Crystalline states

Matter usually exists in one of the three basic states gaseous, liquid, or solid. At fixed temperature and pressure, only one of the states is typically stable for any given substance, except for certain combinations of these thermodynamic variables, where two or all three states may co-exist in equilibrium. By decreasing temperature and/or increasing pressure, a gas may be condensed into a liquid and then into a solid, although in some cases gas - solid transitions occur without formation of a liquid phase. The most 

Gases are formed by weakly interacting, nearly isolated particles - atoms or molecules. Interatomic or intermolecular distances continuously change and as a result, gases have no fixed shape or volume and gaseous matter occupies all available space. As far as macroscopic properties of a gas are concerned, they remain identical in any direction because its structure, more precisely, the absence of long- or short-range order, is isotropic. 

State of matter Fixed volume Fixed shape Order Properties 

Short-range order is over a few atoms. Long-range order extends over -A system has different properties in different directions. 

One of the most distinct properties of the crystalline state is, therefore, the presence of long-range order, or in other words, a regular and in the simplest case periodic repetition of atoms or molecules in space. In theory, periodic crystals are infinite, but in practice, their periodicity extends over a distance fi om 10 to 10 ° atomic or molecular dimensions, which occurs because any crystal necessarily has a number of defects and may contain impurities without losing its crystallinity. Furthermore, a crystal is always finite regardless of its size. 

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If the entropy of each element in some crystalline state be taken as zero at the absolute zero of temperature, every substance has a finite positive entropy, but at the absolute zero of temperature the entropy may become zero, and does so become in the case of perfect crystalline substances. 

The boron so obtained is an amorphous powder. It can be obtained in the crystalline state by reducing the vapour of boron tribromide with hydrogen, either in an electric arc or in contact with an electrically-heated tungsten filament ... 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Urease is one of the enzymes which have been obtained in the crystalline state. This has been done by stirring jack bean meal with 30°o aqueous acetone, filtering and allowing the filtrate to remain at o for several hours. The urease which crystallises out is separated by centrifuging and is then recrystallised. Like crystalline pepsin and trypsin, it is a protein. 

In order to reach a crystalline state, polymers must have sufficient freedom of motion. Polymer crystals nearly always consist of many strands with a parallel packing. Simply putting strands in parallel does not ensure that they will have the freedom of movement necessary to then find the low-energy con-former. The researcher can check this by examining the cross-sectional profile of the polymer (viewed end on). If the profile is roughly circular, it is likely that the chain will be able to change conformation as necessary. 

Some of the distinctions that we shall have to examine in more detail before proceeding much further are the considerations of order versus disorder, solid versus liquid, and thermodynamics versus kinetics. These dualities are taken up in the next section. With those distinctions as background, we shall examine both the glassy and crystalline states from both the experimental and modelistic viewpoint. 

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