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ISNS 4371 - Phenomena of Nature
The state or phase of matter is
determined by its temperature.
Consider water:
Below 32º F - ice - relatively low
KE - each molecule tightly bound
to it neighbors - solid
At 32º F molecules have enough
energy to break solid bonds of ice
- remain together but move
relatively freely - liquid
At 212º F water boils and turns to
gas - molecules break free of all
bonds with neighbors - move
independently of other molecules
- gas
ISNS 4371 - Phenomena of Nature
Phase/State Changes
Heat transfer always occurs whenever a substance changes phase
Melting - when a solid changes to a liquid
Energy is
absorbed
Evaporation - when a liquid changes to a gas
Sublimation - when a solid changes directly to a gas
Energy is
released
Condensation - when a gas changes to a liquid
Freezing - when a liquid changes to a solid
ISNS 4371 - Phenomena of Nature
Change of State (Phase Change)
Latent heat of fusion - amount of thermal energy required to change
a substance from a solid to a liquid (melting).
Latent heat of solidification - amount of thermal energy given up
when a substance changes from a liquid to a solid (freezing).
Latent heat of fusion for water is 80 calories per gram.
Latent heat of solidification for water is -80 calories per gram.
Energy is released.
ISNS 4371 - Phenomena of Nature
Change of State (Phase Change)
Latent heat of vaporization - amount of thermal energy required to
change a substance from a liquid to a gas (vaporization or
evaporation).
Latent heat of condensation - amount of thermal energy given up
when a substance changes from a gas to a liquid.
(condensation).
Latent heat of vaporization for water is 539 calories per gram.
Latent heat of condensation for water is -539 calories per gram.
Energy is released.
ISNS 4371 - Phenomena of Nature
Phase Change Diagram
ISNS 4371 - Phenomena of Nature
Specific Heat Capacity, c: Thermal inertia
Specific Heat Capacity is the quantity of heat required to change the
temperature of 1 gram of a substance by 1° C.
Q units of of thermal energy added to 1 gram of a substance produces a
temperature change of ∆T,
Q = c x ∆T
Specific heat , c, of a substance is the heat capacity per unit mass.
For m grams of a substance,
Q = cm ∆T or ∆T = Q/cm
Water has high specific heat capacity - used as a cooling fluid.
Specific heat capacity of water is 1 calorie/gram-deg. C
ISNS 4371 - Phenomena of Nature
Heat of Fusion Measurement
Add 10 grams of ice (at 0º C) to 100 grams of water.
What is the heat of fusion of water?
Mass of water=M
Mass of ice =m
Hf= heat of fusion of water
To = initial temperature of ice
Tw = initial temperature of water
Tf = final temperature of water
Heat required to melt the ice = mHf
Heat required to raise the temperature of melted ice to final
temperature of water = cm ∆T = cm(Tf - To)
Heat absorbed from water = cM ∆T = cM(Tw-Tf)
ISNS 4371 - Phenomena of Nature
Heat of Fusion
Heat Absorbed by Ice
Heat Transferred from Water
Q1 = cm(Tf - To) +mHf
Q2 = cM(Tw - Tf)
Equate heat absorbed by ice to heat transferred from water
cm(Tf - To) + mHf = cM(Tw - Tf)
To = 0 and c = 1, so
mHf = M(Tw - Tf) - m Tf
Hf = M/m((Tw - Tf) - Tf
ISNS 4371 - Phenomena of Nature
Heat of Fusion
Heat Absorbed by Ice
Heat Transferred from Water
Q1 = cm(Tf - To) +mHf
Q2 = cM(Tw - Tf)
Equate heat absorbed by ice to heat transferred from water
cm(Tf - To) + mHf = cM(Tw - Tf)
To = 0 and c = 1, so
mHf = M(Tw - Tf) - m Tf
Hf = M/m((Tw - Tf) - Tf
Hf = 80 cal/gr
ISNS 4371 - Phenomena of Nature
Change of State (Phase Change)
Example:
Add 10 grams of ice at 0º C to 100 grams of water at 30 º C.
What is the final temperature of the water?
