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CRYSTALS!! !! West Point Schools Slide 2 Is it a Crystal? Yes NO Slide 3 What is a Crystal? A crystal is a solid substance made of atoms, molecules or ions that form regular repeating patterns called a crystal lattice. The regular patterns of atoms in crystals cause crystals to grow or cleave in geometric shapes. The atoms in glass are not arranged in regular patterns. Slide 4 Crystals formed from Sublimation Phenol CrystalsIodine Crystals Slide 5 Crystals From Solution Nickel (II) sulfate Ferrous Ammonium Sulfate Chrome Alum Slide 6 Crystals from Evaporation NaCl crystals Vanadyl Sulfate crystals Ferrous Ammonium Sulfate Slide 7 Atoms, Ions, Molecules and the Unit Cell The unit cell is the smallest portion of the entire crystal that contains all of the identifying characteristics of the crystal. Crystals form as a result of bonds (attractions) between atoms, ions and molecules. The particles align themselves to maximize the number of attractions (for example, Cl and Na + attract each other). Slide 8 Symmetry of Crystals Crystals form in seven geometric forms called crystal systems. The crystal systems are named cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral. Each crystal system has a unique shape. Slide 9 Cubic Cubic crystals have all edges of equal length. Cubic crystals have all angles at 90 0. These crystals also have 3-fold axes. Pyrite Crystals Halite, NaCl Slide 10 Tetragonal Crystals that form the tetragonal crystal structure have 2 of their 3 edges having the same length. All of the angles are 90 0. These crystals also have one 4-fold axis. Slide 11 Hexagonal Crystals that form a hexagonal shape have 2 of 3 edges that are the same lengths. The angles are of different degrees. One angle is 90 0, another at 60 0, and the third at 120 0. Slide 12 Orthorhombic In a crystal showing the orthorhombic formation, all three sides have different lengths however all angles are 90 0. Slide 13 Monoclinic A crystal exhibiting a monoclinic structure will have different lengths for each side, will have two angles equaling 90 0 and one angle that does not. Slide 14 Triclinic A crystal that exhibits a triclinic structure will have all sides of different lengths and all of the angles are also different. Slide 15 Rhombohedral Crystal showing this structure will have all sides equal however, none of the angles are equal. Slide 16 Optical Properties of Crystals Fluorescence the crystal will glow in the dark Above: Crystals in white light. Left: Crystals in UV light. Slide 17 Optical Properties of Crystals Interference the making of rainbows Above: A bismuth crystal that shows color due to the accumulation of oxides. Left: A portion of mica that forms rainbows as light goes through it. Slide 18 Optical Properties of Crystals Birefringence the splitting of light to cause double vision. Slide 19 Setting Up the Experiment Measure 50 mL of water into a graduated cylinder. Make sure the meniscus, thats the bottom of the curve at the top of the liquid, is touching the 50 mL line. Slide 20 Setting Up the Experiment To start the experiment, you will need to obtain a graduated cylinder and a plastic cup. You will be making a device for a standard measurement. Slide 21 Setting Up the Experiment Pour the 50 mL of water into the plastic cup. Using a permanent marker, draw a line around the plastic cup at the top of the water level. You will now have your standard liquid measurement for most of the crystal labs. Slide 22 Setting Up the Experiment Put spoonfuls of solute into the styrofoam cup. The amount of spoonfuls are given in the lab write up. Pour water from clear plastic cup into styrofoam cup. Stir water and solute until all solute is dissolved. Slide 23 Setting Up the Experiment Cut a piece of string slightly longer than the height of the cup. Tie one end of the string to the plastic spoon. Place the other end of the string into the solution. Rest the spoon across the cup as shown. Cover the cup with a piece of paper or paper towel and leave undisturbed overnight to one week. Slide 24 Results of the Experiment Remove string from solution and examine crystals. Slide 25 Crystals Made of Two Compounds Chrome Alum is a crystal that is formed from the simultaneous growing of potassium chromium sulfate and potassium aluminum sulfate. Here the chromium and the aluminum are distributed throughout the crystal based on the concentration of the potassium chromium sulfate. Slide 26 Chrome Alum By examining the crystals, a color difference is seen. The intensity of the purple increases with increased concentration of KCrSO 4. Slide 27 Substitution of Cr for Al in Alum Crystals The deep purple dots stand for Cr, the pale dots stand for Al Slide 28 Crystals!!!