Most of Earth's water is found in oceans, which cover most of the planet's surface. The water cycle describes the continuous movement of water on, above, and below the Earth's surface, including evaporation, condensation, precipitation, and runoff. Evaporation occurs when water is heated by the sun and turns into water vapor, which rises up into the atmosphere and can form clouds and precipitation that falls back to Earth. [END SUMMARY]

1. Evaporation
Heat of Reaction
Energy Balance
Mrs Nadia Junaid

2. Distribution of Water on Earth
• Most of the Earth’s water is in oceans.
• Oceans contain salt water and covers most of the earth’s
surface.
• Of the final 1%, only 4/10 of water comes from rivers, a major
source of the water we use.
Click on the graph to learn more!

3. Most of Earth’s water is in the oceans, which
cover most of Earth’s surface.
• Liquid water evaporates, turns into water vapor and back into a
liquid when cooled or as a solid if cooled below the freezing point
of water.
• Water vapor in the air moves and can form fog or clouds, and can
fall to Earth as rain, hail, sleet, or snow.
• The amount of fresh water located in rivers, lakes, underground
sources, and glaciers is limited and that its availability can be
extended by recycling and decreasing water use.

5. Clouds Formation

6. What is a cloud?
~ It is a thick mass of suspended water drops or
ice crystals

7. Clouds Identification
• 1. The altitude at which they occur
2. Color
3. Density
4. Shape
5. Degree of cover

8. FOG

9. Fog : Clouds at ground level

11. Evaporation, Condensation and Precipitation:
The main processes of the water cycle

12. WATER CYCLE

13. Energy and Phase Changes

15. Evaporation
• The transformation of a liquid into a gas.
• The sun’s heat raising the temperature of
water and turning it into a gas.
• Water from earth’s rivers, and oceans that is
heated and travels through the air as a gas.

16. Stages of Evaporation
1. Water is warmed
by the sun
3. Water particles rise as a gas
or vapor
2. As the water
heats, it turns
into a gas.

17. Condensation
• Water vapor particles rising through the air.
• You can't see it, but air contains a lot of water.
• When air gets cold, the water vapor condenses into
clouds.
• Warm air can hold more water vapor than cold air.
– When warm air holds a lot of water vapor, the air feels sticky
and damp.
• The amount of water in the air is called humidity.

18. The Stages of Condensation
1. Water vapor particles rising
2. Particles forming groups
3. Water particles grouped to form bonds

19. Precipitation is…
• …the process by which
water falls back down to
earth as rain, snow, or
hail.
• Depending on the
temperature, water may
fall to earth as:
– liquid (rain),
– semi-solid (snow),
– solid (hail)

20. Factors affecting evaporation
• evaporation the escape of more energetic
molecules from the surface of a liquid.
• Relate evaporation to the consequent cooling.
• Demonstrate understanding of how
temperature, surface area and air flow over a
surface influence evaporation

21. Evaporation

22. Evaporation
Consider a beaker of water at room
temperature

23. Evaporation
The molecules of water are moving around at
different speeds, some fast, some slow.
speed of
molecule (m/s)
# of molecules at
a particular speed
Average
speed

24. Evaporation
If a molecule is at the surface, and moving fast
enough, it may escape the liquid. This is called
evaporation.
Freedom!

25. Evaporation
Since the average speed of the remaining molecules must
now be lower, the temperature of the liquid drops (since
temperature is a measure of the kinetic energy of the
molecules).
Freedom!

26. Evaporation
Evaporation can thus take place at any
temperature.

27. Increasing the rate of evaporation
Increasing the
temperature.

28. Increasing the rate of evaporation
Increasing the
temperature
means that more
molecules are
moving fast
enough to escape.

29. Increasing the rate of evaporation
Increasing the
surface area

30. Increasing the rate of evaporation
Increasing the
surface area
means that more
molecules are at
the surface.

31. Increasing the rate of evaporation
Increasing the air
flow over the
surface

32. Increasing the rate of evaporation
Increasing the air
flow over the
surface so that
molecules are
carried away
before they can
fall back into the
liquid

33. Increasing the rate of evaporation
Decreasing the
humidity of the
surrounding
atmosphere

34. Increasing the rate of evaporation
Decreasing the
humidity of the
surrounding
atmosphere to
stop water
molecules from
the atmosphere
entering the liquid.

