Crystal Formations

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Research Questions

1.  What is a crystal?
Crystals are solids that form by a regular repeated pattern of molecules connecting together. In some solids, the arrangements of the building blocks (atoms and molecules) can be random or very different throughout the material. In crystals, however, a collections of atoms called the Unit Cell is repeated in exactly the same arrangement over and over throughout the entire material.

2.  What substances can be used to make crystals?
Common Chemicals Used for Crystal Growing
Aluminum Potassium Sulfate
Ammonium Chloride
Calcium Chloride
Sodium Nitrate

3.  What are some examples of crystals in nature? What household items can you also grow crystals from?
Garnet, Quartz, Apophyllite, Danburite, Staurolite and Amazonite are all crystals that occur naturally.
Water, salt, string, pencil, container. Boil about 1 cup (250 ml) water.Add 1/4 cup salt.The solution will be supersaturated when salt crystals begin to collect on the bottom of the pan.

4.  Explain the process of crystal growth.
Crystal growth is a major stage of a crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of a crystalline Bravais lattice. The growth typically follows an initial stage of either homogeneous or heterogeneous (surface catalyzed) nucleation, unless a "seed" crystal, purposely added to start the growth, was already present.

5.  Explain how crystals can grow in different shapes and sizes.
Crystals are atoms that are arranged in regular geometric shapes. The varying shapes of crystals has to do with its formation. Factors that go into the formation of crystals, which lead to differing shapes, are temperature, pressure, and chemical conditions.

6.  Outline a few different types of crystals.
Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange
Shapesmonoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids
Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together)
Tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms
Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division
Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon
Cubic - not always cube shaped!
You'll also find octahedrons (eight faces) and dodecahedrons (10 faces).

Another way to categorize crystals by their physical/chemical properties. In this classification you have four types of crystals:
Covalent Crystals:This is a crystal which has real chemical covalent between all of the atoms in the crystal. So really a single crystal of a covalent crystals is really just one big molecule. An example of this is a crystal like diamond or zinc sulfide. Covalent crystals can have extremely high melting points.
Metallic Crystals:Individual metal atoms sit on lattice sites while the outer electrons from these atoms are able to flow freely around the lattice. Metallic crystals normally have high melting points and densities.
Ionic Crystals:This is a crystal where the individual atoms don't have covalent bonds between them, but are held together by electrostatic forces. An example of this type of crystal is sodium chloride (NaCl). Ionic crystals are hard and have relatively high melting points.
Molecular Crystals:This is a crystal where there are recognizable molecules in the structure and the crystal is held together by non-covalent interactions like van der Waals forces or hydrogen bonding. An example of this type of crystal would be sugar. Molecular crystals tend to be soft and have lower melting points.

7.  What effect do crystals have on light travelling through them? (relate to reflection, refraction, dispersion and diffraction)

8.  What are the optimum conditions for crystal growth?
Because warmth is key to forming crystals, the jar's surroundings should be warm also for optimum crystal growth. Warm air temperature aids water evaporation, causing the crystals to grow more quickly. Crystals will still grow in cooler temperatures, but it will take much longer for the water to evaporate.

Crystal growth also requires light. Again, the crystals will eventually grow in the dark, but it will take a very long time. Light evaporates water as heat does; combine them by placing your jar on a warm, sunny windowsill and you should have crystals in a few days.

9.  Make a plan for next week (as a list);

-  Tuesday – method + materials
Make sure all apparatus is clean and dry.Place approximately 25 g of potash alum in a beaker (250 ml or bigger) and add approximately 170 ml of hot water. Stir the mixture until all the crystals have dissolved. The solution may have a slightly cloudy ormilky appearance due to impurities in the technical grade of alum used.(NOTE: don’t use all the alum you have been given – keep a few crystals back in reserve in case you need to “seed” the solution later.)Filter the warm solution through a filter funnel (in which filter paper has been inserted) into another clean beaker. Cover the beaker of solution with the watch glass or fresh filter paper and set aside in a cool sheltered place and allow it to stand undisturbed overnight. The first beaker and funnel can be washed and dried and the residue and filter paper used for filtration discarded.

-  Wednesday – method + materials
The next day, observe the beaker of solution. The bottom should have become covered with a layer of smallish crystals which formed spontaneously as the solution cooled. Carefully decant (pour off) the clear solution above the crystals into a clean beaker and set it aside for later, leaving the crystals behind in the other beaker. If for any reason, no crystals have formed after leaving the solution to stand overnight, the solution can be “seeded” by adding a crystal from the original alum. Let it stand overnight again and observe the formation of crystals.

-  Thursday – method + materials
From the bed of crystals, one good symmetrical crystal or group of crystals needs to be selected to act as “seed” for your big crystal. Using a plastic spoon, spatula, tongs or tweezers, transfer the selected crystal to the beaker containing the decanted solution, trying to place it centrally in the beaker. A good technique to promote uniform growth is to suspend the crystal with a nylon thread tied round a stirring rod or pencil resting on the rim of the beaker. This step is not essential, however, and good a crystal can usually be obtained just by leaving the crystal on the bottom of the beaker.Page 3 of 4Take a picture (if possible) of the crystal, preferably close to a ruler so that you can measure its size, and mark the level of the liquid in the beaker. Record the date in your diary.Cover the solution with a loose-fitting paper hat that permits water to evaporate slowly whilst keeping out dust. Allow the solution to stand in a draft free location, not in direct sunlight or near a heater. The aim is to keep the temperature as constant as possible.

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