The following lesson is one lecture in a series of Chemistry Programs developed by Professor Larry Byrd Department of Chemistry Western Kentucky University

PART 2 Temperature Scales

Why do we use three different temperature scales? In calculations involving gases and in many other chemical problems, we must use Kelvin temperatures. In the medical field in the United States, both Fahrenheit and the Celsius scale are used, but the everyday citizen uses only the Fahrenheit scale. The Celsius Scale is used by most people in the world as their everyday temperature scale. International scientists used both Kelvin and Celsius when reporting their research efforts

Since all three scales are used, we need to be able to convert from one into another. As we found in the last section, it is easy to convert a Celsius value into a Kelvin value by simply adding to the Celsius value You need to memorize that: K =  C

Also, you need to memorize the expression:  C = K Let us look at two examples to see how the signs are so important:

EXAMPLE ONE: Normal air, which is mainly nitrogen and oxygen gases, can be converted into liquid air if high pressure is used and at the same time the very low temperature of 80.0 K is also used. To convert 80.0 K into  C we would do the following: 1.  C = K °C =  C = Since 80.0 K is to the nearest tenth, then the answer must be to the nearest tenth.

EXAMPLE TWO: The temperature at which table salt (sodium chloride, NaCl) melts ( solid to liquid ) is K What is this value in  C? 1.  C = K  C =  C = =  C = 800. Since the is a positive value and the is negative value, thus algebraically the answer is Next, round off to a whole number! The 800.  C is to the nearest whole number just as is a value to the nearest whole number.

Conversions between Fahrenheit and Celsius temperatures are more difficult than Kelvin-Celsius conversions since we have to take into account two changes. The differences between the boiling and freezing points of water are 180 division- change (212  F – 32  F) on the Fahrenheit scale, but only 100 division-change [100  C – 0  C] on the Celsius scale.

The "RATIO" between the two scales can be found by dividing the number of divisions found in each scale between the boiling and freezing points of water: 180 Fahrenheit degrees = 100 Celsius degrees

The second thing that we must take into consideration in Fahrenheit-Celsius conversions is that the locations of 0  differs by 32  on these scales. Thus, to convert a temperature from degrees Celsius to degrees Fahrenheit we must first multiply the Celsius reading by 1.8 (ratio used to adjust Celsius degrees to Fahrenheit degrees) and then add 32 to that value to adjust for their difference in reference points.

This relationship may be written algebraically as:  F = (1.8) (  C) + 32 or just as  F = 1.8  C + 32

 F = (1.8) (  C) + 32 or just as  F = 1.8  C + 32 In words, the above equation means that to convert a value given in Celsius into one in Fahrenheit, we must do the following steps: (1) First multiply the Celsius value by 1.8 (2) Next, add 32 to that value (3) Then, round-off and give the new value with units of  F

 F = (1.8) (  C) + 32 or just as  F = 1.8  C + 32 It is important that you memorize the above equation so you can convert from Celsius to Fahrenheit. Study the following examples:

EXAMPLE THREE: Convert 0.  C into  F. [Notice, that the given temperature is a whole number; thus, the final answer must be a whole number] 1.  F = (1.8) (  C ) + 32 Write the needed equation you had to learn 2.  F = (1.8) ( 0. ) + 32 Substitute the given value 3.  F = Do math 4.  F = Thus, 0.  C = 32.  F [ The answer is given as a whole number ]

EXAMPLE FOUR: Convert  C into  F. [ give value to nearest tenth ] 1.  F = (1.8) (  C) + 32 Write the needed equation you had to learn 2.  F = (1.8) (-60.6) + 32 Substitute the given value: 3.  F = Do math algebraically Observe, that since both 1.8 and 32 are both Exact Numbers, they are not involved in the number of significant figures that will be used.

EXAMPLE FOUR: Convert  C into  F. [ give value to nearest tenth ] 1.  F = (1.8) (  C) + 32 Write the needed equation you had to learn 2.  F = (1.8) (-60.6) + 32 Substitute the given value: 3.  F = Do math algebraically Just remember (just like the Celsius-Kelvin conversions) that if the temperature you are going to convert is to the nearest whole number then you must "ROUND-OFF" the final temperature to the nearest whole number.

EXAMPLE FOUR: Convert  C into  F. [ give value to nearest tenth ] 1.  F = (1.8) (  C) + 32 Write the needed equation you had to learn 2.  F = (1.8) (-60.6) + 32 Substitute the given value: 3.  F = Do math algebraically In the above example, since the starting temperature (  C ) was to the nearest tenth, we need to round off the final answer to the nearest tenth. Thus, when we multiply 1.8 times -60.6, we need to round off the resulting value to degrees.

EXAMPLE FOUR: Convert  C into  F. [ give value to nearest tenth ] 1.  F = (1.8) (  C) + 32 Write the needed equation you had to learn 2.  F = (1.8) (-60.6) + 32 Substitute the given value: 3.  F = Do math algebraically 4.  F = ( In this step round off to the nearest tenth ) 5.  F = ( Then, we must add algebraically ) 6.  F = Thus,  C is equal to  F