Q: How do crystals form in borax ornaments?
When I was younger, I used to make ornaments for my family members every year. Sometimes they were Popsicle reindeer; others were acrylic paint in clear bulbs. One of my favorite ornament batches involved some borax and a bit of chemistry.
Full directions to make borax ornaments can be found here, but the gist is to dissolve borax in hot water, hang a pipe cleaner shape in the borax solution, and wait for the borax to crystallize as the water cools.
What’s the science behind these?
First, borax is another name for sodium tetraborate decahydrate, also known as sodium borate, sodium tetraborate, or disodium tetraborate. Its chemical formula is Na2B4O7 • 10 H2O. (This means that for every Na2B4O7 compound, there are ten water molecules.) Borax is used in laundry boosters, hand soaps, and toothpaste.
When in a powdered form, borax consists of tiny crystals, which can easily dissolve in water. To make the borax ornaments, you dissolve as much borax into water as you can. This is done by manipulating the solubility of borax in water.
Solubility, simply put, is the ability for a substance to dissolve in another. We dissolve a solute into a solvent. Solutes and solvents may be solid, liquid, or gas. In this example, borax is the solute and water is the solvent- we dissolve borax into water. Temperature and pressure affect how much solvent can be dissolved within a solvent. The solubility of most solids increases with temperature, and more of the solute can be dissolved. At higher temperatures with increased kinetic energy, solvent molecules can break apart solute molecules more easily, as the solute molecules experience increased vibration and less able to stick together.
By heating up the water, more borax can be dissolved in the warm water than in cold water. The idea is to add borax until the solution is completely saturated, meaning no more borax can be dissolved into the water. As the water cools, the molecules slow down and come closer together, forcing the borax out into crystals. This process is called crystallization, in which molecules or atoms form into crystals, organized structures with a specific repeated pattern. The pipe cleaners provide a source for the crystals to cling to. If you make these borax creations, you’ll likely find borax crystals along the internal surface of the cup as well as on the pipe cleaner.
Borax dust may cause skin and eye irritation, and it should not be inhaled. For the most part, as long as you don’t lick it, breathe it in, or coat your skin in it, borax is safe to use. Still, it’s always better to be safe, so use caution when handling.
Keep calm and science on.