Supersaturation
Supersaturation is when water holds more dissolved material than it normally can, so crystals form as it cools.
Hot water holds more sugar than cold water does. Stir a lot of sugar into hot water and it all dissolves. As the water cools, it can no longer hold that much sugar. The extra sugar falls out and forms crystals on the bottom of the cup.
Explaining supersaturation by grade level
When you stir Borax powder into very hot water, it all disappears. But as the water cools down, it cannot hold that much anymore. The extra Borax has to go somewhere, so it stacks up into tiny, shiny crystals you can see and touch. The cooler the water gets, the more crystals appear.
Projects that explore supersaturation
When water is supersaturated, it holds more dissolved material than it normally can — and crystals form as it cools. You dissolve Borax in hot water, then suspend pipe cleaners in the solution. As the mixture cools, the dissolved Borax comes out of the water and stacks into solid crystal structures along the pipe cleaner. After 12 to 24 hours, sparkling crystals coat the shape you bent.
Hot water can hold more dissolved material than cool water — that is what supersaturation means. When you heat water and keep adding sugar until no more will mix in, you have a supersaturated solution. As the liquid cools, the extra sugar can no longer stay dissolved. Those sugar molecules need somewhere to go, so they stack onto the string you have hung in the jar and slowly build into large crystals you can eat.
When water is supersaturated, it holds more dissolved Borax than it normally can. You dissolve Borax in hot water and pour it into a hollow eggshell — the eggshell acts as the rock cavity where crystals grow. As the solution cools, it can no longer hold all the Borax, so the extra Borax forms crystals along the inside walls. The result looks like a miniature geode with sparkling crystal points.