|
STATES OF MATTER -
properties of gases and liquids (fluids) and solids
12. Describing the special case of
SUBLIMATION (solid <==> gas)
using kinetic particle theory and its use as a separation technique
[Author
©
Dr WP Brown PhD:
Doc Brown's chemistry exam revision notes on
states of matter - physical properties of gases, liquids and solids,
suitable for students of UK GCSE
level and international
IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated
Nov 29th 2025]
INDEX of all my notes on the states of matter
GCSE (~US grades 8-10) level multiple choice QUIZ on
the states of matter: gases, liquids & solids
12.
What is sublimation?
Examples of sublimation explained using the kinetic particle theory of gases and
solids
-
This is when a
solid, on heating, directly changes into a gas without melting, AND the
gas on cooling re–forms a solid directly without condensing to a
liquid. Sublimation usually just involves a physical change BUT its not always that simple
(see ammonium chloride!).
The opposite of sublimation is sometimes referred to as
deposition
or 'reverse
sublimation'.
Theory in terms of particles:
-
When the solid is heated
the particles vibrate with increasing force from the added thermal
energy.
-
If the particles have enough kinetic energy of vibration to
partially overcome the particle–particle attractive forces you would
expect the solid to melt.
-
HOWEVER, if the particles at this point have
enough energy at this point that would have led to boiling, the liquid
will NOT form and the solid turns directly into a gas.
-
On cooling, the particles
move slower and have less kinetic energy.
-
Eventually, when the particle kinetic energy is low
enough, it will allow the particle–particle attractive forces to produce
a liquid.
-
BUT the energy may be low enough to permit direct formation of
the solid, i.e. the particles do NOT have enough kinetic energy to
maintain a liquid state!
Examples of sublimation:
-
IODINE
-
Even at room temperature
bottles of solid iodine show crystals forming at the top of the bottle
above the solid. The warmer the laboratory, the more crystals form when
it cools down at night!
-
I2 (s)
I2 (g) (physical
change only)
-
If you gently heat iodine in a test tube you see the
iodine readily sublime and recrystallise on the cooler surface near
the top of the test tube.
ICE
-
The formation of a particular
form of
frost involves the direct freezing of water vapour (gas).
Frost can also evaporate directly back to water vapour (gas) and this
happens in the 'dry' and extremely cold winters of the Gobi Desert on a
sunny day.
CARBON DIOXIDE
- Solid carbon dioxide (dry ice)
is
formed on cooling the gas down to less than –78oC. On warming
it changes directly to a very cold gas!, condensing any water vapour in
the air to a 'mist', hence its use in stage effects.
CO2
(s)
CO2 (g) (physical change only)
Dry ice is added to water where
the sublimation causes evaporation of water which is cooled to fine
droplets to give a 'smoke effect' in stage effects.
AMMONIUM CHLORIDE
- On heating strongly in a test tube, white solid ammonium chloride, decomposes into a mixture of two
colourless gases ammonia and hydrogen chloride. On
cooling the reaction is reversed and solid ammonium chloride reforms at
the cooler top surface of the test tube.
-
Ammonium chloride +
heat energy
ammonia + hydrogen chloride
-
NH4Cl(s)
NH3(g) + HCl(g)
-
This involves both chemical and
physical changes and is so is more complicated than examples 1. to 3,
but in all cases total mass is conserved.
-
In
fact the ionic ammonium chloride crystals change into covalent
ammonia and hydrogen chloride molecule gases which are naturally,
and normally, far more
volatile than ionic solids like salt crystals (covalent substances generally have much lower melting and
boiling points than ionic substances).
5.
The formation of hoar frost - the reverse of
sublimation
Frost is a thin layer of ice on a solid
surface.
Hoar frost forms directly from water vapour
in air above 0oC, coming in contact with a solid surface whose
temperature is below freezing (<0oC).
The water vapor changes directly from gas
(vapour) to solid (ice) as it comes into contact with the solid surface.
PLEASE
NOTE
The liquid particle picture
does not figure here, but the other models fully apply apart from state
changes involving liquid formation.
GAS
particle model and
SOLID
particle model links.
You can also use the reverse reaction to illustrate diffusion and the fact
that the rate of diffusion depends on the molecular mass See
section 4.
Examples
of demonstrating diffusion in gases
At a higher level of study, you
may study the g–l–s
phase diagram for water and the vapour pressure curve of ice at particular
temperatures.
For example, if the ambient vapour pressure is less than the
equilibrium vapour pressure at the temperature of the ice, sublimation can
readily take place.
The snow and ice in the colder regions of the Gobi
Desert do not melt in the Sun, they just slowly 'sublime' and disappear!
Sublimation
can be used as a separation technique in chemistry and five examples are
described below.
Examples of sublimation in the context of a separation technique
(a) and (b) can be done as
simple school laboratory experiments.
All you need is the chemicals
to make the mixture and heat it in a pyrex boiling tube.
(a)
Separating iodine from
sand
Heating a mixture of iodine
crystals and sand heated.
The iodine sublimes → vapour
and crystals condense on a cool surface.
The sand remains behind, it
has a very high melting point.
(b)
Separating ammonium chloride from salt
Heating a mixture of ammonium
chloride and sodium chloride heated.
The ammonium chloride sublimes
→ collected separately.
Sodium chloride does not
sublime, it has a very high melting point.
(c)
Separating naphthalene from impurities
Naphthalene mothballs sublime,
leaving behind non-sublimable impurities.
This technique is used in
organic chemistry to purify compounds.
(d)
Dry ice (solid CO2)
Sublimes directly to gas,
leaving behind non-sublimable contaminants.
(e)
How
freeze drying food works
Freeze-drying is a process in which water in the form of ice
under low pressure is removed from a material by sublimation.
