# Temperature and Temperature Scales

### Learning Objectives

• Define temperature.
• Describe the Fahrenheit temperature scale.
• Describe the Celsius temperature scale.
• Describe the Kelvin temperature scale.

### What is temperature?

Touch the top of the stove after it has been on and it feels hot. Hold an ice cube in your hand and it feels cold. Why? The particles of matter in a hot object are moving much faster than the particles of matter in a cold object. An object’s kinetic energy is the energy due to motion. The particles of matter that make up the hot stove have a greater amount of kinetic energy than those in the ice cube.

Temperature is a measure of the average kinetic energy of the particles in matter. In everyday usage, temperature indicates a measure of how hot or cold an object is. Temperature is an important parameter in chemistry. When a substance changes from solid to liquid, it is because there was an increase in the temperature of the material. Chemical reactions usually proceed faster if the temperature is increased. Many unstable materials (such as enzymes) will be viable longer at lower temperatures. Figure 1. The glowing charcoal on the left represents high kinetic energy, while the snow and ice on the right are of much lower kinetic energy.

## Temperature Scales Figure 2. Daniel Gabriel Fahrenheit.

The first thermometers were glass and contained alcohol, which expanded and contracted as the temperature changed. The German scientist, Daniel Gabriel Fahrenheit used mercury in the tube, an idea put forth by Ismael Boulliau. The Fahrenheit scale was first developed in 1724 and tinkered with for some time after that. The main problem with this scale is the arbitrary definitions of temperature. The freezing point of water was defined as 32°F and the boiling point as 212°F. The Fahrenheit scale is typically not used for scientific purposes. Figure 3. Anders Celsius.

The Celsius scale of the metric system is named after Swedish astronomer Anders Celsius (1701–1744). The Celsius scale sets the freezing point and boiling point of water at 0°C and 100°C respectively. The distance between those two points is divided into 100 equal intervals, each of which is one degree. Another term sometimes used for the Celsius scale is “centigrade” because there are 100 degrees between the freezing and boiling points of water on this scale. However, the preferred term is “Celsius.” Figure 4. Lord Kelvin.

The Kelvin temperature scale is named after Scottish physicist and mathematician Lord Kelvin (1824-1907). It is based on molecular motion, with the temperature of 0 K, also known as absolute zero, being the point where all molecular motion ceases. The freezing point of water on the Kelvin scale is 273.15 K, while the boiling point is 373.15 K. Notice that here is no “degree” used in the temperature designation. Unlike the Fahrenheit and Celsius scales where temperatures are referred to as “degrees F” or “degrees C,” we simply designated temperatures in the Kelvin scale as kelvins.

As can be seen by the 100 kelvin difference between the two, a change of one degree on the Celsius scale is equivalent to the change of one kelvin on the Kelvin scale. Converting from the Kelvin scale to the Celsius scale or vice versa is easy, as you simply add or subtract 273. Figure 5. Comparing the three different temperature scales.

Converting between the Celsius and Fahrenheit temperature scales is a little bit trickier but still not too difficult.  To convert from Fahrenheit to Celsius, first multiply the temperature in Celsius (TC) by 1.8 and then add 32, in that order.

$\text{T}_{\text{F}}=1.8\times\text{T}_{\text{C}}+32$

To convert from Celsius to Fahrenheit, first subtract 32 from the temperature in Fahrenheit then divide by 1.8, in that order.

$\displaystyle\text{T}_{\text{C}}=\frac{\text{T}_{\text{F}}-32}{1.8}$

### Summary

• Temperature is a measure of the average kinetic energy of the particles in matter.
• The Fahrenheit scale defines the freezing point of water as 32°F and the boiling point as 212°F.
• The Celsius scale sets the freezing point and boiling point of water at 0°C and 100°C respectively.
• The Kelvin scale is based on molecular motion, with the temperature of 0 K, also known as absolute zero, being the point where all molecular motion ceases.

### Practice

http://www.visionlearning.com/library/module_viewer.php?mid=48

1. What mixture did Fahrenheit use to set his thermometer at zero degrees?
2. Why is the Celsius scale preferred in scientific work over the Fahrenheit scale?
3. What was the idea behind the establishment of the Kelvin temperature scale?
4. What is the advantage of using the Kelvin scale at low temperatures?

### Review

1. What is absolute zero on the Celsius temperature scale?
2. What are the freezing and boiling points of water in the Celsius scale?
3. Convert the following Kelvin temperatures to degrees Celsius.

1. 188 K
2. 631 K

4. Temperature in degrees Fahrenheit can be converted to Celsius by first subtracting 32, then dividing by 1.8. What is the Celsius temperature outside on a warm day (88°F)?
5. Why is the Celsius scale sometimes called “centigrade”?

## Glossary

• kinetic energy: The energy due to motion
• temperature: A measure of the average kinetic energy of the particles in matter. In everyday usage, temperature is how hot or cold an object is
• temperature scale: A way of measuring temperature quantitatively. There are three major scales used today.