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If you’ve ever baked from a pre-packaged cake or cookie mix, you’ve probably noticed the warning that baking times will vary. That’s because the elevation of the area in which you’re doing the baking makes a difference in the baking time required. Living in New Orleans? Then you’re at or below sea level. In Colorado? Then you’re above sea level. Your cake will be in the oven more or less time at the prescribed temp, depending on your location. Last time we learned how the heat energy absorbed by water determines whether it exists in one of the three states of matter, gas, liquid, or solid. We also learned that at the atmospheric pressure present at sea level, which is about 14.7 pounds per square inch (PSI), the boiling point of water is 212°F. At sea level there are 14.7 pounds of air pressure bearing down on every square inch of water surface. Again, I said sea level for a reason. The boiling point of water, just like cake batter baking times, is dependent upon the amount of pressure that’s being exerted on its surface from the surrounding atmosphere. When heat energy is absorbed, it causes the water or cake batter molecules to move around. In fact, the temperature measured is a reflection of this molecular movement. As more heat energy is absorbed, the molecules move more and more rapidly, causing temperature to increase. When the water temperature in our tea kettle reaches its boiling point of 212°F at sea level, the steam molecules in the bubbles that form have enough energy to overcome the atmospheric pressure on the surface of the water. They become airborne and escape in the form of steam.
If we’re up in the Rockies at say an altitude of 7000 feet above sea level, the atmospheric pressure is only about 10.8 PSI. There’s just less air up there. That means there’s less air pressure resting upon the surface of the water, so it’s far easier for steam molecules to form into bubbles and leave the surface. As a result the boiling point is much lower in the Rockies than it is at sea level, 196°F versus 212°F.
So what if the water was boiling in an environment that had even higher pressures exerted upon it than just atmospheric? We’ll see how to put this pent-up energy to good use next week, when we begin our discussion on how steam is used within electric utility power plants.
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Posts Tagged ‘air pressure’
Forms of Heat Energy – Boiling Water and Atmospheric Pressure
Sunday, July 21st, 2013
Forms of Heat Energy – Latent
Monday, July 15th, 2013|
If you took high school chemistry, you learned that water is created when two gases, hydrogen and oxygen are combined. You may have even been lucky enough to have a teacher who was able to perform this magical transformation live during class. Depending primarily on the amount of heat energy absorbed, water exists in one of the three states of matter, gas, liquid, or solid. Its states also depend on surrounding atmospheric pressure, but more about that later. For our discussion, the water will reside at the atmospheric pressure present at sea level, which is around 14.7 pounds per square inch. Last time we learned that the heat energy absorbed by water before it begins to boil inside our example tea kettle is known as sensible heat within the field of thermodynamics. The more sensible heat that’s applied, the more the water temperature rises, but only up to a point. The boiling point of water is 212°F. In fact this is the maximum temperature it will achieve, no matter how much heat energy is applied to it. That’s because once this temperature is reached water begins to change its state of matter so that it becomes steam. At this point the energy absorbed by the water is said to become the latent heat of vaporization, that is, the energy absorbed by the water becomes latent, or masked to the naked eye, because it is working behind the scenes to transform the water into steam. As the water in a tea kettle is transformed into steam, it expands and escapes through the spout, producing that familiar shrill whistle. But what if we prevented the steam from dispersing into the environment and continued to add heat energy? Ironically enough, under these conditions temperature would continue to rise, upwards of 1500°F, if the stove’s burner were powerful enough. This process is known as superheating. Now hold your hats on, because even more ironically, the heat added to this superheated steam is also said to be sensible heat. Confused? Let’s take a look at the graph below to clear things up.
Sensible heat is heat energy that’s added to water, H2O, in its liquid state. It’s also the term used to describe the heat energy added to steam that’s held within a captive environment, such as takes place during superheating. On the other hand, the latent heat of vaporization, that is the heat energy that’s applied to water once it’s reached boiling point, does not lead to a further rise in temperature, as least as measured by a thermometer. Next time we’ll see how surrounding air pressure affects water’s transition from liquid to steam. ___________________________________________
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