The food material is cooled to below 0oC so that any liquid
water is frozen to ice.
The ice is vapourised directly by sublimation (solid ==>
gas) under low pressure to dry the food and avoid the presence of liquid
water.
This process has found many applications for the production
of high quality food and pharmaceuticals.
Freeze drying is widely used for the stabilization of
high-quality food, biological materials, and pharmaceuticals, such as
proteins, vaccines, bacteria, and mammal cells. In the freeze drying
process, the quality of the dried product is preserved because freezing
water in the material inhibits chemical, biochemical, and microbiological
degradation processes. This improves retention of the taste, smell, and
content of various nutrients inhibiting change.
KEY
POINTS about sublimation
via a set of syllabus-aligned
revision notes on sublimation explained with the kinetic
particle model, tailored for WJEC, CCEA, CIE IGCSE, AQA, Edexcel,
OCR Gateway, and OCR 21st Century GCSE Chemistry.
Kinetic Particle
Model: Sublimation
Definition of
sublimation
- Sublimation:
The direct change of state from solid → gas (without
passing through the liquid phase).
- Reverse process:
Gas → solid (sometimes called deposition).
Explanation of
sublimation using the Kinetic Particle Model
- In a solid, particles are closely packed
and vibrate in fixed positions.
- When heated, particles gain kinetic
energy.
- At the sublimation point, particles gain
enough energy to overcome intermolecular forces completely
and escape directly into the gas phase.
- No liquid phase is formed because the
forces are overcome before particles can move freely as a liquid.
Examples of
sublimation
- Iodine crystals:
Purple vapour forms when iodine is gently heated.
- Solid carbon dioxide (dry ice):
Sublimes at –78 °C, producing carbon dioxide gas.
- Naphthalene mothballs:
Slowly sublime at room temperature, releasing vapour.
Key Features about
sublimation
| Feature |
Sublimation |
| State change |
Solid → gas (direct) |
| Energy change |
Absorbs heat (endothermic) |
| Particle behaviour |
Vibrations → overcome forces → escape
freely |
| Temperature |
Occurs at sublimation point (specific
to substance) |
| Examples |
Iodine, dry ice, naphthalene |
Typical Exam Board
Requirements about sublimation
- Requires particle theory explanation and
everyday examples (dry ice).
- Emphasise particle diagrams showing solid
lattice → dispersed gas particles.
- Know and describe sublimation examples
(iodine, ammonium chloride) and links to separation technique contexts.
- Describe in terms of energy transfer and
particle motion.
- Apply sublimation to real-world contexts
(dry ice in theatre effects).
Student Exam Tips
for sublimation
- Use particle diagrams:
Show solid lattice breaking directly into dispersed gas particles.
- Always state “no liquid phase”:
Examiners look for this distinction.
- Give specific examples:
Iodine and dry ice are the most commonly accepted.
- Link to separation techniques:
Sublimation can be used to purify substances (e.g., separating iodine from
sand).
- Mention energy transfer:
Sublimation requires heat input (endothermic).
Common
Misconceptions about sublimation
- “Sublimation is the same as
evaporation” → Evaporation is liquid → gas; sublimation is solid → gas.
- “All solids can sublime” →
Only certain solids (e.g., iodine, dry ice, naphthalene).
- “Sublimation skips energy
transfer” → Energy is still absorbed to overcome intermolecular forces.
- “Sublimation happens at any
temperature” → It occurs at a specific sublimation point.
- “Sublimation creates new
substances” → It’s a physical change, not chemical.
Quick Recap about
sublimation
- Sublimation:
Solid → gas, bypassing liquid phase.
- Kinetic particle model link:
Heating increases kinetic energy → particles overcome intermolecular forces
→ escape directly as gas.
- Examples:
Iodine, dry ice, naphthalene.
- Exam tip:
Always emphasise “no liquid phase” and give a named example.
Learning objectives
for sublimation reverse sublimation
(deposition)
Know what we mean by sublimation, the
interchange of vapour/gas and solid without an intermediate liquid state of
matter.
Be able to draw particle pictures to
illustrate and explain sublimation and its reverse i.e. deposition..
Be able to use the kinetic particle theory
of matter to explain the state of matter change of sublimation,
Know that thermal energy must be absorbed
by a solid before it can be sublimed.
Know that iodine and ammonium chloride
are good examples of solids to heat and observe the phenomenon of
sublimation.
All my
UK GCSE level (~US grade 8-10) school chemistry revision
notes
All my UK advanced level (~US grades 11-12)
pre-university chemistry revision notes
This is a BIG
website, you need to take time to explore it [ SEARCH
BOX]
Email doc
brown - comment? query?
  Website
content © Dr Phil Brown 2000+. All copyrights reserved on Doc
Brown's Chemistry revision notes, images, quizzes, worksheets etc.
Copying of website material is NOT permitted.
GCSE level and advanced pre-university level
revision notes. Detailed
notes on the states of matter and their properties.
Based on the syllabus-specifications for students taking the IGCSE/GCSE
level physics examinations summary revision notes and key points about
describing and explaining what sublimation is using kinetic particle
theory and its use as a separation technique,
for students taking the WJEC gcse
chemistry/physics, CCEA gcse chemistry/physics, CIE igcse chemistry/physics, AQA
igcse/gcse physics, Edexcel gcse chemistry/physics, OCR 21st century chemistry/physics, OCR gateway
chemistry/physics or any other GCSE or IGCSE level chemistry/physics
exams e.g. US grade 9-10 physics courses
INDEX of all my notes on the states of matter
GCSE (~US grades 8-10) level multiple choice QUIZ on
the states of matter: gases, liquids & solids |
|
