### LBC Unit3 Section3

```LIVING BY
CHEMISTRY
Unit 3: WEATHER
Phase Changes and Behavior of Gases
In this unit you will learn:
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thermometers work
the effects of changing temperature, pressure,
and volume on matter
how to read weather maps and make weather
predictions
Section III: Concentrating Matter
•Lesson 15 n is for Number
•Lesson 16 STP
•Lesson 17 Take a Breath
•Lesson 18 Feeling Humid
•Lesson 19 Hurricane!
•Lesson 20 Stormy Weather
Lesson 15: n is for Number
•Pressure and Number Density
ChemCatalyst
•Look at the diagram of the
Earth’s atmosphere.
•Compare the atmosphere at
sea level and at 34,000 ft, the
altitude at which airplanes fly.
1. Describe at least three
differences.
2. Explain why it is difficult
to breathe at 34,000 ft.
Key Question
•How is the number of gas molecules in a sample related to
pressure?
You will be able to:
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define the number density of a gas
describe the number density of the atmosphere as it
relates to altitude
explain the relationship between number density and
gas pressure
describe one way to measure gas pressure
Prepare for the Lab
•Work in groups of four.
•Wear safety goggles at all times during the lab.
Discussion Notes
•The gas pressure increases as the number of
gas molecules per unit of volume increases.
n
•
P = k(V)
•Number density: The number of gas particles
per unit volume. Number density = n/V
•The height of the water levels in a U-tube
indicates differences in pressure.
Discussion Notes (cont.)
•In order to make the water levels uneven, it is
necessary to seal off one end with a finger, a
syringe, a cork, or a balloon.
Discussion Notes (cont.)
•Air pressure can be determined by measuring
the difference in height of a liquid.
Wrap Up
How is the number of gas molecules in a sample
related to pressure?
• The composition of Earth’s atmosphere is not
uniform. The density of the gas molecules in the
air decreases with increasing altitude. This
causes the pressure of the atmosphere to
decrease with increasing altitude.
• The number density of a gas is the number of
gas molecules per unit of volume, n/V.
• The pressure of a gas is directly proportional to
the number of gas molecules per unit of volume.
This relationship is written as P = k·(n/V).
Check-in
•A balloon is filled with helium, tied off, and then
released. As it climbs into the air, its volume
slowly increases.
•Explain what is going on with the helium atoms
inside the balloon and the air molecules outside
the balloon in terms of number density and
pressure.
Lesson 16: STP
ChemCatalyst
•There are two balloons. One is filled with helium,
He, and the other with carbon dioxide, CO2.
1. Describe what happens when the balloons are
released.
2. For the two balloons, state whether these
properties are the same or different, and
explain why:
• pressure, P
• number density, n/V
• temperature, T
• number, n
• volume, V
• mass, m
• density, m/V
Key Question
•How do chemists keep track of the number of gas
particles?
You will be able to:
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define a mole
define standard temperature and pressure
Prepare for the Activity
•Work individually.
•Chemists use a unit called a mole to describe the number
of gas particles in a sample.
•1 mole = 602,000,000,000,000,000,000,000
Discussion Notes
•Standard temperature and pressure, STP:
One atmosphere of pressure and a
temperature of 273 Kelvin.
•Equal volumes of gases contain equal
numbers of gas particles if the temperature
and pressure are the same.
Discussion Notes (cont.)
•There are exactly
602,000,000,000,000,000,000,000 particles in
22.4 L at STP.
•Mole: A unit invented by chemists to count large
numbers of gas particles. There are
602,000,000,000,000,000,000,000 particles in
1 mole. This is 602 sextillion.
Wrap Up
How do chemists keep track of the number of gas
particles?
• Avogadro’s law states that equal volumes of
gases contain the same number of particles if
they are at the same temperature and pressure.
This holds true for all gases.
• Gases are often compared at a standard
temperature and pressure of 1 atm and 273 K.
This is also referred to as STP.
• At STP, any gas will occupy 22.4 L and consist
of exactly 602 sextillion, or
602,000,000,000,000,000,000,000, particles.
Check-in
•One balloon contains 22.4 L of Ar, argon gas,
and another balloon contains 22.4 L of Ne, neon
gas. Both balloons are at 273 K and 1 atm.
1. Do the balloons contain the same number of
atoms? Why or why not?
2. Will the balloons have the same mass? Why
or why not?
Lesson 17: Take a Breath
•Ideal Gas Law
ChemCatalyst
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2.
3.
Describe how you can determine the volume
of a breath of air.
Name four factors that might affect the
volume you measure.
What do you need to know in order to
determine the number of molecules in a
breath of air?
Key Question
•How can you calculate the number of moles of a gas if you
know P, V, and T?
You will be able to:
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define the ideal gas law
define the universal gas constant, R
complete calculations for finding n, using the ideal gas
law
Prepare for the Lab
•Work in groups of four.
•Wear safety goggles.
Prepare for the Lab (cont.)
•The equation for the ideal gas law is
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PV = nRT
•where R is equivalent to the proportionality constant, k, for
this equation.
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R=
R = 0.082 L · atm/mol · K
PV
nT
Discussion Notes
•We should see differences in the volume of one
breath of air from group to group.
•There is more than one way to figure out the
volume of air that was exhaled into the bottle.
Discussion Notes (cont.)
•The ideal gas law allows scientists to relate gas
pressure, volume, moles of particles, and
temperature.
•Ideal Gas Law: The ideal gas law states that
PV = nRT, where R, the universal gas constant,
is equivalent to the proportionality constant, k,
for this equation.
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R = PV
R = 0.082 L · atm/mol · K
nT
Discussion Notes (cont.)
