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Phase Diagram for Water

Using a pressure/temp graph to examine the properties of water

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All Three Phases AT Once!

All Three Phases AT Once!

Credit: Nina Matthews
Source: http://commons.wikimedia.org/wiki/File:Outdoor_pool_Chena_Hot_Springs.jpg
License: CC BY-NC 3.0

I’m sure you have already been told how amazing the substance the water is. Normally this is within the context of how it is a universal solvent. Another amazing property of water is that under easily accessed temperatures and pressures all three phases of matter (solid, liquid and gas) can be achieved simultaneously. At a hot spring, during the winter, you can observe solid water (snow or ice), liquid water (the water in the pool) and gaseous water (the steam above the surface of the liquid water – steam is invisible so don’t confuse it with liquid water vapor above the pool). 

For what other substance can you access all three phases of matter at the same time?

Amazing But True!

  • The phases of any substance are influenced by the ambient pressure and temperature. What does this complicated sentence mean? It means that for a substance to go from a solid to a liquid to a gas the temperature and the pressure are important. Taking this further, it means that the boiling point of water is not always 100°C. In fact, with the proper apparatus you can boil water at room temperature! Interestingly, in the same apparatus you can remove enough energy from a liquid sample of water to even freeze the water at room temperature. Pressure matters as much as temperature!
  • Credit: Steven Depolo
    Source: http://www.flickr.com/photos/stevendepolo/3762686904/
    License: CC BY-NC 3.0

    When ice melts at room temperature you only see the liquid and solid phases of water. Where in the phase diagram does this happen? [Figure2]

  • Seeing is believing: 


Explore More

With the links below, learn more about phases and phase diagrams. Play with the phase change simulation (Phase Changes tab) and make sure to vary the temperature and the pressure. Then answer the following questions.

  1. How can you boil (convert a liquid to a gas) water without increasing the temperature?
  2. Water has an unusual phase diagram. If you look at the border between the solid and the liquid phase you will notice that the line has a negative slope (goes from top-left to bottom-right). This has profound implications. What is one of the counter-intuitive predictions you could make about applying extreme pressure to ice (solid water)? Why is this a counter-intuitive or unexpected prediction? (Note: This is a pretty deep question. Think about it first).

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Image Attributions

  1. [1]^ Credit: Nina Matthews; Source: http://commons.wikimedia.org/wiki/File:Outdoor_pool_Chena_Hot_Springs.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Steven Depolo; Source: http://www.flickr.com/photos/stevendepolo/3762686904/; License: CC BY-NC 3.0

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