Chapter 1 The Nature of Science and Physics
1.9 Other Units: Temperature and Density
Learning Objectives
- Learn about the various temperature scales that are commonly used in chemistry.
- Define density and use it as a conversion factor.
(Originally from OpenStax College Chemistry 1st Canadian Edition)
There are other units in chemistry that are important, and we will cover others in the course of the entire book. One of the fundamental quantities in science is temperature. Temperature is a measure of the average amount of energy of motion, or kinetic energy, a system contains. Temperatures are expressed using scales that use units called degrees, and there are several temperature scales in use. In the United States, the commonly used temperature scale is the Fahrenheit scale (symbolized by °F and spoken as “degrees Fahrenheit”). On this scale, the freezing point of liquid water (the temperature at which liquid water turns to solid ice) is 32 °F, and the boiling point of water (the temperature at which liquid water turns to steam) is 212 °F.
Science also uses other scales to express temperature. The Celsius scale (symbolized by °C and spoken as “degrees Celsius”) is a temperature scale where 0 °C is the freezing point of water and 100 °C is the boiling point of water; the scale is divided into 100 divisions between these two landmarks and extended higher and lower. By comparing the Fahrenheit and Celsius scales, a conversion between the two scales can be determined:
Using these formulas, we can convert from one temperature scale to another. The number 32 in the formulas is exact and does not count in significant figure determination.
Example 12
- What is 98.6 °F in degrees Celsius?
- What is 25.0 °C in degrees Fahrenheit?
Solution
-
Using the first formula from above, we have
°C = (98.6 – 32) × 5/9 = 66.6 × 5/9 = 37.0 °C
-
Using the second formula from above, we have
°F = (25.0 × 9/5) + 3/2 = 45.0 + 32 = 77.0 °F
Check Your Understanding 1
- Convert 0 °F to degrees Celsius.
- Convert 212 °C to degrees Fahrenheit.
The fundamental unit of temperature (another fundamental unit of science, bringing us to four) in SI is the kelvin (K). The Kelvin temperature scale (note that the name of the scale capitalizes the word Kelvin, but the unit itself is lowercase) uses degrees that are the same size as the Celsius degree, but the numerical scale is shifted up by 273.15 units. That is, the conversion between the Kelvin and Celsius scales is as follows:
K = °C + 273.15
°C = K − 273.15
For most purposes, it is acceptable to use 273 instead of 273.15. Note that the Kelvin scale does not use the word degrees; a temperature of 295 K is spoken of as “two hundred ninety-five kelvins” and not “two hundred ninety-five degrees Kelvin.”
The reason that the Kelvin scale is defined this way is because there exists a minimum possible temperature called absolute zero. The Kelvin temperature scale is set so that 0 K is absolute zero, and temperature is counted upward from t