<text><span class="style10">an-made Products (1 of 5)</span><span class="style7">Our chemical knowledge of natural compounds will ultimately be dwarfed by that relating to man-made, or unnatural, products. The reason for this is that chemistry is not only the science that deals with molecules, but also an exercise in design and engineering at the molecular level. In so far as chemistry creates its own world, its practice in an abstract form has infinite possibilities. Synthetic zeolites, which have important applications in the petrochemical industry, and ceramics, which have been found to behave as high-temperature superconductors, are typical examples of man- made products that are totally inorganic in their atomic composition. Likewise, liquid crystals - although based on organic compounds - are for the most part wholly synthetic compounds. Man-made products are also beginning to show promise in other areas, notably as </span><span class="style26">artificial enzymes</span><span class="style7">, i.e. tailor-made biological catalysts.</span><span class="style10">Liquid crystals</span><span class="style7">Discovered in 1888 by the Austrian botanist Friedrich Reinitzer, liquid crystals have been heralded as the fourth state of matter, occurring at the interface between the solid and the liquid phases. About 5% of crystalline compounds do not simply melt when heated: they form turbid (cloudy) liquids, which may also exhibit marked color changes as the temperature is raised, before becoming normal liquids. The process is reversible: upon cooling, the liquid passes back through the so-called `liquid-crystalline' state before solidifying again. Generally, it is rod-shaped molecules, such as 4-n-hexyl-4'-cyano biphenyl, that exhibit liquid-crystal behavior.Because of the relatively weak forces between the molecules of a liquid crystal, the interactions between the molecules and hence their relative orientations can be changed not only by temperature and pressure but also by electric and magnetic fields. The effect of temperature on the color of liquid crystals has led to their use in the detection of tumors that are `warmer' than surrounding healthy tissue. Their most familiar application is in the liquid-crystal displays (LCDs) used in watches and calculators, where their optical properties are controlled by applying electric fields to change the orientation of the molecules in the liquid crystal.</span></text>
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<text><span class="style10">he liquid-crystal phase</span><span class="style7"> occurs between the solid (ordered) and liquid (disordered) phases. The intermediate phase may take a number of slightly different forms, of which 'smectic' and 'nematic' are examples. The characteristic orientations of the molecules of a liquid-crystalline substance are due to the long, rod-like shape of the molecules, which allows a weak, long-range order to develop between each molecule and its neighbors. A typical example is 4-n-hexyl-4-cyanobiphenyl.</span></text>
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<text>ΓÇó ELECTRICITY IN ACTIONΓÇó NATURAL COMPOUNDSΓÇó CHEMICALS IN EVERYDAY LIFEΓÇó RUBBER AND PLASTICSΓÇó CHEMICALS AND BIO TECHNOLOGY</text>
