Another important concept is shown in the illustration to the right: absolute zero. At this temperature, molecules (in essence) cease moving.
Reaching this temperature is not theoretically possible, but temperatures quite close to this are being achieved. Absolute zero is 0 K, or
í273.15°C.
To standardize temperatures, scientists have agreed on a common reference point called the triple point. The triple point is the sole
combination of pressure and temperature at which solid water (ice), liquid water, and gaseous water (water vapor) can coexist. It equals
273.16 K at a pressure of 611.73 Pa. The triple point is used to define the kelvin as an SI unit. One kelvin equals 1/273.16 of the difference
between absolute zero and the triple point.
If you are a sharp-eyed reader, you may have noticed the references to both 273.16 and 273.15 in this section. The freezing point of water is
typically stated as 273.15 K (0°C) because this is its value at standard atmospheric pressure, but at the triple point pressure, water freezes at
273.16 K (0.01°C).18.3 - Temperature scale conversions
Since the Celsius and Kelvin scales have the same number of units between the
freezing and boiling points of water, it takes just one step to convert between the two
systems, as you see in the first conversion formula in Equation 1. To convert from
degrees Celsius to kelvins, add 273.15. To convert from kelvins to degrees Celsius,
subtract 273.15.Since water freezes at 32° and boils at 212° in the Fahrenheit system, there are 180
degrees Fahrenheit between these points, compared to the 100 units in the Celsius and
Kelvin systems. To convert from degrees Fahrenheit to degrees Celsius, first subtract
32 degrees (to establish how far the temperature is from the freezing point of water) and
then multiply by 100/180, or 5/9, the ratio of the number of degrees between freezing
and boiling on the two systems. That conversion is shown as the second equation in
Equation 1. If you further needed to convert to kelvins, you would add 273.15.
To switch from Celsius to Fahrenheit, you first multiply the number of degrees Celsius
by 9/5 (the reciprocal of the ratio mentioned above) and then add 32.In Example 1, you see the conventionally normal human body temperature, 98.6°F,
converted to degrees Celsius and kelvins.Temperature scales:
conversions
TK = TC + 273.15
TC = (5/9)(TFí 32)
TK = Kelvin temperature
TC = Celsius temperature
TF = Fahrenheit temperature
Convert 98.6°F to Celsius and
Kelvin.
TC = (5/9)(TFí 32.0)
TC = (5/9)(98.6°F í 32.0)
TC = (5/9)(66.6) = 37.0ºC
TK = TC + 273.15
TK = 37.0 + 273.15 = 310 K
(^336) Copyright 2007 Kinetic Books Co. Chapter 18