|MadSci Network: Chemistry|
Greetings, Sara: This is a very complex issue, which has been under study for decades. The answer I offer here will cover considerable territory, because many things in science are related -- and it is fun to digress. Still, I hope you will find it satisfactory. The first thing to keep in mind is that all atoms and molecules are constantly vibrating. Some of this vibration is an inherent part of the existence of any particle, and some of it is simply an indication of how little freedom a particle has to move about. Consider a tiny tiny box that contains just one particle. If the box is only slightly larger than the particle, then an overall motion of the particle, bouncing off the walls, will be practically indistinguishable from a stationary vibration. If the box is considerably larger than the particle, then the particle might be imagined able to smoothly move about the interior of the box. However, real particles will vibrate anyway, even while moving significant distances before hitting the walls of a container. For many molecules, an altogether different type of vibration is possible. Let me portray a water molecule as < (oxygen on the left, with two hydrogens extending towards the right). WITHOUT ROTATING, it is possible for the molecule to vibrate to the opposite > position. And it need not stop there; it is able to vibrate back and forth: from < to > to < to > ... This type of vibration is frequently associated with the absorption or emission of radiant energy (radio microwaves, usually, but it depends on the molecule). The next thing to keep in mind is that the overall rate of all types of vibration is highly dependent upon the temperature: the more heat, the more vibes, and that includes overall physical motion of particles. Inside a typical object, say food in an ordinary oven, if its surface is heated, the heavily vibrating particles there will collide with interior and less-active neighboring molecules, and transfer heat to them. Then those molecules can transfer heat to THEIR neigbors, and gradually the whole chunk of food gets warmer via this 'conduction' process. A microwave oven is designed to generate radiant-energy waves that cause water molecules to vibrate as described in the previous paragraph; this vibration is indistinguishable from heat to the neighboring molecules in food. Microwave energy penetrates the food completely, is absorbed by water molecules throughout, causes them to vibrate against and thereby heat up their neighboring molecules -- and the longer this goes on, the more the food cooks. Since it bypasses the time needed for heat to conduct from the surface to the interior of food, a microwave is much more efficient than a conventional oven. Now let's consider a solid substance about to be melted. Its atoms or molecules are all very close to each other, and for any one particle in the middle of the solid, this can easily be imagined as being the equivalent of confinement inside a tiny box. The REASON all the particles are so close together is the third thing to remember: everything attracts. All atoms and molecules experience an attraction for each other, and depending on the type of particles, the amount of attraction may be large or small. (This attraction is electrostatic, not gravitational. Even though an atom may be electrically neutral, it still consists of distinct particles with distinct electric charges, which are seldom evenly distributed. 'Holes' in the distribution of electrons allows the positively-charged nucleus of one atom to attract the negatively-charged electrons of neighboring atoms. These attractions are known as 'van der Waals forces', after the fellow who first figured it out.) Atoms and molecules possessing well-distributed electrons don't attract each other much, and are generally gases even at fairly low temperatures; particles that attract each other a lot generally stay clumped into solids even at pretty high temperatures. SOME of the rest of the time, groups of particles may exist together in the liquid state. The liquid state is relatively uncommon in the Universe, compared to the other six known states of matter. [In order of increasing density of particles, there are plasmas, gases, liquids, and solids, followed by degenerate matter (found in white-dwarf stars), neutronium (in neutron stars, of course), and 'singularitum' (is there a better name for matter in the state of infinitely-shrinking-towards-a-single-mathematical-point that occurs inside a black hole?). I might note that plasmas, which are mixtures of ionized atoms and loose electrons, often exist in a wide range of densities: Where the Sun's outer atmosphere becomes the Solar Wind is a very-low-density plasma; if one journeyed to the center of the Sun, one would encounter plasma all the way, gradually increasing in density. The very core of the Sun consists of plasma at the maximum possible density, known as degenerate matter -- a cubic centimeter might contain a metric ton of mass -- and the overall average density of all the Sun's mass of plasma is about 1.4 times that of water on Earth.] The main reason why liquids are less common in the Universe than solids or gases is because they require two things which are not always found together in the right amounts: temperature and pressure. If one starts with a cool solid substance and heats it, individual particles in it will start vibrating faster and faster. Eventually some of them will break loose from the attractions of nearby other particles. What is to keep them from flying out of this description altogether? For some substances, such as frozen carbon dioxide (dry ice), it is NORMAL on Earth for the solid to directly dissipate (or 'sublime') into the gaseous state, as it is heated. For other substances, such as water, the Earth's atmosphere provides one aspect of a barrier. A water molecule that has broken free from an ice cube will most certainly bounce off some air molecules, and very likely find itself heading right back to the ice cube! The second aspect of the barrier consists of the