Why does it take so much longer to boil an egg in Denver than in San Francisco?

3 responses so far ↓

• 1 cpnpicard1 // Nov 9, 2008

  The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure. For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100°C. Since the vapor pressure increases with temperature, it follows that for pressure greater than 760 mmHg (e.g., in a pressure cooker), the boiling point is above 100°C and for pressure less than 760 mmHg (e.g., at altitudes above sea level), the boiling point will be lower than 100°C. As long as a vessel of water is boiling at 760 mmHg, it will remain at 100°C until the phase change is complete. Rapidly boiling water is not at a higher temperature than slowly boiling water. The stability of the boiling point makes it a convenient calibration temperature for temperature scales.

• 2 Andrew // Nov 9, 2008

  I’d imagine it’s because of the altitude in Denver as opposed to San Francisco

  higher altitudes = less boiling temperature

• 3 Dr. Buzz // Nov 9, 2008

  Maddy,

  I found a reference (see below) that is pretty thorough. The short story is that Denver at about one mile altitude has a boiling point about 5 degrees Celcius (9 degrees Fahrenheit) lower than at sea level (San Francisco).

  If reactions double in rate for every 10 degress Celcius, cooking an egg would take about 50% longer in Denver than in San Francisco.

  “At standard atmospheric pressure (1 atmosphere = 0.101325 MPa), water boils at approximately 100 degrees Celsius. That is simply another way of saying that the vapor pressure of water at that temperature is 1 atmosphere….. When the surrounding pressure is lower (such as at high altitudes), the vapor pressure reaches that pressure at a lower temperature. For example, in the Denver, Colorado area of the U.S. where the elevation above sea level is approximately one mile (1600 meters), the atmospheric pressure is about 83% of a standard atmosphere, and water boils at approximately 95 degrees Celsius.”

  Hope that helped!