Phase changes
What is a phase change?
A phase change is a phenomenon of
transition of a thermodynamic system from
one phase to another.
A phase is a state of matter.
The states can be of various types:
liquid, solid or gas.
and… what about Plasma state?
In many books we can find only three states of matter: solid, liquid and gas.
The fourth state of matter is plasma.
It is made up of two types of intermixed gas: a gas of ions and a gas of
electrons.
When a liquid turns into gas and its temperature of 100°C increases, the
electrons and the nucleus that constitute an atom no longer can stay together.
The atoms are striped of their electrons and a plasma is formed.
Plasma is a collection of positively and negatively charged particles.
Melting (solid to liquid)
ENDOTHERMIC PHASE (energy absorbed)
Vaporization (liquid to gas)
Sublimation (solid to gas)
Condensing (gas to liquid)
EXOTHERMIC PHASE (energy released)
Freezing (liquid to solid)
Deposition (gas to solid)
Melting
When a solid changes into liquid, we are talking about
Melting.
The solid absorbs heat and its temperature rises till a point called Melting
Point. At this point the temperature stops rising. As more energy the solid
absorbs, solid particles change into liquid ones.
Q = m · Lfus
Freezing
When the liquid changes into solid by realising heat energy,
the phase change is known as freezing.
The heat the liquid realeses to turns into a solid is the same
it absorbed when he became a liquid.
Q = m (-Lfus)
Vaporization
Vaporization happens when liquid state of matter turns
into gaseous state.
The liquid absorbs heat energy and its temperature increases till a level
called Boiling Point. Now the temperature stops rising and the liquid starts
boiling. As more heat energy the liquid absorbs, the liquid particles convert
into gas.
Q = m · Lvap
Saturated steam
The steam is in equilibrium with liquid state: the number of
particles that from liquid change into gaseous ones is equal
to the number of gaseous particles that condens into liquid.
The pressure of saturated steam is the pressure, the steam applies on the wall
of the container, when the evaporation of the liquid stops happening.
Sublimation
When the solid turns directly into gas, we have the
Sublimation.
Sublimation happens in solid compounds like: ammonium chloride (NH4 Cl),
carbon dioxide (CO2) or iodine. Chemists use this phase change to purify solid
compounds.
CONDENSING
Condensing is the process of transition from GAS TO
LIQUID.
When the heat is removed from the gas, its temperature
reduces to a certain level.
After that level, the gas start changing from gaseous phase
to liquid.
This phase change occurs at almost constant temperature.
DEPOSITION
This is the process of the direct transition from
GAS TO SOLID.
It appears in nature and
its opposite is the sublimation..
Relative humidity
A great abundance of water vapor in a determined zone and instant is described by a physical
quantity
called: relative humidity.
Relative humidity is the ratio of water
vapor pressure (Pw) to the saturation vapor
pressure (Pws), at the same temperature.
To measure the relative humidity, we can use a tool called:
hygrometer.
There are different types of hygrometer, but one of the most
common used is the hair hygrometer that makes use of hair
characteristics to contract when there is humidity.
Temperature for bodies
Evaporation of perspiration on skin is a natural mechanism to regulate internal
temperature.
- 
A very hight relative humidity slows down the evaporation of
perspiration, making this process less efficient.
The skin, for this reason, feels cold.
-  A relative humidity of 100 % stops completely the evaporation of
perspiration. Hence the skin feels hot.
-  A combination of cold and wind makes body feel colder than the
actual temperature. That happens because wind distances from the
skin the air exposed to it ,which is damper, changing it with air more
dehydrate that comes from other zones. In this case, the evaporation
of perspiration can be a very efficient process to defend our
organism.
Phase change diagram
A phase change diagram is a common way to represent the different phases of a substance.
At the point A, there is a solid with a temperature of
-25°C. The temperature increases to 0°C.
At the point B, there is the Melting Point, because the
solid turns into water.
At the point C, water’s temperature increases to 100°C.
At the point D, the temperature stops rising and the
water turns into steam. It is called Vaporisation Phase.
Once the water has completely vaporised, the
temperature of the steam increases. (Point E/F)