Lesson 2.9 Evaporation
Overview
This lesson deals with
evaporation, condensation and boiling. On completion of
the lesson, you should be able to describe the process of
evaporation and explain the difference between
evaporation and boiling.
MINI
LAB
1) Wrap a small piece of
cloth on a thermometer bulb, moisten the cloth and
observe the change in temperature when it is placed in
front of a fan
2) Place a piece of wet
cloth inside a clear plastic bag, put a piece of wire
inside the bag to provide an air space in the bag. Place
the bag in the sun and observe the process of evaporation
from the cloth – and condensation on the inside of
the bag.
Evaporation
Evaporation is defined as the process by which a liquid (usually
water) is converted into a gaseous state. When
evaporation of water into the air occurs, it requires
that the humidity of the atmosphere be less than the
evaporating surface (at 100 % relative humidity there is
no more evaporation). The evaporation process also
requires large amounts of energy.
Equilibrium
Evaporation occurs when molecules of a liquid, near the
surface, gain sufficient energy to be able to break free
from the forces of attraction that hold them in the
liquid phase. All molecules in the liquid phase (and the
gas phase) move at different rates and have different
amounts of kinetic energy. They continually gain and lose
energy by colliding with other molecules. Molecules in
the gas phase may lose energy and combine with other
slower moving molecules to form liquid droplets (condense)
or they may collide with the surface of the liquid and
return to the liquid phase.
The processes of
evaporation and condensation occur simultaneously. At
equilibrium, the rate of evaporation equals the rate of
condensation. Both processes continue but because there
is no net increase in the amount of vapor, the
evaporation process appears to have stopped.
Volatility
The ease with which a liquid (or solid such as
naphthalene or solid CO2) evaporates varies
from material to material. Some substances are more
volatile than others. Gasoline, for example, evaporates
more easily than water does. Propane evaporates more
easily than gasoline etc. A more volatile substance
exerts a higher vapor pressure than a less volatile
substance at the same temperature.
Heat of Vaporization
The amount of heat required to vaporize a certain amount
of a liquid at its boiling point with no change in
temperature. Usually expressed in J/kg. (The molar heat
of vaporization is the amount of heat required to
vaporize one mole of liquid at its boiling point with no
change in temperature and usually expressed ion J/mol.)
Condensation
The process whereby a vapor changes to a liquid. This
requires a cooling effect to draw heat away from the
vapor. When the temperature of the vapor reaches a
certain point, droplets of liquid (condensate) begin to
form.
Distillation
Distillation is the process of heating a mixture and
condensing the resulting vapor to produce a more pure
substance.
The term "Evaporation" is often used instead of
distillation when a pure liquid (such as water) is
separated by evaporation and condensation from a solid
that does not evaporate.
Distillation is often used
to partially separate a mixture of two or more liquids.
Nature uses an evaporation
(distillation) process called the "Hydrologic Cycle".
This natural distillation / evaporation process is where
the sun heats the water on the earth's surface. The water
is turned into a vapor and rises, leaving contaminants
behind, to form clouds. As the upper atmosphere drops in
temperature the vapors cool and is converted back to
water to form water droplets. When the water droplets
reach the earth's surface, as snow or rain (precipitation),
the cycle repeats again.
Vapor Pressure
The pressure exerted by a vapor in equilibrium with the
solid or liquid phase of the same substance. Also, the
partial pressure of the substance in the atmosphere above
the solid or the liquid.
Gases exert pressure on
the inside of a container because the molecules collide
with the inside of the container.
The amount of pressure
depends on the temperature of the system
If the temperature is
increased, more vapor is formed. This exerts a greater
pressure.
The pressure that water
vapor exerts is fixed by the temperature. If other gases
are present, the pressure exerted by each gas contributes
to the total pressure. Each gas exerts a partial pressure
Vapor Pressure Of Water
If we increase the temperature of the system, more water
goes into the vapor phase and the vapor pressure
increases.
This table shows how the vapor pressure of water
increases with temperature.
Vapor Pressure
of Water
T (oC
|
P (mm Hg)
|
0
|
4.58
|
10
|
9.21
|
20
|
17.54
|
30
|
31.82
|
40
|
55.3
|
50
|
92.5
|
60
|
149.4
|
70
|
233.7
|
80
|
355.1
|
90
|
525.8
|
100
|
760.0
|
150
|
3570.4
|
200
|
11659.2
|
Boiling
When the vapor pressure of a material (water) reaches the
surrounding pressure (atmospheric pressure), boiling
occurs.
While it is being heated,
some of the water evaporates but when the vapor pressure
reaches the same value as the pressure around the liquid,
the temperature stops rising and all the energy goes to
changing the liquid to vapor. The temperature at which a
liquid boils depends on the pressure above the liquid.
At sea level, water boils
at 100 ºC. Higher up, the pressure is less and water
boils at a lower temperature.
We can find the
temperature from the table above. Whenever the vapor
pressure equals the atmospheric pressure, boiling will
occur.
Humidity
The mass of water vapor in a fixed total mass of air.
Humidity is an indication
of how much more water could evaporate into the air
before it becomes completely saturated.
Air is often less than 100%
humid because evaporation takes time, temperatures change
constantly and air currents move air from regions in
contact with water to dryer regions.
Relative Humidity
The relative humidity is the ratio of the amount of water
vapor actually in the air compared to the amount of water
vapor required for saturation at that particular
temperature and pressure. Increasing or decreasing the
amount of water vapor in the air changes the relative
humidity.
A change in temperature
will also bring about a change in relative humidity.
Dew Point
The dew point is the temperature to which air would need
to be cooled (with no change in air pressure or water
content) for saturation to occur.
Saturation Vapor
Pressure
The pressure that water vapor molecules exert when the
air is saturated at a given temperature. Saturation vapor
pressure increases with rising temperature.
Questions
- If a cup of water is
left for a number of hours at room temperature in
dry air, some of the water will evaporate. Why
does the water evaporate and not boil?
- If the temperature of
the water is increased, will it evaporate faster?
- If a cup of gasoline
at the same temperature as the water is placed in
the room, will the gasoline also evaporate? If
so, will there be a difference in the rates of
evaporation? Why?
- Are the molecules in
a liquid constantly moving? If so, do they all
move at the same speed?
- Why do some molecules
escape from the liquid surface?
- Do some of the vapor
molecules return to the liquid?
- Is the average
kinetic energy of vapor molecules greater than
that of liquid molecules at the same temperature?
What is this difference in energy known as?
- When a liquid
evaporates into a closed container, does the
vapor have an effect on the pressure of the gas
in the container?
- If a liquid at the
bottom of a small closed container evaporates
until the air in the container is saturated with
vapor, will the liquid continue to evaporate?
- What is the increase
in pressure caused by the vapor called?
- If the temperature of
the liquid is increased, will the pressure in the
closed container increase?
- Why does the liquid
boil when it reaches a particular temperature?
- How does the pressure
of the air in the closed container affect the
temperature at which the liquid will boil?
- Why does water boil
at a lower temperature at higher elevations?
- Why do pressure
cookers speed up the cooking process?
|
