236 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME shown in Fig. 73A. Such a curve is called a probability cnri'e. Similar curves are found in the graphical expression of the grades made in college by a large number of students; in fact, many schools require that the greides of large classes conform to the probability curve. One 2500 - E 2000 34 36 38 40 42 44 46 48 60 12 Chest Measurement 3 Velocity of Molecules Fig. 73A. The curve of variation Fig. 73B. The theory of probaof the chest measurements of Scottish bility shows that molecules subject soldiers. to chance collisions may be divided into groups, each one with a different range of velocity. can expect to find in almost any large group of people a large number of ordinary people and a small number of idiots and men of genius. Similar curves may be drawn to illustrate the distribution of hits on a target or the heights and weights of men of a certain age. The theory of probability can be applied to the speed of molecules and the frequency of their collision. Calculations show that when the average speed is one mile per second, then — About one molecule in every 700,000 5 2 50,000 150,000,000 less than less than has a speed of 0.01 miles per second 0.5 miles per second 0.7-1.3 miles per second more than 3 miles per second more than 4 miles per second 7. In what directions do molecules move? Answer: Molecules move along straight lines in all conceivable directions. 8. Do molecules ever collide with each other, and if so, what is the result? Answer: Molecules of gases collide with each other and with the walls of a container very frequently. Although molecules of a gas may be moving w^ith a speed of one mile per second, they actually travel an average of only 1/60,000 centimeter before hitting another molecule at atmospheric pressure. This means that an average hydrogen molecule is hit by other molecules in one second as many times as a clock ticks in seventy years.
THE KINETIC-MOLECULAR THEORY 237 The total number of the molecules of a given volume of a gas neither lose nor gain energy when kept at a given temperature. This means, of course, that when molecules hit each other their individual speeds are changed but no energy is lost or gained in the process. 9. What causes molecules to move? Answer: Heat causes molecules to move, and the larger the amount of the heat the more rapidly do the molecules move. Heat, therefore, is not a fluid substance but energy of motion which can be transferred from one body to another. At absolute zero the molecules are motionless. 10. Do light and massive molecules move at the same average speeds at the same temperature? Answer: No. Light molecules move more rapidly than heavy molecules. At a given temperature, the average kinetic energy (energy of motion) of molecules of all dififerent gases will be the same, because light molecules make up in velocity squared what they lack in mass. The kinetic energy of molecules may be expressed as follows: Kinetic energy = 3^ mass X velocity squared. IL Do molecules obey Newton's law of universal gravitation? Answer: Yes. 12. What causes the gaseous pressure on the walls of a container? Answer: The bombardment of the walls of a container by the molecules is the cause of the pressure. 13. Do containers of the same size contain the same number of molecules when the temperature and pressure are the same? Answer: Yes. Avogadro' s Law states that equal volumes of gases measured under the same conditions of temperature and pressure contain the same number of molecules. Liquids and solids are composed of molecules just as gases are, except that the molecules are closer together in liquids and solids and therefore attract each other with greater force. There is evidence that many crystals are huge molecules. The attraction of the molecules of liquids and gases produces the resistance to flow, called viscosity. For vitreous solids this attraction is greater and is known as plasticity. In crystalline solids the attraction between the molecules is so great that they are held in a definite arrangement; the resistance to flow in this case is called hardness. All solids which have an odor must sublime, for the sense of odor is sensitive only to gases, and the solid which has an odor must therefore form a gas without passing through an intermediate liquid state. Such solids as naphthalene (mothballs) or dichlorobenzene (deodorant) must be composed of molecules a few of which are continually breaking away into the air, where they are jostled in every direction by the air