Heat Absorbed by Ice
Heat Transferred from Water
Mass of ice =m
Mass of water=M
Hf= heat of fusion of water: 80 calories/gram
To = initial temperature of ice
Tw = initial temperature of water
Tf = final temperature of water
Q=cm(Tf - To) +mHf
Q=cM(Tw-Tf)
ISNS 4371 - Phenomena of Nature
Change of State (Phase Change)
Heat Absorbed by Ice
Heat Transferred from Water
Mass of ice = m = 10 grams
Mass of water = M = 100 grams
Hf= heat of fusion of water:
80 calories/gram
Q = cm(Tf - To) +mHf
Q = cM(Tw - Tf)
Q = 1 x 10(Tf - 0) + 10 x 80
Q = 1 x 100(30 - Tf)
Equate heat absorbed by ice to heat transferred from water
10Tf + 800 = 3000 - 100Tf
110Tf = 3000 - 800
Tf = 2200/110
ISNS 4371 - Phenomena of Nature
Change of State (Phase Change)
Heat Absorbed by Ice
Heat Transferred from Water
Mass of ice = m = 10 grams
Mass of water = M = 100 grams
Hf= heat of fusion of water:
80 calories/gram
Q = cm(Tf - To) +mHf
Q = cM(Tw - Tf)
Q = 1 x 10(Tf - 0) + 10 x 80
Q = 1 x 100(30 - Tf)
Equate heat absorbed by ice to heat transferred from water
10Tf + 800 = 3000 - 100Tf
110Tf = 3000 - 800
Tf = 2200/110
Tf = 20º C
ISNS 4371 - Phenomena of Nature
Atmospheric Pressure
Atmosphere - layer of gas surrounding a
world
Atmospheric pressure - collisions of individual
atoms or molecules in atmosphere
Air molecules in a balloon exert pressure as
they collide with the walls pushing outward.
Air molecules outside balloon collide with wall
and exert pressure inward. Balloon stays
inflated when pressures are balanced.
Adding molecules to balloon (blow it up)
causes balloon to expand (increases its
volume) until pressures are balanced again.
Heating it also increases pressure (increases
the speed of the molecules). The balloon
expands until pressures are equalized again
ISNS 4371 - Phenomena of Nature
Gas in an atmosphere is held down by gravity.
Atmosphere above presses downward,
compressing atmosphere below.
At the same time, fast moving molecules exert
pressure in all directions, including upward tends to make atmosphere expand.
Planetary atmospheres exist in balance
between downward weight of their gases and
upward push of their gas pressure
The higher you go, the less the weight of gas
above you, and the less the atmospheric
pressure.
1 bar - atmospheric pressure at sea level equal to weight of a column of gas extending
upward from Earth’s surface from sea level
ISNS 4371 - Phenomena of Nature
Vapor Pressure and Boiling Point
Evaporation in a closed container will proceed until there are as many
molecules returning to the liquid from the vapor above the liquid as there
are escaping - the vapor is then said to be saturated. The pressure of that
vapor is called the saturated vapor pressure.
Molecular kinetic energy is greater
at higher temperature - more
molecules can escape the surface
and the saturated vapor pressure is
correspondingly higher. If the liquid
is open to the air, then the pressure
of the air opposes the escape of
the molecules. The temperature at
which the vapor pressure is equal
to the atmospheric pressure is
called the boiling point.
ISNS 4371 - Phenomena of Nature
Evaporation vs Boiling
Both start with a liquid and end with a gas. But they are different processes.
Evaporation:
Strictly a surface phenomena
Occurs at any temperature
Some hotter (faster)-than-average particles overcome the forces
they feel from their neighbors and escape the liquid, taking
their heat energy with them.
Forces only felt from particles beneath them
Boiling:
Happens throughout the liquid
Occurs at the boiling point/temperature
Average motion of particles is fast enough to overcome the forces
holding them close together - all the particles are trying to escape liquid turns to vapor
Forces felt from particles all around them
Boiling point dependent on atmospheric pressure - steam bubbles form
in liquid only when vapor (steam) pressure exceeds atmospheric
pressure (plus pressure of water pushing down)
ISNS 4371 - Phenomena of Nature
The Boiling Point Depends on the Liquid Temperature and
the Atmospheric Pressure
Boiling (evaporation) cools the liquid
- when 100º C water is boiling, it is in thermal equilibrium
- it is being cooled by the boiling as fast as it is being heated by the
heat source - if not the water temperature would continue to rise
ISNS 4371 - Phenomena of Nature
Carbon Dioxide
People breathe in oxygen and breathe out carbon dioxide. Plants breathe in
carbon dioxide and breathe out oxygen.
