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Unit 2: The Behavior of Atoms—Phases of Matter and the Properties of Gases

Section 3: Measuring Temperature

Before chemists could analyze phase changes, they had to be able to measure temperature accurately. Although early scientists did not realize it, temperature is a measure of the average kinetic energy of the molecules in a substance, so it is directly related to states of matter.

In 1597, Italian physicist Galileo Galilei (1564–1642) demonstrated the idea that water in a tube could be used to measure temperature, because the water expanded when heated and contracted when cooled. However, the device Galileo created was a "thermoscope," not a thermometer: It did not have a temperature scale, so it simply registered that the amount of heat was changing. Over the next century, other European scientists built enclosed glass thermometers that contained alcohol (which has a lower freezing point than water) or oil to measure temperature, but none were widely adopted. Moreover, without a fixed temperature scale, no two scientists' measurements were directly comparable.

Fahrenheit and Celsius Temperature Scales

Figure 2-4. Fahrenheit and Celsius Temperature Scales

© Science Media Group.

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Fahrenheit and Celsius Temperature Scales

Figure 2-4. Fahrenheit and Celsius Temperature Scales

The Fahrenheit and Celsius scales both measure temperature scales, but one degree Celsius represents a larger increment of heat than one Fahrenheit degree.

In 1724, Dutch-German physicist Daniel Fahrenheit (1686–1736) created the temperature scale that bears his name, using a mercury thermometer that he had developed a decade earlier. Fahrenheit set zero at the temperature of a mixture of ice, water, and salt (adding salt lowers the freezing point of water)—the coldest condition he could produce. The next point, 32°F, was the temperature at which ice began to form on the surface of cold water. Fahrenheit's third fixed point was the body temperature of a healthy adult male, which he initially measured at 96°F and later corrected to 98.6°F. From these points, other scientists calculated the boiling point of water to be 212°F.

Swedish astronomer Anders Celsius (1701–1744) created another new scale in 1742 that set the freezing point of water at 0° and the boiling point at 100°. He called the scale "Centigrade," reflecting that it was based on a 1-to-100 scale; the system was renamed Celsius in his honor after his death. The Fahrenheit and Celsius scales are equal at -40°, but diverge from that point, since each one-degree increase Celsius represents a much larger increment of temperature than one degree Fahrenheit. (Figure 2-4)

In 1854, William Thomson, 1st Baron Kelvin, an Irish-Scottish physicist (1824-1907), produced yet another temperature scale that was intended to be more useful than the Fahrenheit or Celsius scales. In Thomson's "absolute" scale, every increase from one degree to the next represented an equivalent amount of energy. The only fixed reference point on the scale was absolute zero, which represented the temperature where no kinetic energy remained in any substance—that is, the temperature where everything was frozen. This was an abstract concept at that time. Thomson knew from work by early gas chemists that the volume of a gas at constant pressure was directly proportional to its temperature, and he used this linear relationship to calculate the temperature where pressure would equal zero.

Fahrenheit, Celsius and Kelvin Scales

Figure 2-5. Fahrenheit, Celsius and Kelvin Scales

© Science Media Group.

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Fahrenheit, Celsius and Kelvin Scales

Figure 2-5. Fahrenheit, Celsius and Kelvin Scales

Unlike the Fahrenheit and Celsius scales, the Kelvin scale was developed starting from absolute zero, so it does not have negative values.

This value, -273°C, represents absolute zero, or 0 on the Kelvin scale—so named because Thomson received the title of 1st Baron Kelvin in 1892. Today, the Kelvin scale is widely used by scientists, who consider it more useful than other systems because it is based on an absolute value. When a substance's temperature is measured in Kelvins, its value is directly proportional to the average kinetic energy of the molecules of the substance. And since the Kelvin scale starts at absolute zero, it does not require use of negative numbers, which simplifies calculations. (Figure 2-5)

Glossary

Directly proportional

Occurs if, as the value of one variable increases, the value of another variable increases at the same rate.

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