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Vapor Pressure Lowering

Introduction to colligative properties

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Putting on the Pressure

Putting on the Pressure

Credit: Petr Kratochvil
Source: https://commons.wikimedia.org/wiki/File:LA2-gutbrod-pressure-cooker-1864.png
License: CC BY-NC 3.0

Imagine eating fish, bones and all. Or pigeon the same way (it’s hard to imagine eating pigeon in any way). These were part of the menu for a dinner served to King Charles II and several members of the Royal Society in 1682.  The occasion: a demonstration by Denis Papin of his new cooking device. It was described as “A New Digester or Engine, for softaing bones, the description of its makes and use in cookery, voayages at see, confectionary, making of drinks, chemistry, and dying, etc.

Why It Matters

  • The first pressure cookers were dangerous devices. Based on the idea that trapped steam would raise the cooking temperature of the food, these cookers allowed for quicker preparation of meals. However, there was no way to regulate the pressure or temperature. As a result, the early models experienced a number of explosions. In addition, the pots were made out of sections of metal, so the high pressure would cause cracks along the seams, also leading to explosions.
  • Credit: Julie Magro
    Source: http://www.flickr.com/photos/magro-family/4323640485/
    License: CC BY-NC 3.0

    Modern pressure cookers have specialized lids that allow you to regulate the internal pressure to avoid accidents [Figure2]

  • A pressure cooker operates on the principle that an increase in the pressure on the surface of a liquid will increase the boiling point of the liquid. More pressure on the surface decreases the vapor pressure because more liquid molecules will stay in the liquid phase and not escape to the vapor phase. As a result, a higher temperature is needed to boil the liquid.
  • Steam is a more effective heat transfer system than air, so the material under pressure will cook faster. In addition, no liquid is lost using a pressure cooker, so food does not dry out. Essential nutrients will not be lost as fast because the cooking time is shorter. The higher pressure and closed system forces steam through the food, allowing more complete cooking.
  • Watch a video at the link below to learn about pressure cookers:


Show What You Know

Use the links below to learn more about vapor pressure. Then answer the following questions.

  1. Why are the vapor pressures of ethyl ether and ethanol so different?
  2. Explore vapor pressure relationships at the iastate.edu web site above.
  3. Why does water boil at a lower temperature in Denver, Colorado than it does at sea level?
  4. Why do firefighters dike the area around a leaking tank?

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Image Attributions

  1. [1]^ Credit: Petr Kratochvil; Source: https://commons.wikimedia.org/wiki/File:LA2-gutbrod-pressure-cooker-1864.png; License: CC BY-NC 3.0
  2. [2]^ Credit: Julie Magro; Source: http://www.flickr.com/photos/magro-family/4323640485/; License: CC BY-NC 3.0

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