Changes of Phases
The Foundation Course on Physics
Professor Vojtěch Mornstein
Phases
Gas – energy of thermal motion of particles is greater than the energy of
different bonds between the particles.
Liqiud - energy of thermal motion of particles is about the same like the
energy of different bonds between the particles (van der Waals bonds –
see next slide).
Solid - energy of thermal motion of particles is smaller than the energy of
different bonds between the particles. The bonds can be covalent
(diamond), ionic (salt crystal), metallic (metals), van der Waals (see next
slide).
Plasma – highly ionised gas is considered 4th phase. It exists at very high
temperatures.
Changes of phases (i.e. phase transitions) are always accompanied by
absorption or release of heat!!
Overview of weak chemical bonds
(thea are responsible for e.g. stable shape of our
bodies!)
• Hydrogen
bonds
• Hydrophobic
interaction
• Van der Waals
bonds
Also called London forces, sometimes not
classified as proper van der Waals bonds
Hydrogen bonds between water molecules –
stronger than van der Waals bonds
Liquid water
Pictures: http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/11.html
Ice and
hexagonal
shape of
snow flakes
About five molecules of
water form the so called
cluster – a relatively stable
body resembling a
molecule.
Phase changes
A general rule:
1. To break some bonds we need some energy.
Heat is absorbed/delivered from surroundings.
2. A formation of some bond leads to release of
some energy. Heat is produced/released into the
surroundings.
These rules are valid also for the van der Waals
intermolecular forces!!!
Latent Heats
a) Latent heat of fusion (melting) = latent heat of solidification
For unit mass: Specific latent heat of fusion (melting) = specific latent heat of solidification.
It is the heat required per unit mass to melt the substance without change of temperature.
If = hf = [J·kg-1]
b) Latent heat of vaporization = latent heat of condensation:
For unit mass: Specific latent heat of vaporization = specific latent heat of condensation.
Heat required per unit mass to vaporize the liquid without change of temperature.
Iv = hv = [J·kg-1]
c) Sublimation and desublimation, the direct changes of a solid to a gas, and vice versa,
are also accompanied by consumption or production of heat.
m
Qf
m
QV
Mysterious liquids
• Superheated liquid
• Supercooled liquid
Start of solidification or boiling
needs a nucleation center (a
seed crystal, dust particle,
microbubble etc.)
Very similar phenomenon
Sodium acetate heating pads
Sodium acetate is used in heating
pads, hand warmers, and hot ice.
Sodium acetate trihydrate crystals melt
at 58 °C dissolving in their water of
crystallization. When they are heated
past the melting point and
subsequently allowed to cool, the
aqueous solution becomes
supersaturated. This solution is
capable of cooling to room
temperature without forming crystals!
By pressing on a metal disc within the
heating pad, a nucleation center is
formed, causing the solution to
crystallize back into solid sodium
acetate trihydrate. The process of
crystallization is exothermic (latent
heat of fusion is about 264 –289 kJ/kg).
Thermoregulation due to sweating
• One of the ways how to decrease our body
temperature, e.g., in hot days, is production of
sweat by small glands we have in our skin.
Water is evaporated and heat of vaporisation
is absorbed.
• Imagine a hot day with the air temperature of
about 30 °C. Compare the body
thermoregulation in the Amazonian forest or
Sahara dessert at this temperature!
Salt on streets
(it will appear in next days!)
What happens when salt comes in contact with
snow or ice? Keep in mind that dissolving of a
substance in water lowers its fusion temperature.
1. The snow is molten in presence of dissolving salt.
Temperature decreases.
2. Heat is absorbed from ambient medium
(surroundings) so that more snow can be
dissolved. Street become free of snow or ice.
3. This mechanism can work up to about –10 °C!!
Freezing mixtures
• How to prepare ice
without a
refrigerator?
About −20 °C can be achieved with
a 1 to 3 ratio by weight of sodium
chloride to ice.
In the past, ice was collected in
winter and sold in summer by e.g.
Breweries.
Liquefaction
of gases
• How to produce liquid gas?
• The most common process for the preparation of liquid air is the
Linde cycle using the Joule-Thomson effect. Keep in mind that
expansion of a real gas leads to a temperature decrease even
(seemingly) without doing mechanical work.
• However, there are some attraction forces between molecules and
we need to do some work against them when the distances
between molecules increase.
End of lecture – thank you for your
attention!
Version 2020