Growing Crystals, Supersaturation and the Beauty of Science
I wish I could sort through every single belonging of mine more often.
I’m moving house soon and that’s been filling my days with getting sidetracked with toys from my childhood I haven’t thought about in years. On the top shelf, at the very back of my cupboard, I found a crystal-growing kit gifted to me many Christmases ago. (Earlier that day I also found my first ever microscope, and spent too many hours staring at various parts of pea pods and was disappointed to discovered the 200x and 300x lenses were too dirty to function anymore — but that wouldn’t have been as interesting a story.)
Curiosity. That’s how my first inklings of scientific interest began.
Back in the day, with my competency low and my ignorance high, I would grow crystals in small dishes with a solution of salt and water, and wait days for the water to evaporate and leave a couple of wimpy crystals and feel cheated by the pictures of amazingly large crystals on the box. If I couldn’t resist staring at pea pod cells, how could I resist putting my increased knowledge about chemistry into practise? This time around, I decided to supersaturate the solution.
Saturation is a common term in chemistry. In relation to crystal solutions, it means you have reached the threshold of adding salt into the water: it can’t hold any more dissolved salt, and whatever salt is left remains solid.
Supersaturation, on the other hand, once brought to mind an image of a french fry with a cape around its shoulders, underpants over tights and a swoon-worthy smile. But in reality, it just means more salt was dissolved in the water than would have been possible in normal circumstances. These aren’t all that freaky, really — carbonated water is a common supersaturated solution of carbon dioxide and water, which is created through adding the gas at high pressure. (This is the reason soft drinks slowly go flat after you open them — lower pressures let the gas escape the supersaturated solution.)
My means of supersaturation, however, was heat: the wonderful stove top. And the first step to supersaturation? Saturation.
My salt was monoammonium phosphate (with added red food colouring for some aesthetics by the manufacturer), and I just poured (a suprisingly large amount of) the salt into water bit by bit until it stopped dissolving. Easily saturated. With this combination of salt and water, it was approximately half salt and half water at this point.
To reach supersaturation, I heated the saturated solution on the stove, gradually adding more salt as the temperature increased. I think the water was around 70ºC before I ran out. Once heated, water holds a ridiculous amount of salt: I believe I reached at least double the original concentration before my maroon monoammonium phosphate had all disappeared into the rapidly-reddening liquid. (I’ll have to use another colour next time.) I switched the stove off and went to change the volume on the TV, leaving my fully-dissolved red solution for a moment. By the time I came back, a floating disk of crystals had formed over the top of the jar, and no doubt small crystals were popping up on the bottom of the jar as well.
Supersaturation is a delicate state, not very stable and quick to find another option. Salt doesn’t like to be packed all together like that with so little water, hence the pretty rapid crystal formation. In fact, super-anything is hard to achieve and maintain — chemicals enjoy being at ease; in their comfort zones, if you will. You need careful conditions for supersaturation, or super-heated steam, or superconductivity.
So now I have some pretty crystals, which I can use to create bigger pretty crystals! And everyone knows that once you have big, pretty crystals, you have to stare at them and simply marvel at science.
[Creative Commons licensed Flickr photo by jonrb]