35. Factors Influencing Evaporation
• Energy supply for vaporization
(latent heat)
– Solar radiation
• Transport of vapor away from
evaporative surface
– Wind velocity over surface
– Specific humidity gradient above
surface
• Vegetated surfaces
– Supply of moisture to the surface
– Evapotranspiration (ET)
• Potential Evapotranspiration (PET)
– moisture supply is not limited
nR
E
Net radiation
Evaporation
Air Flow
u

36. Boiling
• Boiling is a cooling process
• Temperature is 100
degrees C no matter how
much heat we give the
water.
• Water boils as fast as it is
being warmed by heating.
• The steam carries away the
heat.

37. Boiling Water
• At what temperature does water boil in the
mountains?

38. Energy in Condensation/Vaporization
• The Heat of Vaporization is 540 calories per
gram of condensed water at 100 degrees.

39. EVAPORATION
• Definition: Process by which water is changed from the liquid
or solid state into the gaseous state through the transfer of
heat energy (AMERICAN SOCIETY OF CHEMICAL, 1949).
• It occurs when some water molecules attain sufficient kinetic
energy to break through the water surface and escape into
the atmosphere (~ 600 cal needed to evaporate 1 gram of
water).
• Depends on the supply of heat energy and the vapor pressure
gradient (which, in turn, depends on water and air
temperatures, wind, atmospheric pressure, solar radiation,
etc).

40. Under which of the following conditions will
water evaporate the slowest?
a) Humidity = 95% b) Humidity = 80%
c) Humidity = 30% d) Humidity = 70%

41. At which of the following ground temperatures will
the WATER evaporate the fastest?
a) 50°C b) 15°C
c) 29°C d) 87°C

42. Your little sister forgot to turn off the water in the bathroom sink. The
water overflowed and soaked the carpet. You want to dry the carpet
out before your mother gets home. At which level should you turn on
the fan?
a) low b) high
c) medium d) The carpet would dry
out faster without a fan.

43. Students are investigating whether wind affects the rate at
which the sponge will dry. Which experiment is best to use
in answering their question?
a) b)
c) d)

44. Each of the following containers hold exactly10
ounces of water. From which container will water
evaporate the fastest?
a) b)
c) d)

45. Which letter represents evaporation?

46. In which state is the fog?
a) Melting
b) Freezing
c) Boiling
d) Evaporating
e) Condensing
f) Sublimating

47. In which state is the moisture on the
bottle?
a) Melting
b) Freezing
c) Boiling
d) Evaporating
e) Condensing
f) Sublimating

48. In which state is the moisture on the leaf
petals?
a) Melting
b) Freezing
c) Boiling
d) Evaporating
e) Condensing
f) Sublimating

49. In which state is the water in the tea pot?
a) Melting
b) Freezing
c) Boiling
d) Evaporating
e) Condensing
f) Sublimating

50. FACTORS affecting Evaporation

51. Factor 1 - Temperature
The higher the temperature the faster the rate
of evaporation.

52. Which part will evaporate faster?
A B

53. Factor 2 - Wind
• The more wind or air circulation the faster the
rate of evaporation.

54. Which part will evaporate faster?
A B

55. Factor 3 – Exposed Surface Area
The larger the exposed surface area the faster
the evaporation rate.
The water will evaporate more quickly from the
beaker on the right with the wider opening.
3 inches
1 inch

56. If each beaker contains 8 ounces of water, which
beaker will evaporate the quickest?

57. Factor 4 - Humidity
• When humidity (the amount of water vapor in
the air) is high, evaporation is slower.

58. In which location would laundry hanging
on a line dry the quickest?
A) Desert
Humidity = 30%
B) Nice Day
Humidity =60%
C) Rainy Day
Humidity = 100%
D) Tropics
Humidity = 90%

59. Heat of REACTION

60. Thermochemistry
• Thermodynamics is the science of the
relationship between heat and other forms of
energy.
• Thermochemistry is the study of the quantity of
heat absorbed or evolved by chemical reactions.

61. Energy
• There are three broad concepts of energy:
• Kinetic Energy is the energy associated with an
object by virtue of its motion.
• Potential Energy is the energy an object has by
virtue of its position in a field of force.
• Internal Energy is the sum of the kinetic and
potential energies of the particles making up a
substance.
We will look at each of these in detail.

