Molecules in Gases and Liquids
Text 17 A Molecules in Gases and Liquids - раздел Образование, Part One THE PREDICATE According To Avogadro's Principle, Equal Volumes Of Gases Regardless Of Compo...
According to Avogadro's principle, equal volumes of gases regardless of composition, contain the same number of molccules at the same temperature and pressure. As a consequence of the principle, the gram- molccular weight of any gaseous substance occupics 22.4 litres at standard temperature (0°C) and pressure (760 mm of mercury). The number of molcculcs per gram-mole has been calculated by different methods of increasing refinement through the years, and is now considered to be 6.023xl0u atoms per gram-atom, or molecules per gram-mole), and it is accurate within 0.1%. For example, one mole of ammonia gas (NHj — weighs 17.073 grams, occupics a volume of 22.4 litre at standard temperature and pressure, and contains 6.023x102* molccules).
At the same temperature, molccules of a liquid move at the same rate as those in a gas. In a liquid, however, the extent of motion must be more restricted. Liquids flow as a stream and tend to form drops to a greater or less extent, thus giving evidence of the importance of the force of cohesion between the molccules in a liquid. Heating liquids, as a rule, results in their expansion, an effect explained by the tendency of the molcculcs to occupy more space when they move at a faster rate. Also, increase in pressure has but slight effect on the volume compressible. From this evidence it is argued that molcculcs in a liquid arc ajacent, close enough to flow in a continuous stream.
The molecules in a liquid, like those in a gas, are not all moving at the same velocity, but at the same average velocity at a given temperature. The molccules at the surfacc of a liquid, unlike those below the surface
layer, have no force of attraction from molcculcs above. Some of the more rapidly moving molcculcs overcome the cohcsivc force of their neighbour and leave the surface. The tendency to leave the surfacc or to evaporate varies from liquid to liquid, and it increases when the temperature is raised. The pressure causcd by the evaporation of molecules from a liquid, measured at equilibrium with the returning molecules at a given temperature, is callcd the vapour pressure. In general, vapour pressure increases when the temperature rises. With continued addition of heat the vapour pressure rises still more until the vapour pressure reaches the vapour pressure of the atmosphere above the liquid. The evaporation goes on throughout the liquid, and the liquid is boiling. Obviously the act of boiling can be accomplished either by raising the temperature of the liquid or by reducing the pressure of the atmosphere above the liquid.
