210 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME Charles' Law Deals with Temperature-volume Relationships. The expansibility of gases is another property of gases stated as a law, named after Charles, who first observed that under constant pressure the volume of a gas is very nearly proportional to the absolute temperature. This property is likewise familiar qualitatively, because, as most people have observed, the volume of a gas in a closed container will expand when heated, provided that it is capable of expansion. Thus the sides of a gasoline tin will bulge out and a balloon will swell to the bursting-point as it is heated. The quantitative aspects of this law are not so well recognized, inasmuch as the absolute-temperature scale is unfamiliar. The three common temperature scales are shown in Fig. 68. It was observed that the volume of a given weight of gas would decrease 1/273 for each degree of decrease in temperature below 0" C. Water Boils Water Freezes Absolute Zero op op OA If the temperature were lowered @ @ @ 273° C, the decrease in the volume 212° 32° -459.4° -273° 373° 273° of the gas would thus produce zero volume if the gas did not condense earlier. (All gases do condense above absolute zero.) This temperature, called absolute zero, is the lowest temperature theoretically possible to obtain. It has been approached experimentally within a small fraction of a degree. Standard Temperature. In making many measurements, the results must be expressed at the same temperature for purposes of comparison. The standard reference temperature is 0° C. Temperature measurements depend upon the measurement of the Fig. 68. The three temperature scales. amount of expansion of gases or liquids, such as mercury or alcohol, in thermometers. Temperatures may be measured by the electric current produced when one of two metals in contact with each other (a thermocouple) is heated. In the case of the resistance thermometer, the temperature is measured by the change in the electrical resistance of a coil of wire in response to changes in temperature. Inasmuch as electrical resistance can be
MATTER IN THE GASEOUS STATE 211 easily measured with great precision, the resistance thermometer is widely used in scientific investigations. Any temperature-measuring device must be calibrated in terms of certain standards; the usual standards for calibrated temperature-measuring devices are the boilingand freezing-point of pure water, which are always the same at standard pressure. The people of Florence used the body temperature of cows for the upper standard. Later the temperature of the human body was accepted as the standard temperature and called 100°. This was the origin of the Fahrenheit scale, but somewhat fevered subjects must have been used inasmuch as the normal body temperature is usually considered to be only about 98.6° F. Gases Diffuse Readily. When gases are mixed, the pressure or the volume of the resulting mixture is equal to the sum of the original pressures or volumes of the original gases, provided that the temperature is kept constant. In other words, they behave just as if they were present alone. Dalton's Law of Partial Pressure states this behavior as follows : Each gas in an ideal gaseous mixture exerts the same pressure as it would if it occupied the whole space alone. The rates at which gases will diffuse in each other depend on their densities. is A convenient way to measure the rates of diffusion of gases to determine the time required for equal volumes of different gases to pass through a small orifice or porous surface. Such experiments have shown that gases diffuse at rates which are inversely proportional to the square roots of their densities. This is a statement of Graham's Law. STUDY QUESTIONS 1. Why does the pressure in an automobile tire increase on a hot day? 2. Explain the action of the simple lift pump, and show why it will not lift water more than thirty-four feet above the free surface. 3. Define and contrast the different states of matter. 4. State the laws which deal with the effect of temperature and pressure changes on the volume of'a gas. 5. Why does hydrogen gas diffuse more rapidly than carbon dioxide gas? 6. How could you prove that the atmosphere exerts a pressure? 7. How could you measure the atmospheric pressure? 8. Explain the use of a barometer to estimate the altitude. 9. Differentiate between physical and chemical properties. 10. Give several illustrations of each type of property. 11. What are the values of a knowledge of the properties of a substance? 12. State Graham's Law. 13. State Dalton's Law of Partial Pressure and work out an illustration.