Growing your own crystals on a string is a popular science experiment that demonstrates fundamental principles of chemistry. This accessible project involves dissolving a crystalline substance in hot water and then allowing the dissolved particles to slowly precipitate onto a suspended surface. The method uses a simple cotton string or pipe cleaner as a scaffold, offering a rough texture that encourages the initial formation of solid structures.
Essential Supplies and Preparation
To begin this experiment, you will need a few simple household items, starting with the crystal-forming compound, sodium tetraborate, commonly known as borax. The solvent is ordinary tap water, which must be heated to allow maximum material dissolution. You will also need a heat-safe glass container, such as a mason jar or beaker, and a stirring utensil. Select a piece of cotton string or a pipe cleaner for the growth surface. Finally, a pencil or dowel is required to suspend the string within the container.
Creating the Saturated Solution
The most important step is creating a supersaturated solution, which means dissolving more of the borax solute than the water would normally hold at room temperature. Start by bringing the water to a full boil on a stovetop, as the increased heat makes more space for the solid particles to dissolve. For an effective ratio, aim to dissolve approximately three tablespoons of borax powder for every one cup of boiling water used. Add the borax one spoonful at a time, stirring continuously until it completely disappears into the hot liquid. Continue adding the powder until you reach the point of saturation, where excess material begins to collect at the bottom of the jar.
Pour the clear liquid portion of the supersaturated solution into your clean glass jar, being careful not to include any of the undissolved powder. If you wish to grow colored crystals, add a few drops of food coloring to the hot solution, stirring gently to ensure the color is evenly distributed. Allowing the solution to rest for a few minutes before adding the string prevents the excessive heat from potentially damaging the growth surface.
Setting Up the Growth Environment
Once the solution has cooled slightly, set up the string, which acts as the nucleation site for the crystalline growth. Tie one end of the cotton string or pipe cleaner to the center of a pencil, skewer, or dowel. The other end of the string should hang freely. Carefully place the pencil across the rim of the glass container so that the string is completely submerged in the liquid. It is essential that the string does not touch the sides or the bottom of the container, as this will result in crystals forming on the glass instead.
The entire setup must be moved to an area that is cool and free from vibrations, drafts, or sudden temperature fluctuations. A slow, steady cooling process over many hours encourages the formation of larger, more defined crystalline structures. To minimize dust contamination and slow the cooling rate further, the jar should be loosely covered with a piece of paper or a thin cloth. For the best results, the string should be left undisturbed for at least 12 to 24 hours. After the desired growth has been achieved, gently lift the crystal-covered string from the solution and set it on a paper towel to dry completely before handling.
Understanding Crystal Formation
The success of this experiment is rooted in two chemical principles: supersaturation and heterogeneous nucleation. Supersaturation is a temporary, unstable state where a liquid contains more dissolved solute than it can normally hold at that temperature. The high heat of the water initially allows the borax molecules to remain suspended in the solution.
As the liquid cools, the water molecules slow down and move closer together, which reduces their ability to hold the large amount of dissolved borax. This forces the excess borax molecules out of the solution. The suspended string provides a rough, foreign surface, known as a nucleation site, which allows the excess molecules to easily precipitate out of the liquid and begin forming a solid structure. The molecules arrange themselves in an organized, repeating pattern characteristic of the specific crystalline material. The slow cooling allows the crystal lattice to grow steadily, resulting in visible, geometric forms.