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A beaker of water being heated over a flame. The beaker is shown at three different times. In the first, at twenty degrees C, a small bubble sits on the bottom of the beaker. In the second step, the water temperature is fifty degrees C and the bubble is larger, though still sitting on the bottom of the beaker. In the third step, the water temperature is one hundred degrees C. The bubble is larger and is rising toward the surface.
(a) An air bubble in water starts out saturated with water vapor at 20 º C size 12{"20"°C} {} . (b) As the temperature rises, water vapor enters the bubble because its vapor pressure increases. The bubble expands to keep its pressure at 1.00 atm. (c) At 100 º C size 12{"100"°C} {} , water vapor enters the bubble continuously because water’s vapor pressure exceeds its partial pressure in the bubble, which must be less than 1.00 atm. The bubble grows and rises to the surface.

Freeze drying is a process in which substances, such as foods, are dried by placing them in a vacuum chamber and lowering the atmospheric pressure around them. How does the lowered atmospheric pressure speed the drying process, and why does it cause the temperature of the food to drop?

Decreased the atmospheric pressure results in decreased partial pressure of water, hence a lower humidity. So evaporation of water from food, for example, will be enhanced. The molecules of water most likely to break away from the food will be those with the greatest velocities. Those remaining thus have a lower average velocity and a lower temperature. This can (and does) result in the freezing and drying of the food; hence the process is aptly named freeze drying.

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Phet explorations: states of matter

Watch different types of molecules form a solid, liquid, or gas. Add or remove heat and watch the phase change. Change the temperature or volume of a container and see a pressure-temperature diagram respond in real time. Relate the interaction potential to the forces between molecules.

States of Matter: Basics

Section summary

  • Relative humidity is the fraction of water vapor in a gas compared to the saturation value.
  • The saturation vapor density can be determined from the vapor pressure for a given temperature.
  • Percent relative humidity is defined to be
    percent relative humidity = vapor density saturation vapor density × 100 . size 12{ size 11{"percent relative humidity"= { { size 11{"vapor density"}} over { size 11{"saturation vapor density"}} } times "100" "." }} {}
  • The dew point is the temperature at which air reaches 100% relative humidity.

Conceptual questions

Because humidity depends only on water’s vapor pressure and temperature, are the saturation vapor densities listed in [link] valid in an atmosphere of helium at a pressure of 1 . 01 × 10 5 N/m 2 size 12{1 "." "01"´"10" rSup { size 8{5} } " N/m" rSup { size 8{2} } } {} , rather than air? Are those values affected by altitude on Earth?

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Why does a beaker of 40 . 0 º C size 12{"40" "." 0°C} {} water placed in a vacuum chamber start to boil as the chamber is evacuated (air is pumped out of the chamber)? At what pressure does the boiling begin? Would food cook any faster in such a beaker?

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Why does rubbing alcohol evaporate much more rapidly than water at STP (standard temperature and pressure)?

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Problems&Exercises

Dry air is 78.1% nitrogen. What is the partial pressure of nitrogen when the atmospheric pressure is 1 . 01 × 10 5 N/m 2 size 12{1 "." "01"´"10" rSup { size 8{5} } " N/m" rSup { size 8{2} } } {} ?

7 . 89 × 10 4 Pa size 12{ size 11{7 "." "89" times "10" rSup { size 8{4} } " Pa"}} {}

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(a) What is the vapor pressure of water at 20 . 0 º C size 12{"20" "." 0°C} {} ? (b) What percentage of atmospheric pressure does this correspond to? (c) What percent of 20 . 0 º C size 12{"20" "." 0°C} {} air is water vapor if it has 100% relative humidity? (The density of dry air at 20 . 0 º C size 12{"20" "." 0°C} {} is 1 . 20 kg/m 3 size 12{1 "." "20"" kg/m" rSup { size 8{3} } } {} .)

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Pressure cookers increase cooking speed by raising the boiling temperature of water above its value at atmospheric pressure. (a) What pressure is necessary to raise the boiling point to 120 . 0 º C size 12{"120" "." 0°C} {} ? (b) What gauge pressure does this correspond to?

(a) 1 . 99 × 10 5 Pa size 12{ size 11{1 "." "99" times "10" rSup { size 8{5} } " Pa"}} {}

(b) 0.97 atm

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Questions & Answers

the diagram of the digestive system
Assiatu Reply
allimentary cannel
Ogenrwot
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
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discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
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Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed
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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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