Crystals Crystals What is a crystal? A crystal is a solid substance with definite geometric shapes and molecules that are arranged in a repeating pattern (Comptons ). Crystals have fixed angles between its faces, which have distinct edges. If the faces of a crystal can reflect light, then it will sparkle (Stangle). Crystals have constant angles because of the regular arrangement of its particles. There are seven types of crystals: cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic (Dr.
Boyle). Crystals are classified by the shapes of their lattice or the regular, periodic configuration of particles (American Heritage). There are two different types of lattices. There are primitive lattices where there is only one lattice point per unit cell. There are also non-primitive lattices. They are the ones with more than one lattice point per unit cell. Crystals can also be classified by their physical/chemical properties.
There are four types of crystals classified this way. They are covalent, metallic, ionic, and molecular (Dr. Boyle). A covalent crystal is a crystal which is one big molecule. They usually have extremely high melting points. An example of a covalent crystal is a diamond and zinc sulfide (Dr.Boyle).
Metallic crystals have high melting points and densities. They are metal atoms which sit on lattice sites while the outer electrons from these atoms flow freely around the lattice (Dr. Boyle). Ionic crystals are crystals where the individual atoms do not have covalent bonds among themselves. These atoms are held together by electrostatic forces. They are also very hard and have a relatively high melting point. Sodium chloride (NaCl, salt) is an example of an ionic crystal (Dr.
Boyle). Molecular crystals are crystals where there are recognizable molecules in the structure and the crystal is held together by non-covalnet interactions such as hydrogen bonding (Dr. Boyle). Sugar is an example of a molecular crystal. These crystals tend to have low melting points and are soft (Dr.
Boyle). Crystals start growing by a process called nucleation. They are produced in solutions. The temperature and pH balance of the solutions must be controlled well. Crystals start growing one of two ways: unassisted nucleation, crystals that start with the molecules themselves or assisted nucleation, with the help of some solid matter already in the solution (Dr. Boyle).
In unassisted nucleation, molecules of the solute are in the solution. Most of the time the solute molecules only see the solvent molecules around them. Sometimes the solute molecules can see other solute molecules and they become attracted to one another if the compound is solid and pure (Dr. Boyle). These molecules will stay together for a little while and will eventually be separated by other internal forces. However, sometimes the molecules will stay together long enough to meet up with a third, then a fourth, and then even a fifth solute molecule. When this happens the combined attraction force eventually becomes stronger than the other forces within the solution (Dr.
Boyle). This attraction force tends to disrupt the formation of these aggregates. When this happens a protocrystal or a pre-crystal becomes a nucleation site (Dr. Boyle). As the protocrystal floats around in the solution, it encounters other solute molecules.
These other molecules feel the attractive force of the protocrystal and decide to join the group of molecules (Dr. Boyle). This is when the crystal begins to grow. Crystals grow form the outside instead of the inside. The crystal molecule grows until it can no longer remain dissolved in the solution and falls out of the solution (Dr. Boyle).
After this happens, the other solute molecules grow on the surface of the crystal. The crystal gets bigger until there is an equilibrium, or a state of a chemical reaction in which a forward and reverse reaction occur at equal rates so that the concentration of the reactant and product do not change with time (American Heritage). When there is equilibrium between the solute molecules in the crystal and the solute molecules in the solvent the crystal no longer get bigger (Dr. Boyle). In assisted nucleation the same process is followed as in unassisted nucleation.
The only difference is that a solid surface such as a stone or brick acts as a meeting for all of the solute molecules. The solute molecules encounter the surfaces and stays on it for a certain amount of time before randomizing forces of the solution knock it off. If the solute molecules remain, they start to form a crystal. This is where protocrystals are formed (Dr. Boyle). If theres more solute molecules in a given volume, then there will be a larger chance that theyll meet one another. The solution should not be heated up because it acts as the major randomizing force in which causes the aggregates of the molecules to break up (Dr.
Boyle). This means that if the solution in which a crystal is to be produced is heated up, the crystals will not grow or they will start to grow and eventually break apart. When making crystals at home, the easiest thing to do is to grow them from a solution. There are two different types of solutions that can be used . They are solids such as salt or sugar in water and a liquid in water such as ammonia (Stangle).
When growing crystals like this, the crystal is produced by a process of dissolving, absorbing, evaporating, and crystallizing . The ammonia speeds up the evaporation process which allows the crystals to form more rapidly (Stangle). When making assisted nucleation crystals at home, rock, charcoal, paper towels, aluminum foil, porous rock, or sponge can be used as the porous material or solid surface (Dr. Boyle). When salt is dissolved in water it is absorbed by the porous material.
Then, the salty solution is evaporated and the mineral residue or salt is left behind and recrystallizes. This process is repeated until there is not any more liquid to be evaporated (Dr. Boyle). The shape of a homemade crystal is determined by the type of substances used in the solution. Salt crystals are cube like, with flat sides.
Alum crystals have eight triangular faces and sugar crystals are oblong in form and sharply slanted at either end (Stangle). To grow alum crystals boil a cup of water. Stir in three tablespoons of alum, a white powder used as baking soda, until it dissolves. Pour the solution into a clean jar and cover it with plastic wrap. Place the jar in a place where it will not be disturbed for several days (Growing Crystals). To grow a salt crystal, places charcoal in a dish.
Stir salt into some warm water until no more salt can be dissolved. Add a spoonful of vinegar to the solution and pour it over the charcoal. The vinegar will degrees the charcoal stones, allowing capillary action to carry the salt water to the surface so it can evaporate, leaving salt crystals (Growing Crystals). To grow a crystal garden by using the assisted nucleation process mix three tablespoons of water, laundry bluing, and salt in a container. Then, slowly add one tablespoon of ammonia to the mixture and mix well.
Then, carefully spoon the solution over charcoal, paper towels, or any other porous material (Stangle). The scientific definition of a crystal is based on its internal structure rather than its outward appearance. In order for a substance to be a crystal its molecules must be arranged in a repeating pattern. A single crystal grows in size as additional molecules of the material settle on it (Comptons). Sometimes the atoms can combine singly instead of first forming molecules to make a large crystalline mass (Crystals).
Seed crystals are immersed in a solvent that contains typically about ten to thirteen percent of the desired solution. This means that the protocrystals are subsided in ten to thirteen percent of the desired solution (Britanica). The growth of a crystal is faster than vapor growth because there is a higher concentration of molecules at the surface in a liquid as compared to a gas , but it is still relatively slow. Crystals can be grown under moderate conditions from all ninety-two naturally occurring elements except helium. Helium can be crystallized only at low temperatures by using twenty-five atmospheres of pressure (Britanica). For a crystal to be created in nature can last from ten years to ten centuries.
These uniquely structured substances are used for many things. They are used in food, cutting, building, in the field of medicine, and in many other things. Not only are crystals useful in the everyday life of humans, but they are also beautiful creations of mother nature. Bibliography American Heritage Electronic Dictionary. 3 ed. Houghton Mifflin Company.
1992 Britanica Online. vers. 98. 2. 2000. Encyclopedia Britanica.
14 May 2000. . (crystals). Comptons Encyclopedia Online. v3.0. 1998.
Comptons Encyclopedia.14 May 2000. . (crystals). Crystals. Comptons Encyclopedia. 1969 ed.
Dr. Boyle. Frequently Asked Questions about Crystals for Students. 14 Aug. 1998 .
Growing Crystals. . Stangle, Jean. Crystals and Crystal Gardens You Can Grow. New York. 1990.