The Foundations of Chemistry

In the SI system, “degrees Kelvin” are abbreviated simply as K rather than °K and are

called kelvins.

Any temperature changehas the same numerical value whether expressed on the Celsius

scale or on the Kelvin scale. For example, a change from 25°C to 59°C represents a change

of 34 Celsius degrees. Converting these to the Kelvin scale, the same change is expressed

as (27325)
298 K to (59273)
332 K, or a changeof 34 kelvins.

Comparing the Fahrenheit and Celsius scales, we find that the intervals between the

same reference points are 180 Fahrenheit degrees and 100 Celsius degrees, respectively.

Thus a Fahrenheit degree must be smaller than a Celsius degree. It takes 180 Fahrenheit

degrees to cover the same temperature intervalas 100 Celsius degrees. From this infor-

mation, we can construct the unit factors for temperature changes:

or and or

But the starting points of the two scales are different, so we cannot converta temperature

on one scale to a temperature on the other just by multiplying by the unit factor. In con-

verting from °F to °C, we must subtract 32 Fahrenheit degrees to reach the zero point

on the Celsius scale (Figure 1-17).

?°F
x°C 32°F and ?°C
(x°F32°F)

EXAMPLE 1-16 Temperature Conversion

When the temperature reaches “100.°F in the shade,” it’s hot. What is this temperature on the

Celsius scale?

Plan

We use the relationship __?°C


1

1

.

.

0

8

°

°

C

F

(x°F32°F) to carry out the desired conversion.

Solution

__?°C
 (100.°F32°F)
 (68°F)
 38°C

EXAMPLE 1-17 Temperature Conversion

When the absolute temperature is 400 K, what is the Fahrenheit temperature?

Plan

We first use the relationship __?°C
K273° to convert from kelvins to degrees Celsius, then

we carry out the further conversion from degrees Celsius to degrees Fahrenheit.

Solution

__?°C
(400 K273 K)

1

1

.

.

0

0

°

K

C


127°C

__?°F
127°C

1

1

.

.

0

8

°

°

C

F

32°F
 261°F

You should now work Exercise 46.

1.0°C



1.8°F

1.0°C



1.8°F

1.0°C



1.8°F

1.8°F



1.0°C

1.0°C



1.8°F

100°C



180°F

1.8°F



1.0°C

180°F



100°C

The numbers in these ratios are exact
numbers, so they do not affect the
number of significant figures in the
calculated result.

These are often remembered in
abbreviated form:

°F
1.8°C32°

°C

Either of these equations can be
rearranged to obtain the other one, so
you need to learn only one of them.

A temperature of 100.°F is 38°C.

(°F32°)



1.8

36 CHAPTER 1: The Foundations of Chemistry