Back To CourseCollege Chemistry: Tutoring Solution
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Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.
Have you ever heard of the three-minute egg timer? It was developed long ago to tell people how long to keep an egg in boiling water for a soft boil. It works great at sea level with standard air pressure, but the results can vary widely at other altitudes and/or pressures. Consider this chart, based on different altitudes, for soft-boiling a medium egg that starts off at room temperature:
Why would there be so much variation if the egg is always dropped in boiling water? The answer lies in when boiling point occurs. When water boils, the water temperature has reached a critical point where:
The biggest variable in all of this is the surrounding air pressure. As altitude increases, air pressure decreases because there is simply less atmosphere above to cause the pressure. It's just like feeling a lot of water pressure at the bottom of the swimming pool and then feeling much less pressure as you rise to the top. When air pressure decreases, water boils at a lower temperature (it happens sooner). Once the water boils, the lower boiling temperature levels off, so it takes longer to boil the egg!
Much has been published about this subject and there are a lot of complicated formulas to consider. Fortunately for us, there are two very simple equations that get you close enough to the boiling point. The first equation gets you in the ballpark (within a degree or two) based on altitude and the second equation is for you chefs that really want to fine-tune the value based on non-standard atmospheric conditions:
Equation 1 gives you the boiling point in degrees Fahrenheit (F) and is very easy to use. Substitute your local elevation in feet, and it gives you the local boiling point for a so-called standard day (that is, a day with standard air pressure). For example, at the top of Mt. Washington, New Hampshire, we have an elevation of 6289 feet (') above sea level. Let h = 6289 in Equation 1, and you get water boiling at T = 200.4 degrees F. In Leadville, Colorado, (h = 10,152') you obtain T = 193.3 degrees F. At both of these locations, the water will begin to boil sooner than at sea level, and it will continue to boil at a lower temperature.
Equation 2 gives you the additional correction (delta T) to apply to the result of Equation 1 based on non-standard local air pressure differences. Standard pressure is 29.92, and the record low for the U.S. (lower 48 states) is somewhere in the neighborhood of 28.1. Let's say that a massive low pressure system has settled over Mt. Washington and the local air pressure drops to a whopping 28.5 inches of mercury (Hg). If you let P = 28.5 in Equation 2, you will get a temperature correction of -2.4 degrees F. So you subtract 2.4 from your original value of 200.4 to get a boiling point of 198 degrees F.
Hopefully, you can see that in most cases Equation 1 will be sufficient. It is rare that local air pressure extremes change the boiling point by much more than one degree. But, if you really want to be precise, you can usually get the local air pressure in inches of mercury from your TV station or a reputable weather website.
Some people like using a table to find boiling point; this one is based on Equation 1 (altitude is in feet above sea level):
The boiling point may change due to other things, such as:
You may have a pressure cooker at home or remember your family using one when you grew up. The idea is to seal off the air above the boiling water and allow it to increase in pressure as the water temperature increases. A vent is provided to control the air pressure inside and keep it from increasing indefinitely (which could be dangerous).
Good pressure cookers allow you to approximately double the value of outside air pressure on the inside of the cooker. For this calculation, it is better to use PSI (pounds per square inch). Standard air pressure at sea level is about 14.7 PSI. This is a chart that allows you to find the boiling point of water based on various PSI values inside the cooker:
From the chart, you should be able to see that if you double the standard air pressure (2 x 14.7 PSI) you can get a boiling point of nearly 250 degrees F. This means that food will cook faster.
All of our calculations so far have been for pure water. But every solution has its own distinct boiling point. For example, R410-A, the chemical that is replacing chlorofluorocarbons (CFCs) as the refrigerant in air conditioning systems, has a boiling point of about -61 degrees F. Hydrogen peroxide, on the other hand, has a boiling point of 302 degrees F.
There is a popular myth that you can change the boiling point of water significantly by dissolving salt or sugar in it. The truth is it only raises it slightly, not enough to make a difference. But it is worth showing what the calculation looks like. This is a simple equation to show how the boiling point of water can change based on the concentration of another substance dissolved in the water:
In this equation, the increase in boiling point is in degrees F and the parameter m represents the concentration (called the molality) of the substance (e.g., salt or sugar) in the water. Let's stick with salt as the substance for now. Here is another handy equation you can use to determine m based on the number of tablespoons of salt (TS) and the number of quarts (Q) of water in which it is dissolved:
Let's do one quick example using these equations. Suppose you want to bring 2 quarts of water to a boil. Before heating it, you dissolve 10 tablespoons of salt in the water. From the second equation, m = (0.308) x (10) / 2 = 1.54. Substituting m = 1.54 into the first equation, you get a change in boiling point of (0.922) x (1.54) = 1.42 degrees. You add this value to whatever boiling point of water you had already calculated. Not much change!
The boiling point of water can change significantly based on altitude and the difference between local air pressure and the standard day. This can especially affect cooking times because the water boils at a lower temperature when air pressure decreases, and this lower temperature levels off as the water continues to boil. Lower air pressure always means lower boiling point.
Calculating the boiling point of water is easy with the equations or table provided. Using the altitude equation gets you within a degree or two of the correct boiling point. Using the equation based on the deviation of your location from standard air pressure can fine-tune the value.
The boiling point of water can be increased significantly using a pressure cooker. The table shows the boiling point based on air pressure in pounds per square inch inside the cooker. The boiling point of water can also be increased slightly by dissolving a substance, such as salt or sugar, into the water. The equations provided show how to calculate the increase in boiling point temperature based on adding a fixed quantity of salt.
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Back To CourseCollege Chemistry: Tutoring Solution
13 chapters | 138 lessons