How to Calculate Boiling Point

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  • 0:25 Boiling Point
  • 1:20 Boiling Point Calculations
  • 3:45 Changing The Boiling Point
  • 6:45 Lesson Summary
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Lesson Transcript
Instructor: Scott van Tonningen

Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.

In this lesson, we look at how to calculate the boiling point of water based on a variety of factors. Altitude, air pressure, and adding substances can all affect the boiling point. A quiz is provided to check your understanding.

Boiling Point

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:

Soft boiled egg chart

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 vapor pressure of the water is equal to the surrounding air pressure
  • The liquid begins to convert to a gas (steam)
  • The temperature of the water does not increase any more (it levels off)

Image of water boiling

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!

Boiling Point Calculations for Water

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):

Boiling temperature chart

Other Ways to Change Boiling Point

The boiling point may change due to other things, such as:

  • Change in the surrounding atmospheric pressure

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).

Pressure cooker

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:

PSI boiling point chart

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.

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