Discovering Crystals

In this lesson, students observe dramatic changes that have occurred in the filtration dishes from Lesson 5 the liquid in the dishes has evaporated, and crystals have appeared in the dishes that contained solutions. Through the experiences in Lessons 5 and 6, students discover that, depending on an unknown s solubility in water, a solid can be separated from water by either evaporation or filtration. Students continue to learn that some changes take place over periods of time and that observing these changes can give them new information about the properties of the five unknowns. 

Many students find the study of crystals interesting. They often want to learn more about them. Let your students know that they will learn more about crystals in a reading selection at the end of the lesson. 

When sharing results, students may ask why the yellow unknown now looks like a crystal and not a powder. Ask for their ideas. Some students have said it was a crystal because something happened when it mixed with the water. This is partially true. Because of evaporation, the unknown, which already is composed of tiny crystals, reappears as larger crystals. Evaporation is the process by which a liquid (in this case, water) changes into a gas (hi1 this case, water vapor). 

Red Sugar Filter paper empty crystalline sticky mass in dish separated by evaporation 

Yellow Alum Filter paper has some residue crystals in dish separated by evaporation and filtration 

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Wohler was actually attempting to synthesize ammonium cyanate when he discovered crystals of urea in his samples. He first... 

The existence of the Periodic Table of the chemical elements and the concept of the atomic number motivated A. van den Broek to assert that the nuclear charge in a neutral atom was exactly equal to its atomic number 20). This cogent speculation was given experimental support by the brilliant experimental work of H.G.J. Moseley 21). He measured the frequency, v, of the characteristic x-rays for most of the known elements using the recently discovered crystal monochromator. A relationship was established ... 

Wohler was actually attempting to synthesize ammonium cyanate when he discovered crystals of urea in his samples. He first prepared urea in 1824, but he did not identify this product and report his findings until 1828. In a note written to Berzelius he proclaimed I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea. Although Wohlers synthesis of urea signaled the birth of organic chemistry, it... 

The crystallization of a biological macromolecule is realized by manipulation of one or more chemical and thermodynamic variables, such that the solubility of a target molecule in a concentrated solution is reduced, thereby promoting a transition to the solid phase in the form of a well-ordered crystal. In principle, any thermodynamic variable that may directly, or indirectly, affect protein solubility may be used to induce crystallization. Variables that are most often manipulated include macromolecule concentration, ionic strength, identity and concentration of precipitating agents, pH, temperature and small-molecule additives. Together, these variables comprise a vast multi-dimensional chemical phase space that must be systematically explored to discover crystallization conditions. 

The literature on the composition of foods is tremendous and has origins as early as any scientific literature. For instance, in 1743, Marggraf discovered crystals of... 

As practitioners discover (or should soon discover), crystallization skills are paramount among the skills set of the bulk process development (and manufacturing) function. 

The entropically driven disorder-order transition in hard-sphere fluids was originally discovered in computer simulations [58, 59]. The development of colloidal suspensions behaving as hard spheres (i.e., having negligible Hamaker constants, see Section VI-3) provided the means to experimentally verify the transition. Experimental data on the nucleation of hard-sphere colloidal crystals [60] allows one to extract the hard-sphere solid-liquid interfacial tension, 7 = 0.55 0.02k T/o, where a is the hard-sphere diameter [61]. This value agrees well with that found from density functional theory, 7 = 0.6 0.02k r/a 2 [21] (Section IX-2A). 

The otiier type of noncrystalline solid was discovered in the 1980s in certain rapidly cooled alloy systems. D Shechtman and coworkers [15] observed electron diffraction patterns with sharp spots with fivefold rotational synnnetry, a syimnetry that had been, until that time, assumed to be impossible. It is easy to show that it is impossible to fill two- or tliree-dimensional space with identical objects that have rotational symmetries of orders other than two, tliree, four or six, and it had been assumed that the long-range periodicity necessary to produce a diffraction pattern with sharp spots could only exist in materials made by the stacking of identical unit cells. The materials that produced these diffraction patterns, but clearly could not be crystals, became known as quasicrystals. 

We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

An impressive example of the application of structure-based methods was the design of a inhibitor of the HIV protease by a group of scientists at DuPont Merck [Lam et al. 1994 This enzyme is crucial to the replication of the HIV virus, and inhibitors have bee shown to have therapeutic value as components of anti-AIDS treatment regimes. The star1 ing point for their work was a series of X-ray crystal structures of the enzyme with number of inhibitors boimd. Their objective was to discover potent, novel leads whid were orally available. Many of the previously reported inhibitors of this enzyme possessei substantial peptide character, and so were biologically unstable, poorly absorbed am rapidly metabolised. 

Discotic blue phases Discotic liquid crystals Discover Disc Tube... 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Examples of chiral smectic Chquid crystals range from 2-methylbutyl (3 -4- -decyloxybenzyhdeneaminocinnamate (12), the first ferroelectric hquid crystal discovered. 

Alkali Metal Perchlorates. The anhydrous salts of the Group 1 (lA) or alkah metal perchlorates are isomorphous with one another as well as with ammonium perchlorate. Crystal stmctures have been determined by optical and x-ray methods (38). With the exception of lithium perchlorate, the compounds all exhibit dimorphism when undergoing transitions from rhombic to cubic forms at characteristic temperatures (33,34). Potassium perchlorate [7778-74-7] KCIO, the first such compound discovered, is used in pyrotechnics (qv) and has the highest percentage of oxygen (60.1%). 

The phenomenon of dye aggregation was discovered in the 1930s (52,53). Polymethine dyes were found to form molecular complexes in solution or on certain crystal surfaces. Molecules within aggregates are bound together by nonvalence bonds, ie, resonance interactions exist between them. 

Diamonds were first discovered in ancient times in India and Borneo and later in Brazil in 1670 in alluvial deposits where water had sorted minerals on the basis of density and toughness. This type of tumbling often concentrates the better quality crystals such as those found in the ocean off the west coast of Africa. Exploration can be done by stream panning or drilling in conjunction with a search for the heavy mineral assemblages that accompany diamond. Alluvial deposits account for about 40% of the diamond found in primary sources. 

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

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