Crystals are any solid material in which the component pieces (atoms, small molecules, or even proteins) are packed together very closely in a regularly ordered, repeating pattern. For example, table salt is made of sodium (Na) atoms and chlorine (Cl) atoms, repeating one after another. For instance:
Na -- Cl -- Na -- Cl -- Na -- Cl
Cl -- Na -- Cl -- Na - Cl -- Na
Na -- Cl -- Na - Cl -- Na -- Cl
and so forth, extending in all 3 dimensions.
Crystals form when conditions are right for the small components to come together in a regular way, usually by cooling them slowly or removing their solvent (the liquid they are suspended in). Sugar is a good example-- Table sugar is composed of a crystal of small molecules of sucrose. These form naturally when you take a solution of sucrose in water and allow the water to evaporate away. The sucrose molecules have some natural attraction to order them, and get pushed slowly closer together as the water leaves, forming a crystal.
When materials are cooled very quickly, or have a mixture of substances which do not easily pack together tightly, sometimes a crystal cannot form, but the material will still be solid. Such materials are called amorphous materials. A good example is glass. Glass is made of a mixture of silicon, boron, and other elements. The mixture is heated to very high heat in an oven, then quickly cooled. The result is clear glass, which doesn't display crystalline edges.
In a sense, science uses crystals almost all the time-- almost all chemicals that one could use exist as crystals when they're sitting on the shelf! But I can mention a few specific ways that crystals are useful to science. First, certain crystals (called piezoelectric crystals) react to electricity by flexing and moving, which has a wide variety of applications from making some loudspeakers to making very precise motors. Piezoelectric quartz crystals are also used in almost every electric clock on earth (except for the atomic clocks) to keep time. Another example of crystals in science is their use in protein crystallography. To study the structure of a protein, biochemists will sometimes cause a large amount of that protein to form a crystal (easier said than done in most cases!). This crystal can then be hit by X-rays. The pattern of x-rays scattering off ofthe protein crystal can be used to deduce the atomic structure of the component proteins.
Crystals grow slowly, atom by atom or molecule. You can do an experiment at home. Take a glass of hot water and stir in salt or sugar until you can't get any more to dissolve. Then let it cool and see the crystals grow. More salt and sugar can dissolve in hot water than cold, so crystals form when the water cools, or evaporates. Little crystals form if the cooling is fast, and big crystals form if the cooling is slow.
Crystals have lots of uses. Jewelry of course. Quartz crystal watches. All our computers and stuff need silicon crystals.
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