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

Excellent Assistance has been provided by: Dr. Robert Wyatt Ms. Elizabeth Romero Ms. Kathy Barnes

PART 1 Temperature Scales

CELSIUS, FAHRENHEIT, AND KELVIN TEMPERATURE SCALES In Chemistry, the terms heat and temperature are often used to describe specific properties of a sample. The sample can be a tiny piece or a large amount of matter. HEAT is the most common form of energy in nature and is directly related to the motion of particles of matter. The faster the motion of particles in a sample the greater its heat content. However, TEMPERATURE is associated only with the intensity of heat and is not affected by the size of the sample.

A forest fire and a lit match may both be at the same temperature, but there is a large difference in the amount of heat each possess. Notice, that heat is a measure of the quantity of energy present but temperature is only related to the heat intensity of a sample.

The temperature of a sample determines its ability to transfer heat or obtain heat from another sample. Heat always spontaneously flows from a hotter system (higher temp.) to a colder system (lower temp.). We may touch an object and describe its temperature as either: neutral, warm, hot, cool or cold. However, if we need to know the precise temperature, we need to use a device known as a thermometer.

The principle of the thermometer depends upon the observation that most substances expand when heated and contract when cooled. For the past 200 years the most common thermometer has been the mercury-in-glass type. Mercury has been used since it is readily available, and it has a nearly linear expansion and contraction with changes in temperature.

The mercury thermometer is constructed by first obtaining a thin capillary tube and sealing at its lower end with a glass bulb. Next, the thermometer is partially filled with mercury. Finally, the air from the device is evacuated, and the top end is sealed. The thermometer is now ready to be calibrated so that the various heights of the mercury column correspond to different temperatures.

Modern mercury thermometers have their scales usually etched on the outside of their glass surfaces. Today electronic thermometers with digital read-outs are replacing glass thermometers in the laboratory and in the medical field. Three different temperatures scales are commonly used today to describe temperature: Fahrenheit (  F), Celsius (  C), and Kelvin (K).

The first temperature scale was invented by Gabriel Daniel Fahrenheit ( ). He used several points for calibration of his mercury-in-glass thermometer. He used 32  F for the freezing point of water and 212  F for the boiling point of water. He placed the bulb of his thermometer in an ice-water mixture and when the height of the mercury column became constant, he made a mark on the glass and assigned its value to be 32  F. Next, the water was heated until it was boiling. He then immersed the bulb of the thermometer into the boiling liquid and allowed the height of the mercury column to become constant. This point was marked, and its value was assigned to be 212  F.

He divided the distance between 32 and 212 into 180 equal divisions. Each division was called a degree of temperature. By expanding the divisions above and below the assigned values, he was able to measure temperatures with his thermometer, that were below 32  F or above 212  F.

The Celsius Scale, which was developed by the Swedish astronomer Anders Celsius ( ), used the same procedures for establishing the freezing point and boiling point of water on his mercury thermometer. However, so that there would be a 100 equal divisions or degrees for his temperature range, he assigned the freezing point of water to be 0  C and the boiling point of water to be 100  C. He also extended his scale divisions above and below the fixed points so he could measure below 0  C or above 100  C. Since the Celsius Scale contained 100 degrees between the reference points, it also became known as the Centigrade Scale when it was adopted in 1795 as the standard temperature scale of the Metric System.

In 1787 the French physicist, Jacques Charles ( ) discovered that the volume of a gas varies in a specific way with its Celsius temperature. From later theoretical calculations about the behavior of gases at low pressures, it was postulated by scientists that the very lowest possible temperature must be  C.

The British physicist and mathematician Sir William Thomason ( ), who was better known as Lord Kelvin, suggested that since there was an absolute lowest possible temperature, why not call it zero. Thus, the Kelvin Temperature Scale came into existence 0 K became the lowest possible temperature on his scale, and it also became known as absolute zero. His temperature scale also became known as the Absolute Temperature Scale. On the MODERN KELVIN SCALE the freezing point of water is given as K and the boiling point of water as K

Notice that the modern Kelvin Scale does not use the superscript degree (  ) symbol. In the modern Kelvin Scale it is, therefore, incorrect to speak of degrees Kelvin. Thus 67 K is read as sixty-seven Kelvin The Kelvin temperature is always larger than the Celsius temperature by a factor. The Kelvin temperature value can always be found by adding to the value of the Celsius temperature: K =  C

The zero Kelvin (0 K) temperature is now considered by scientists to be one of those values in nature which cannot be reached. However, scientists working under strict conditions have been able to reach K (1 x K). Some places in outer space may have temperature as low as 3 K; whereas, the temperature of part of our sun and other stars is as high as 3.6 million Kelvin. It is important to note that the Kelvin Scale is the only one of the three scales that has no negative temperature values.

100.0°C 37.0°C 21.0°C 0.0°C -40.0°C °C 212.0°F 98.6°F 69.8°F 32.0°F -40.0°F °F K Boiling point of water K Normal body temperature K Room Temperature K Freezing point of water K Dry Ice (solid carbon dioxide) 0.0 K Absolute zero °Celsius °Fahrenheit Kelvin units units units Comparison of the Celsius, Fahrenheit, and Kelvin Temperature Scales

The following four equations you must learn: 4. °C = K – 273.2