Air Pressure, Air Pressure, Vapor Pressure, Vapor Pressure,

& Boiling& Boiling

What is Air Pressure?What is Air Pressure?

• It is the Force of all the air molecules around an object pushing on that object.

• More air molecules equals more air pressure

What is Vapor Pressure?What is Vapor Pressure?

• It is the Force of the escaping Liquid molecules during vaporization.

• More Vaporization equals more vapor pressure.– During Evaporation - Low VP– During Boiling - High VP

Measuring Air PressureMeasuring Air Pressure• Air pressure is measured by

using a Barometer.

• It is measured in mm of Hg (Hg = Mercury)

10 mm of Hg

In order to make any In order to make any liquid boil...liquid boil...

•The Air pressure must equal the Vapor pressure.

How to Change APHow to Change AP

• AP Decreases as altitude Increases.

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713

785

Sea LevelPittsburghDenverDeath Valley

How to Change VPHow to Change VP• VP Increases with an Increase in

temperature. (The molecules are moving faster and a have a better chance of escaping)

What are the 2 ways to make a What are the 2 ways to make a liquid boil?liquid boil?

• Lower the ____________ to the _______________

• Raise the ______________ to the _______________

Vapor Pressure

Vapor Pressure

Air Pressure

Air Pressure

OR

What happens to VP with What happens to VP with a Decrease in altitude?a Decrease in altitude?

• Nothing happens to VP when you change Altitude. Remember that only the temperature affects the VP

What happens to AP with What happens to AP with an increase in altitude?an increase in altitude?

• The AP will decrease as altitude increases.

Is the water Boiling?Is the water Boiling?

• Water Temperature = 99.0 0C

• Atmospheric Pressure (AP) = 733 mm of Hg

• Yes the water would be boiling.

• At 990C the VP = 733 mm of Hg

• The VP does equals the AP

What’s the VP?What’s the VP?

• Water is boiling at a temperature of 93.5 oC

• About 600 mm of Hg (Using the graph)

What’s the AP?What’s the AP?

• Water is boiling at 88.5oC

• About 500 mm of Hg

What’s the VP & APWhat’s the VP & AP

• Water is boiling at 96.5 oC

• The VP is About 670 mm of Hg

• The AP is the same as VP since the water is boiling: About 670 mm of Hg

What’s the Altitude?What’s the Altitude?

• The boiling point of water Decreases 1 oC for every 1000 ft Increase in altitude above sea level.

• Water is Boiling at 96.5 oC

How to Solve Altitude problems

• Step 1:Step 1: Normal Boiling point – Actual Boiling Point

• Step 2:Step 2: Multiply with the Conversion Factor

• Step 3:Step 3: Add the answer from #2 to Sea Level Altitude (0 ft)

100 C – 96.5 C = 3.5 C

3.5 C

X1000 ft1 C

= 3500 ft

3500 ft + 0 ft = 3500 ft

How to Solve Altitude Problems –Part 2

•Do the steps in reverse:

•Step 1: Subtract your Current Altitude from Normal Altitude (Sea Level or 0 ft)

•Step 2: Multiply by the Conversion Factor:

•Step 3: Subtract the answer from #2 from the Normal Boiling Point:

Question #9:

• Water is boiling at 6000 Ft above sea level. What is the temperature of the water?

How to Solve Altitude problems

• Step 1:Step 1: Actual Altitude - Normal Altitude

• Step 2:Step 2: Multiply with the Conversion Factor

• Step 3:Step 3: Normal Boiling point – Change in Temp.

6000 ft – 0 ft = 6000 ft

6000 ft

X1000 ft

1 C = 6 C

100 C – 6 C = 94 C

Question #10• The VP of boiling water is 720 mm of HgThe VP of boiling water is 720 mm of Hg

• What is the Temperature of the water?What is the Temperature of the water?

– Using your graph it is close to 98.5 C

• What is the AP?What is the AP?

– Since the water is boiling the AP must = the VP of 720 mm of Hg

• At What Altitude?At What Altitude?

– Using the temperature of 98.5 the Altitude is found to be 1,500 ft