Effect of Surface Texture on the Growth of Crystals?
Category: Chemistry—Physical Changes
Project Idea by: Sarah and Rachel Cathey
A solid is the state of matter in which a substance has no tendency to flow under moderate pressure, resists forces that tend to deform it, and retains a definite size and shape. Simply stated, a solid has a definite shape and volume. Apples, books, rocks, and even your body are examples of solids.
Solids are very difficult to compress (squeeze together). This is because at the microscopic level (the level at which particles are so small that they cannot be seen except by magnification), the particles making up a solid are more closely packed together than liquids and gases. All particles vibrate (move back and forth), but those in solids do not move far from their places and cannot flow over or around one another. This results in solids having a definite shape and volume, and it is the reason that other matter cannot penetrate, or pass through, a solid. Crystals are solids with particles arranged in a regular, repeating pattern with flat surfaces. Most solids are crystalline solids, which are solids made of crystals. For example, when liquid water freezes, the water particles link together, forming ice crystals. A large piece of ice is made up of many small ice crystals that fit together like a puzzle. The shape of a crystal depends on the arrangement of the particles within it.
A unit cell is the smallest group of particles within a crystal that retains the geometric shape of the crystal. For example, the unit cell of a table salt crystal is cubic. For salt crystals to form, the cubic cells must be able to move about so that they can be positioned with their faces (the flat surfaces of a solid) against one another, much like stacked boxes.
One way that crystals form is from a solute precipitating (separating) from a solution. Precipitation occurs when there is more solute than can dissolve in the solvent. As the water evaporates in an aqueous salt solution, the solution becomes more concentrated with salt; that is, as the amount of water decreases, there is more salt mixed with a smaller amount of water. When there is more salt than can dissolve in the remaining water, the salt precipitates. As the salt (solute) precipitates, its particles bond (link) together, forming unit cells that comprise a crystal. The more unit cells that bond together, the larger the crystal.
Crystals are found in many places, including inside rocks, on the ceiling and floors of caves, and in candy. Sugar crystals are grown on strings and sold as candy called rock candy. When a unit cell sticks to a surface and is stationary, it is more likely that other unit cells will bond to it. A project question might be, "What effect does surface texture have on growing crystals?"
Clues for Your Investigation
Decide on the type of crystal to grow, such as table salt, alum, or Epsom salts, as well as the procedure used for crystal growth. Grow the crystals from equal volumes of solution in identical containers. Place different surfaces in the solution such as a coiled wire and a coiled pipe cleaner. Determine how to measure the results; for example, at the end of a predetermined period of time, compare the amount of each test surface that is covered with crystals.
Independent Variable: Surface texture
Dependent Variable: Amount of surface covered with crystals
Controlled Variables: Type of solute; concentration of solute; amount of solution; type of container; temperature, humidity, light, and other environmental factors
Control: Container of solution with no hanging surfaces
Other Questions to Explore
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