•Note that R is the same for all gases but the value
of R does change depending on if the units
change.
•The number of moles can be converted to the total
number of gas molecules by multiplying by 602
sextillion.
•The ideal gas law can be used to solve for other
variables besides n.
Wrap Up
•How can you calculate the number of moles of a
gas if you know P, V, and T?
• The ideal gas law relates volume, pressure,
temperature, and the number of moles of a
gas sample: PV = nRT, where
R = 0.082 L · atm/mol · K.
• R is a number that relates all the different
units to one another in the ideal gas law. Its
value does not change if the units don’t
change. R is called the universal gas constant.
Wrap Up (cont.)
• The ideal gas law can be used to figure out P,
V, n, or T when the other three variables are
known.
Check-in
•You cap a 1.0 L plastic bottle on a mountaintop
where the air pressure is 0.50 atm and the
temperature is 298 K.
1. How many moles of gas are in the bottle?
2. What is the number density, n/V, of the gas
inside the bottle on the mountaintop?
3. At sea level, the volume of the bottle
becomes 0.50 L. What is the number density
of the gas inside the bottle at sea level?
Lesson 18: Feeling Humid
•Humidity, Condensation
ChemCatalyst
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2.
Is there water vapor in the air right now?
What evidence do you have to support your
What do you think humidity means? How
does humidity depend on temperature?
Key Question
•What is humidity and how is it measured?
You will be able to:
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define humidity and relative humidity
explain the relationship between humidity and
phenomena such as cloud formation, fog, rainfall, and
dew
explain the relationship between water vapor density
and air temperature
Prepare for the Lab
•Work in groups of four.
•Humidity: The density of the water vapor in the
air at any given time. Humidity is dependent on air
temperature and pressure.
Discussion Notes
•Humidity is a measure of the amount of water
vapor in the air.
•The condensation procedure provides evidence
that water vapor is present in the air.
•The temperature at which water vapor condenses
indicates how much water vapor is in the air.
Discussion Notes (cont.)
•Water Vapor Density Versus Temperature
Discussion Notes (cont.)
•There is an upper limit to the amount of water
vapor that can be present in the atmosphere at a
given temperature.
•Humidity is sometimes expressed as relative
humidity.
•Relative humidity: The amount of water vapor in
the air compared to the maximum amount of water
vapor possible for a specific temperature,
expressed as a percent.
Wrap Up
•What is humidity and how is it measured?
• Humidity is a measure of the amount of water
vapor in the air. It can be expressed as water
vapor density or as relative humidity.
• Water vapor density is affected by both air
temperature and air pressure.
• There is a limit to the amount of water vapor that
can be present in the air at a given temperature.
The maximum amount possible is called 100%
relative humidity.
Check-in
•On a hot summer day, a firefighter records a
dry-bulb temperature of 30°C and a wet-bulb
temperature of 12 °C. What does this tell you
Lesson 19: Hurricane!
•Extreme Physical Change
ChemCatalyst
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2.
What is a hurricane? What characteristics does it have?
Where do hurricanes form? What can you tell about a
hurricane from the satellite image?
Key Question
•What are hurricanes and what causes them?
You will be able to:
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describe the meteorological conditions that result in a
hurricane
explain the role of phase change, air pressure, and
temperature in hurricane formation
define climate and global warming
Prepare for the Activity
•Work individually.
Discussion Notes
•Hurricanes are destructive storms characterized
by strong winds and large amounts of rainfall.
•Tropical depressions can build to tropical storms,
which can build to hurricanes.
•There are five categories of hurricane, with
category 1 the least intense and category 5 the
most intense.
Discussion Notes (cont.)
•Anatomy of
a Hurricane
Discussion Notes (cont.)
•Tropical storms begin forming when a great deal
of warm water evaporates into the atmosphere.
•As the storm moves over areas of warmer water,
evaporation increases.
•Water vapor density is related to temperature.
Discussion Notes (cont.)
•Water Vapor Density Versus Temperature
Discussion Notes (cont.)
•Global Temperature Changes (1880-2000)
Discussion Notes (cont.)
•There is a great deal of scientific evidence
supporting the idea that our planet is in a warming
cycle, often referred to as global warming.
•Global warming is causing climate change.
Wrap Up
•What are hurricanes and what causes them?
• Hurricanes are intense tropical weather systems
accompanied by strong winds, massive amounts
of rain, and ocean flooding. They form around
low-pressure systems and are characterized by
spiraling clouds and winds.
• Hurricanes form over the ocean in places where
there is extremely warm, moist air.
Wrap Up (cont.)
• Rapid evaporation and subsequent
condensation of moisture set up an air pressure
differential that further feeds the evaporation and
condensation.
• At higher temperatures, small changes in
temperature have dramatic effects on water
vapor density, n/V. Global warming may increase
the frequency and intensity of hurricanes on the
planet.
Check-in
•Why do most hurricanes have their origins near
the equator?
Lesson 20: Stormy Weather
•Unit Review
ChemCatalyst
1.
2.
Describe three different physical changes
involved in weather.
What physical changes of matter affect the
weather?
Key Question
•What does chemistry have to do with weather?
Prepare for the Activity
•Work in pairs.
Discussion Notes
•Phase changes are a form of physical change.
•It is necessary to understand the behavior of
gases of you want to understand the weather.
•The temperature of a gas must be converted to
the Kelvin scale in order to utilize the gas laws.
Discussion Notes (cont.)
•Gas Laws
V
T
•Charles Law k =
Gay-Lussac’s law k = P
T
•Boyle’s Law k = PV
Ideal gas law PV = nRT
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or R = PV
PV
nT
•Combined gas law k =
T