I. Microcrystals while you wait
The idea of this is to grow crystals of coloured metal compounds in drops of water on glass slides, under a biological microscope. A drop of water will completely dry up and crystallize in 30 minutes, so if you start 'em 15 minutes before class, students can observe the progress of crystal growth They don't quite grow visibly, but almost: have the students check them every 2 minutes or so
Materials:
Biological (transmitting) microscopes. Stereoscopic mikes are no good for this. Glass slides (high school biology labs have lots of these). In dropper bottles, saturated solutions of Copper Sulphate and Nickel Chloride (my favourites; but any water- soluble transition metal compound is worth trying, because all you need are nice coloured crystals. Ask the storeroom techy for suggestions). Note: saturated solutions just mean that the water has taken up all the solute that it can. An easy way to guarantee this is to mix enough of the powder into the water so that some sits on the bottom of the bottle.
What to do:
Not much. Put drops of the solutions on the slides and put them on the microscopes. The light from the tight sources on the scopes heats up the water and hastens the drying-up process. At first the crystals will be really small and scattered. At mid- stage (20 minutes) you have the most beautiful array of large ones. In the latest stage, quench crystals with irregular tree-like shapes grow rapidly from the edges of the drop inwards. These are like hoar frost on a window pane. A few minutes after they start, the water is gone and crystallization is complete. Safety considerations: these compounds are poisonous: no one should drink them, lick the slides, or run them on their skin. No panic, but ensure responsible lab behaviour.
What can be learned:
- This experiment models the growth of natural crystals like quartz, only on a very compressed time scale. Quartz crystals in nature probably grow over thousands of years.
- The round water droplet is like a cross-section of a natural geode, especially with the crystals that grow in from the sides. Geodes in nature form in holes in the rock.
- Each compound or mineral has its own characteristic crystal form Size doesn't matter: form does. Get the students to sketch the various crystals that they grow. Another easy couple are salt and sugar, which can be viewed under a stereoscopic mike. Salt forms little cubes, while sugar crystals are less symmetrical, with less right angles between faces. The reason for the one compound-one form rule is because of the way the atoms arrange themselves, which is characteristic of the compound in question.
II. Sugar crystal growing for younger kids
Take a mason jar. Now boil some water on the stove and add as much sugar to it as you can, until no more will dissolve. Let it cool add a bit of food colour "just for fun", and pour it in the jar. To give the baby crystals something to grow on, use a bamboo kebab skewer or a string tied on a stick In either case, place inside the liquid. (I tie the string to a paper clip to ensure that it sinks). Place some paper over the open mouth of the jar to keep out dust (blah) and flies (ecch), because of course this is an edible experiment. Now put the jar on a sunny window sill (to aid in evaporation) and then forget about it for roughly 2- 3 weeks. This is how long it takes for supersaturation to take over. When it does, crystals will start to form on the water surface. Keep breaking them up, they're not what you're after. At 3 weeks, a sheath of fine crystals should appear on your stick or string. Now you just keep breaking the crust and watching your sweet snack grow to edible proportions. No reason not to have multiple sticks/strings in there, more for everybody. If you're more patient than greedy, you will get a fine set of sparkling sugar crystals to, eventually, eat.
Bon appetit!
JoAnne
Nelson
(British
Columbia
Geological
Survey)