attraction of water molecules for each other; it is fairly easy for the returning molecule to become trapped again (and this does not happen to carbon dioxide, because the attraction between those molecules is much less). The third aspect of the barrier is simple Physics: When a fast-moving particle collides with a slower-moving particle, USUALLY the fast-moving particle slows down, and the slower-moving particle speeds up. (It is the fundamental basis of the process of conduction, mentioned earlier.) This phenomenon means that a returning water molecule will often lose enough energy that it can no longer escape the attractions of the neighboring molecules. The returned molecule may still possess enough energy to move fairly freely on the surface of the ice cube; as a result, a growing film of liquid water can cover the cube. One of the more complicated aspects of the preceding description involves the fact that when a moving particle encounters a relatively massive and non-moving object, the particle can often bounce off with very little loss of energy. For carbon dioxide, a molecule returning to a dry ice cube encounters exactly this situation, so the molecule can re-escape, and the cube can sublime. For a regular ice cube, the film of liquid on its surface prevents a clean bounce; the returning molecule encounters a MOVING water-film molecule, so it can lose energy easily and stay confined by the three-aspect barrier. Yet now one can imagine a chicken-and-egg situation: How does the film of water form when it seems to require itself to maintain itself? The answer lies in the gradual rise in temperature, which permits the surface molecules of an ice cube to loosen enough to start forming the film, but not enough to completely escape -- so that when some DO completely escape, and get bounced back by the atmosphere, there are enough loose molecules to provide the third aspect of the barrier. The less the pressure of the atmosphere, the less is its effectiveness as the first aspect of the barrier described above. This is well known to anyone who ever lived at sea level, and then moved to a high altitude. The boiling point of water is the temperature at which a majority of moving surface molecules can overcome the atmosphere's tendency to bounce them back into their container. (When only a minority of molecules can penetrate the atmosphere's barrier, we call the process 'evaporation'.) The higher the altitude, the lower the boiling point. If the atmospheric pressure is low enough, plain old water ice will sublime from solid to gas, just like dry ice. It works the other way, too, of course. Pressure cookers were invented to compensate for the atmosphere's weakness as a barrier to water's boiling, at high altitudes. (At sea level, the boiling point of water inside a pressure cooker will be significantly higher than 100 degrees Celsius, because it effectively adds to the atmospheric barrier.) And if enough atmospheric pressure surrounded dry ice, then heating it will yield liquid carbon dioxide, just as you might expect. As has been stated in lengthy fashion, when a solid substance begins to melt, its constituent particles move faster, and break away from the attraction that previously held them in place. If there is an appropriate container such as sufficient air pressure, then intermittent mutual attractions, as they bounce off each other, can be sufficient to let them form the liquid state. Since each particle moves about more freely than when in the solid state, it is generally true that it takes up more space, and on the whole, the liquid occupies more space than the solid. Even for liquid water, which takes about 10% LESS space than when solid, there is plenty of space between molecules. (Ice floats on liquid water since it is less dense. Out in space near the planet Jupiter is the moon Europa, which has a surface fully covered by ice. The temperature is so cold that the ice does not sublime into the vacuum of space. The available evidence strongly suggests that the interior of Europa is warm enough to melt ice; the liquid state is possible on a moon with little gravity and no atmosphere, because the exterior layer of ice forms an effective container!) The fact that there is significant space between the particles making up a liquid is one of the keys to understanding most solvents. The other key fact concerns the nature of the molecules of the solvent, and the nature of the substance which you might wish to dissolve. While all the particles of any one substance attract each other equally well, different particles from different substances are not always so cooperative. Since water molecules get along well with sugar molecules, a sugar cube placed in a cup of water will gradually disappear, as molecule after molecule breaks off and fits itself into the spaces between water molecules. But water molecules do not get along well with most plastic molecules, so a pile of plastic dust poured into a cup of water will just sit there. Since other substances, such as benzene, do mesh well with various plastics, those substances are used when it is desired to dissolve plastics. And like sugar in water, the process is the same: molecule after molecule breaks off from the solid substance, and fits itself into the spaces between the particles of the liquid. It may be true that just about any substance can be dissolved in some special-enough liquid. The chemical element mercury is well-known for its abiltiy to dissolve gold, silver, and other metals. Bauxite, or aluminum oxide (the stuff of which sapphires are made) is dissolved in a molten salt known as cryolite, before electrolysis separates the aluminum from its ore for our use. And even though most chemistry-lab equipment is made of glass because few chemicals affect it, there ARE substances which can react with it chemically (hydrofluoric acid, for one), and probably there are substances which can dissolve glass like so much sugar in water (although I don't know the name of one offhand -- oxalic acid, perhaps?).
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