62. Energy
• Internal Energy is the energy of the
particles making up a substance.
• The total energy of a system is the sum of its
kinetic energy, potential energy, and internal
energy, U.
UEEE pktot 

63. Energy
• The Law of Conservation of Energy: Energy may
be converted from one form to another, but the
total quantities of energy remain constant.
The First Law of Thermodynamics:
You can’t get something
from nothing!

64. Heat of Reaction
• In chemical reactions, heat is often
transferred from the “system” to its
“surroundings,” or vice versa.
• The substance or mixture of substances under
study in which a change occurs is called the
thermodynamic system (or simply system.)
• The surroundings are everything in the vicinity
of the thermodynamic system.

65. Heat of Reaction
• Heat is defined as the energy that flows
into or out of a system because of a
difference in temperature between the
system and its surroundings.
• Heat flows from a region of higher
temperature to one of lower temperature;
once the temperatures become equal, heat
flow stops.

66. Heat of Reaction
• Heat is denoted by the symbol q.
– The sign of q is positive if heat is absorbed by
the system.
– The sign of q is negative if heat is evolved by
the system.
• Heat of Reaction is the value of q required to
return a system to the given temperature at
the completion of the reaction.

67. Heat of Reaction
• An exothermic process is a chemical
reaction or physical change in which heat is
evolved (q is negative).
• An endothermic process is a chemical
reaction or physical change in which heat is
absorbed (q is positive).

68. Endothermic
and
exothermic.

69. Heat of Reaction
• Exothermicity
– “out of” a system
Dq < 0
• Endothermicity
– “into” a system
Dq > 0
Energy
System
Surroundings
Energy
System
Surroundings

70. 70
Reaction Conditions
Reaction conditions for a chemical reaction
require
 collisions between reacting molecules
 collisions with sufficient energy to break
the bonds in the reactants
 the breaking of bonds between atoms of
the reactants
 the forming of new bonds to give products

71. 71
Chemical Reactions
In the reaction H2(g) + I2(g) 2HI(g),
 the reactants H2 and I2 collide
 the bonds of H2 and I2 break
 the bonds for HI form
H2 + I2 collision bonds break HI
new bonds form

72. 72
Activation Energy
Activation energy
 is the minimum energy required upon
collision for a reaction to take place

73. 73
Heat of Reaction
The heat of reaction
 is the amount of heat
absorbed or released
during a reaction
 is the difference in the
energy of the reactants
and the products
 Has the symbol ΔH
ΔH = ΔHproducts −
ΔHreactants

74. 74
Exothermic Reactions
In an exothermic reaction,
 the energy of the products is
less than the energy of the
reactants
 heat of reaction is released
 heat is a product
C(s) + O2(g) CO2(g) + 394 kJ
ΔH = –394 kJ/mole (heat released)

75. 75
Endothermic Reactions
In an endothermic reaction,
 heat is absorbed
 the energy of the products
is greater than the energy
of the reactants
 heat is a reactant
N2(g) + O2(g) + 180 kJ 2NO(g)
ΔH = 180 kJ (heat added)

76. 76
Summary
Reaction Energy Heat Sign of
Type Change in Reaction ΔH
Endothermic Heat absorbed Reactant side +
Exothermic Heat released Product side –

77. ENERGY BALANCE

78. Law of Conservation of Energy
• The law of conservation of energy states that
energy cannot be created or destroyed, but
only changed from one form to another.

79. Hess’s Law
• Hess’s Law states that the heat change of a
reaction depends only on the initial and final
states of the reaction and is independent of the
route by which the reaction may occur.
• for a reaction described by a given equation, the
heat of reaction equals the sum of the heats of
formation of the products of the reaction less the
sum of the heats of formation of the reactants of
the reaction.
• ΔHr = ΔHf (products) - ΔHf (reactants)

80. ENERGY BALANCE
According to law of conservation of energy : it is always remain conserved.
So Energy is balanced

81. A Problem of energy balance
• Suppose a piece of iron requires 6.70 J of
heat to raise its temperature by one degree
Celsius. The quantity of heat required to
raise the temperature of the piece of iron
from 25.0 oC to 35.0 oC is:
)C0.25C0.35()C/J70.6(TCq ooo
D
J0.67q 

82. • In above problem, Energy added to the system
is calculated, and energy accumulated by the
system is found by the formula , and overall
energy of the system is conserved.