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Temperature Conversions

Some web sites discussing the history of temperature units:

  1. Why does the Fahrenheit scale use 32 degrees as a freezing point?  
     

  2. Subject: How was the Fahrenheit-scale invented?
     

  3. Temperature
     

  4. What Marilyn omitted about the Fahrenheit Thermometer
     

  5. Origins of the Fahrenheit Scale
     

There are three temperature units that you are likely to encounter in chemistry:  degrees Fahrenheit (°F), degrees Celsius (°C) and Kelvin (K).

The Kelvin is related to molecular motion, and is an absolute temperature, so there is no degree sign associated with it.  The lowest possible temperature is zero Kelvin, which is referred to as “absolute zero”.

The Fahrenheit and Celsius scales were both developed by assigning somewhat arbitrary values to the freezing point and boiling point of water.  Then all other temperatures are some number of “degrees” away from those two reference points.

The relationships among the three temperature units are as follows:

Starting Unit

Final Unit

Equation to Use

Celsius

Fahrenheit

F = 1.8 x C + 32

Fahrenheit

Celsius

C = (F - 32) / 1.8

Celsius

Kelvin

K = C + 273.15

Kelvin

Celsius

C = K - 273.15

The 32 and the 1.8 are both exact numbers.  I’m not sure if the 273.15 is exact or not.

Remember your significant figure rules:  when adding or subtracting round to the fewest DECIMAL PLACES and when multiplying or dividing round to the fewest NUMBER OF SIGNIFICANT FIGURES.

Example conversions:

Q:
A:		 Convert 34.8 °C to °F.
1.8 x 34.8 + 32 = 62.6 + 32 = 94.6 °F
Q:
A:		 Convert 190.44 °F to °C.
(190.44 - 32) / 1.8 = 158.44 / 1.8 = 88.022 °C
Q:
A:		 Convert 25.0 °C to K.
25.0 + 273.15 = 298.2 K
Q:
A:		 Convert 204.33 K to °C.
204.33 - 273.15 = -68.82 °C
Q:
A:		 Convert 18 °F to K.
(18 - 32) / 1.8 = -14 / 1.8 = -7.8 °C + 273.15 = 265.4 K
Q:
A:		 Convert 497 K to °F.
497 - 273.15 = 224 °C then 224 x 1.8 + 32 = 403 + 32 = 435 °F

 

last updated 14 December 2008