What is Gas Pressure? What are the factors influencing

Pressure?

Gas Pressure

Volume

Temperature

How do we measure each?

K-W-L :What is gas pressure?• From the KMT, we know that gas particles are always in motion

colliding with each other and other objects.

• These collisions exert a small individual force but collectively exert a large force, termed gas pressure.

THINK! What is happening to the particles inside

this balloon?

Gas particles are bouncing around, hitting each other, hitting the edges of the balloon, and causing the

balloon to inflate.

QUESTION 1 :• If air is made up of a mixture of gases, does air exert a

pressure on Earth?• YES! Air molecules exert a pressure because the force of

gravity is holding air particles in the Earth’s atmosphere.

Atmospheric Pressure - results from collisions of atoms and moleculesin air with objects

QUESTION 2 :• Does atmospheric pressure change as we increase altitude? Or does it always stay the same?

Atm pressure decreases as you increase inaltitude because the density of the air also decreases. Not as many particles colliding way up there!

What about a vacuum of space? How does that work?

• Vacuum- a volume of space that is empty of matter, including air, so that gas pressure is much less than atmospheric pressure

No Air » No Particles » No Collisions » No Pressure

How do we measure atmospheric pressure?• Barometer- device used to measure atm pressure, created in 1643

by Italian scientist Evangelista Torricelli

How it works: Mercury filled tube is inverted into a dish causing some of the mercury to exit the tube into the dish. The amount of mercury exiting the tube or left in the tube is dependent upon the air pressure outside the tube. Therefore, air pressure can be measured using a barometer. At std atm = 760 mm Hg

BUT WAIT, THERE’S MORE!

What about a manometer?• Manometer- used to

measure the pressure of gas in a container

How it works: the measured gas pressure is applied to one side of the tube while the reference pressure (atm pressure) is applied to the other

Left = Gas Pressure less than Atm pressure

Right = Gas Pressure greater than Atm pressure

What units do we use to measure pressure?LOTS!

1 standard atmosphere = 760 mm Hg

760 torr101.325 kPa

101,325 Pa29.92 inches

14.7 pounds/in2 (psi)

Also helpful when measuring….. 1 torr = 1 mm Hg

Let’s Practice converting between units!1. What is 475 mm Hg expressed in atm?

475 mm Hg x 1 atm = 0.625 atm760 mm Hg

2. The pressure of a tire is measured as 29.4 psi. What is the pressure in mm Hg?

29.4 psi x 760 mm Hg = 1.52 x 103 mm Hg 14.7 psi

3. What is 2 atm expressed in torr?

2 atm x 760 torr = 1520 torr 1 atm

Now, we know about gas pressure and how to

measure it, what other factors influence pressure? • Temperature • Volume

Let’s start with Temperature!

Temperature• Temperature- the measure of the average kinetic

energy of the particles in a sample of matter... NOT HEAT! As Temp

Measured using various scales including:1. Celsius (°C) -- freezing point of water (O°C), boiling point of water (100 °C)2. Fahrenheit (°F) -- freezing point of water (32 °F), boiling point of water (212 °F)3. Kelvin (K) -- WHAT IS THIS!

, energy & collisions

Comparisons of Temperature Scales

KELVIN (K):• Known as absolute temperature scale--

as the zero point of the scale is considered the lowest possible temperature of anything in the universe!

• At freezing point of water = 273 K, the boiling point of water = 373 K

• K = °C+ 273 **NOTICE- no degree sign**

• °C = 5/9 (°F -32)

• °F = (9/5 °C) +32

Lord Kelvin

What is volume? How do we measure different types of matter?

• Volume = amount of space occupied by matter,

• As Volume

mL cm3 gallon, quarts, liter

• SI Unit of measurement for volume = m3

Gases typically measured in terms of liters or m3!

, particle collisions

SUMMARY• Gas Pressure results from gas particle collisions with objects

and other particles exerting a force upon contact.

• Atmospheric Pressure results from air particles colliding with each other and objects and is ALWAYS affecting us even though we don’t always notice it!

• Use manometer and barometer to measure gas and atm pressure, respectively in atm, torr, mm Hg, kPa, Pa, inches, bar.

• Temperature measured in Kelvin effects gas particle collisions.

• Volume measured in liters, m3, mL (depending on the substance) also affects gas particle collisions.

Crush the Can Demo• Let’s examine the effect of atmospheric

pressure.

BEFORE we get started:

1. What is inside the empty pop can ? 2. What is happening to the water in the can when it is heated? 3. What will happen to the can when I put it upside down in the cold water?

AFTER we crush the can: 4. Why did the can collapse (using KMT)? 5. How could I make the can expand?

DON’T

Answer the questions on your handout sheet to turn in at the end of class!

Check out your concept map to visualize how it all relates!