Module 5

Explain:  Lets talk a bit about the process of heating water, for it is not as simple as it seems.  First some trivia:

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The calorie: This is an outdated unit of measure of energy based on the properties of liquid water.  It has been around since the early 1800s.  As we learned in this module, 1cal is defined as the amount of energy required to heat one gram of water one Celsius degree.  Please note that this is not the same as a food calorie.  Food calories are Kcal, or “big Calories,” the equivalent of 1000 little calories.  Sometimes a capital C is used to denote Kcal vs. cal.

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The Celsius: temperature scale dates to the 1700s.  Many of us learned that the temperature scale was designed around the properties of water.  Namely, water freezes at 0 degrees Celsius and it boils at 100 degrees Celsius.  (Basically Mr. Celsius marked the spot on his thermometer where water boiled and where it froze and then divided the distance between into 100 equal spaces, called degrees.)  There is a problem though.  See the next item of trivia, below:

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Water boils at different temperatures depending the atmospheric pressure.  If you boil water on a cloudy, wet, and warm (low pressure) day, the water will boil at a lower temperature than it will if you do the same on a cold, dry, clear, (high pressure) day.  Similarly, if you go to Colorado, Tibet, or Denali, you will measure boiling water and find that it is significantly less than 100 degrees Celsius!  Why?

The Boiling Point: of a liquid occurs when the Vapor Pressure of the liquid equals and exceeds that of the surrounding atmospheric pressure.  As you may hypothesize, the vapor pressure of a liquid increases with temperature.  Here is a graph of the vapor pressure of water in nature.  Note that 760Torr is standard pressure, corresponding roughly to the average pressure at sea-level.  If you're at about 15,000 feet elevation, then the atmospheric pressure would be about 400Torr, and water would boil between 80C and 85C. Click the graph to link to the mathematics behind these calculations on its source page:

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It takes more energy to make boiling water evaporate to steam than it does to heat the same liquid water from 0 degrees Celsius to 100 degrees Celsius!  How can this be?  The water is not getting any hotter; it stays at 100 degrees Celsius!  The reason is that changing a liquid to a gas makes the molecules much more energetic.  This means that they have to absorb more energy just to be a gas!  A simplified explanation is that say that the temperature of the water doesn't rise because as each molecule absorbs enough energy to exceed 100C, it pops out of the liquid to become gas (steam).  The inverse effect happens when changing ice to liquid and heat is released into the environment during condensation and freezing.  This extra energy required to change between the states of matter is called Latent Heat.  The following plot shows the heat that must be absorbed by a fixed amount of water to:

(A) heat ice from -50C to 0C

(B) melt the ice to liquid water, (ice and water are equally at 0C)

(C) heat liquid water from 0C to 100C

(D) vaporize the liquid to steam, the boiling liquid and steam remain at 100C

(E) begin heating the steam

The plot links to its source page.

It actually takes 540 cal/gram of water to change it from liquid form to gas form.  This is a Latent Heat of Vaporization.  Click here for a brief explanation about Latent heat, specifically in the case of water.

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Extend:  I enjoyed the Youtube visualizations of thermohaline circulation.  I will use some of these videos in my physics class when we discuss global temperature change.  One theory that alarmed me briefly when I first heard about it is that of Rapid Global Climate Change.  The premise is that the Earth’s thermohaline circulation gets shut down by melting glaciers, which drastically increases (100 times!), the amount of fresh water in the oceans.  The movie The Day After Tomorrow came out in 2004 and was loosely based on the book The Coming Global Superstorm.  The union of concerned scientists has created a short page that debunks the myths of “rapid” global climate change, though I am sure that another Krakatau, nuclear war, or asteroid could cause some rapid consequences due to particulate matter blotting out the sun in the atmosphere.  (Basically all three result in the reflecting of sunlight, or increasing the Earth’s albedo, but we’ll save that stuff for the upcoming modules.)

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There is a Tlingit story about a father and son who left on a fishing trip and did not return for a whole year. When they did, they told stories of a hot land with trees like ferns, thought to be palm trees. I don't know the story well enough to tell it, which is probably good since it isn't mine to tell!

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Evaluate:  Again, I find this module to be a well organized collection of many interrelated ideas into one place.  There is a lot of room for expansion, and I look forward to future modules.  I ended up playing around with Google Earth and did find some examples of waters mixing, but none as striking as the one in Lynn Canal!  It is amazing how much glacial silt a river can carry.  I rafted the Tateshini and Alsek rivers this summer.  The rivers were dark with silt, but we passed many other streams of various colors and clarity.  Sometimes we would find clear water behind gravel bars that acted as a natural filter.

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Three Colleagues:

Kathy records her personal reaction to this module.  She has some fascinating links and a great idea for a lab using a light source and dirt in pans.  I intend to steal it for my classes.

Alicia writes about how new the culture and food of Alaska (fish) is to her.  She mentioned some difficulty incorporating the science into her math classes.  I offered a link to the math behind the calculations of the boiling point of water on my blog.

Tyler describes a study that he and his father have participated in for years.  I have often admired his glass balls, artifacts, and bath toys that he has collected from the beaches around Southeast.