at
least two offered demonstrations (Summerlin, Borgford &
Ealy, 1987; Najdoski & Petruševski, 2002) in the literature
devoted to chemical lecture experiments and demonstrations
where the authors offered suitable experiments to demon-
strate the existence of liquid iodine at atmospheric pressure.
Now we try to strengthen the notion by including a short
video clip. In the latter decision, we were governed by the
common saying that “a picture substitutes a thousand words”.
Consequently a video clip can indeed substitute (even liter-
ally) a thousand pictures, although the effect of live experi-
ments is beyond doubt even stronger. It is a pity that all too
many instructors rely heavily on available video material pre-
pared by others instead of performing real experiments
(Petruševski, Stojanovska & Šoptrajanov, 2009) but that is a
fact. Therefore, if the material we offer here helps in fighting
the misconception about iodine only subliming, but not melt-
ing upon heating, then it will completely serve its purpose.
Confronting the misconception:
the offered experiment and video clip
The most effective chemistry tool among numerous teaching
strategies and techniques used to reduce misconceptions in
science teaching is an experiment or a demonstration. Using
demonstration (or experiment), one can, more or less, easily
test his/hers assumptions and confirm the correctness (or
falseness) of the proposed hypothesis. Demonstrations/expe-
riments are an inextricable component of chemistry teaching
and, if properly preformed, lead to a development of an ac-
tive and creative thinking.
As a means for fighting the discussed misconception, a
laboratory demonstration was devised,3 in which appropriate
apparatus and careful control of the temperature just above
the melting point of iodine is employed.
The experimental setup (Figure 2) for this experiment in-
cludes a beaker filled with glycerol, a thermometer, a heater
and a narrow test tube containing iodine crystals. The test
tube may be sealed (for safer work), but this is not a necessary
precondition for performing the demonstration. Glycerol has
been chosen for this purpose because, on one hand, its boiling
point (290 ºC) is much higher than the melting point of io-
dine (113.5 ºC), and on the other, glycerol is a colorless liquid
unlike oil that has previously been used (Summerlin, Borg-
ford & Ealy, 1987). The glycerol bath is heated to approxi-
mately 140 ºC. As temperature passes over the melting point
of iodine, it can be clearly seen that iodine crystals begin to
melt and, after some time, flow along the test tube inner walls
when the tube is tilted (Figure 3). The process of iodine melt-
ing can be easily noticed on the video clip prepared for this
demonstration. The first part of the clip shows the behavior
of iodine crystals in a test tube and is to be compared with
Figure 1. Iodine (both solid
and vapor) in a hermetically
closed vessel.
3 It is based on modifications of two demonstrations proposed
earlier (Summerlin, Borgford & Ealy, 1987; Najdoski & Petruševski,
2002).

educación química • marzo de 2012
174 emergent topics on chemistry education [experimental teaching]
the behavior of the liquid iodine (obtained a few minutes
later).
Conclusion
The result of the experiment (or, for that matter, the video
clip) shows very clearly and beyond any doubt that iodine
can be liquid under atmospheric pressure. This is only one
example of the fact that experiments (carried out either
by teachers or students) are very powerful tool in chemistry
teaching. They can be used as an introductory or as a conclu-
sion of the lesson, to verify or to explore phenomena, as well
as to serve as a concept building and correcting existent mis-
understandings and misconceptions students may have. An-
other aspect that has to be addressed at this point is caution
while reading experimental procedures and performing the
experiments. There are cases (one of them is dealt with in this
paper) when the result of the experiment does not corre-
spond to the summary or the explanation offered.
If no demonstration is performed, a lot of efforts might be
needed to convince the students to abandon the previously
learned concepts and to finally accept the new knowledge as
valid and correct. Nevertheless, we should continue the search
for suitable (novel or existent) ways to persuade the students
and eliminate the effect of this misconception.
Acknowledgements: The authors would like to express their
sincere thanks to M. Sc. Robert Jankuloski, assistant professor
at the University of Audiovisual Arts European Film and The-
atre Academy ESRA Paris–Skopje–New York, Photography
Department, and to Mr. Vančo Mirakovski, Quasar Film Sko-
pje, for preparing and supplying us with the video clip and
the photograph for Figure 3.
Supplementary material: Liquid_iodine?.mpg (video clip,
available at the URL http://bit.ly/A6F1GS)
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Gracias a la DGAPA-UNAM
Educación Química agradece a la Dirección General de Asuntos del
Personal Académico de la Universidad Nacional Autónoma de México
el apoyo otorgado a través del Proyecto
PAPIME PE200211
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