Carbon dioxide gas doesn’t turn into a liquid when it gets cold like most other
matter, it turns straight into a solid - it sublimates - at -109° F!
Sublimate - to transform directly from the solid to the gaseous state or
from the gaseous to the solid state without becoming a liquid.
Frozen carbon dioxide is called dry ice.
We can make fog with dry ice by putting it in hot water.
- the extreme cold causes the hot water vapor to condense
into clouds
- the dry ice turns into carbon dioxide gas and mixes with the clouds
- the fog is heavier than air and sinks to the floor
ISNS 4371 - Phenomena of Nature
Nitrogen
Most of the air we breathe ( 78%) is made of Nitrogen.
Nitrogen gas turns into a liquid at -320° F and freezes at -346° F.
We can use liquid nitrogen (N2) to freeze things.
Some things become very hard when they are frozen - like a banana, some
become very brittle - like a flower.
The water in them freezes, forming millions of tiny ice crystals. The banana
has fibers in it that hold the crystals together making it hard. The flower doesn’t
have such fibers and becomes brittle.
ISNS 4371 - Phenomena of Nature
Freezing Nitrogen
When we pump the air out of a container with liquid nitrogen in it, we make a
vacuum above the liquid nitrogen.
- the nitrogen starts to boil - because of a reduction in atmospheric
pressure - reduction in boiling point
- evaporation increases - heat for evaporation comes from liquid
nitrogen (remember heat of vaporization)
- N2 molecules are not returning to liquid because they are being
removed by vacuum pump - temperature continues to drop
- nitrogen freezes (quickly and easily in demonstration because
temperature difference between liquid and solid N2 is only 26° F)
Same process by which evaporation of sweat cools your skin or condensation
of steam heats (burns) your skin
Does the nitrogen ice float or sink?
Its solid form is more dense than its liquid form - it sinks.
ISNS 4371 - Phenomena of Nature
Water, Wonderful Water
ISNS 4371 - Phenomena of Nature
Water is unusual because it is
less dense as a solid than as a
liquid.
Most materials contract as they
solidify, but water expands.
At temperatures above 4oC,
water behaves like other
liquids, expanding when it
warms and contracting when it
cools.
Ice is about 10% less dense
than water at 4oC.
ISNS 4371 - Phenomena of Nature
Oceans and Lakes Don’t Freeze Solid Because Ice Floats
Ice floats on the cool water below - has important consequences for life.
– If ice sank, eventually all ponds, lakes, and even the ocean would
freeze solid.
– During the summer, only the upper few inches of the ocean would
thaw.
– Instead, the surface layer of ice insulates liquid water below,
preventing it from freezing and allowing life to exist under the frozen
surface.
ISNS 4371 - Phenomena of Nature
Water and the Environment
Water has a high heat capacity - resists changes in temperature
Compare with:
ethyl alcohol - 0.6 cal/gr-deg
iron - 0.1 cal/gr-deg
Can absorb or release relatively large amounts of heat with only a slight
change in its own temperature.
Moderates temperatures on Earth - stabilizes air temperatures by
absorbing heat from warmer air and releasing heat to cooler air.
ISNS 4371 - Phenomena of Nature
The impact of water’s high specific heat ranges from the level of the whole
environment of Earth to that of individual organisms.
– A large body of water can absorb a large amount of heat from the
sun in daytime and during the summer, while warming only a few
degrees.
– At night and during the winter, the warm water will warm cooler air.
– Therefore, ocean temperatures and coastal land areas have more
stable temperatures than inland areas.
– The water that dominates the composition of biological organisms
moderates changes in temperature better than if composed of a
liquid with a lower specific heat.
ISNS 4371 - Phenomena of Nature
Water and Weather
Very important for weather
Land cools and heats up more quickly than water
Causes on shore/off wind circulation patterns
Weather in northern hemisphere more extreme - more land mass - greater
extremes in temperature
Weather on US west coast
less extreme than on east
coast - prevailing winds blow
west to east - blow over
ocean in west and over land
in east - greater extremes in
temperature in east
Water temperature also
colder on west coast
because of ocean currents