Up Close with Crystals

Can you tell the difference between salt and sugar without tasting them? With a microscope, you can see that their crystals are different shapes. Crystals come in all sorts of shapes and sizes and are fun to look at under a microscope! If you grow your own crystals, they will have more regular shapes than if you just look at them straight out of the saltshaker or sugar bowl, where they have been chipped until they no longer have their natural shapes.

Materials

• Microscope
• Plain glass slides
• Any of the following: salt, sugar, Epsom salts, alum, fresh juice from orange, lemon, rhubarb, etc.
• Polarizing film or the lenses from some polarizing sunglasses (optional)

What To Do:

1. Pour a tablespoon of hot water into a cup and then mix in salt until no more salt will dissolve. Take a drop of this saltwater mixture and put it in the center of a clean glass slide. Repeat this process with other powders you want to test. For fruit juices, just place a drop directly on the slide.
2. Set the slides in a windowsill and let all the liquid evaporate. Depending on the temperature and other factors, this can take anywhere from an hour to over a day.
3. When all the liquid has evaporated, look at each slide with the microscope at different powers. (Adjust the microscope diaphragm to get the best contrast.) Salt forms small cubed-shaped crystals, but other substances may have feathery patterns, hexagons, and more.
4. Many crystals are transparent and can be hard to see. You can see the details better (and sometimes beautiful colors, as well) if you use polarizing film. Place one piece of film over the light source below the microscope stage. Turn the second piece of film so that it is at right angles with the first piece and place it over the slide on top of the stage. You may have to adjust the light intensity and/or the microscope's diaphragm to get the best contrast. Look at the sample at different powers - the crystals will show up on a dark background, sometimes with colors. (If you don't have polarizing film, you can try this with the two lenses from a pair of polarizing sunglasses. Before putting them on the microscope, line up the two lenses until you can't see through them; this may not be exactly right angles like the film is.)

What's Happening?

As the water evaporates, the salt or sugar atoms join together to form regular patterns. These patterns are different depending on what substances they are made from. When crystals are viewed with polarized light, they split the light into different colors. This makes them easier to see, and also helps scientists identify what type of crystals they are based on the colors they have.

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