Crystal Clear

The last couple of activities have talked about crystals, but what are they? Is a crystal just another name for a solid? And are all crystals the same?

Crystals are always solid, but not all solids are crystals. Glass and many plastics are examples of solids that are not crystalline. In a crystal, the atoms or molecules are piled together in a very neat, organized way, and they're often arranged in familiar shapes like these:The names may seem funny to you, but you probably recognize squares, rectangles, and hexagons. Each of the colored balls represents an atom or molecule. For example, table salt (sodium chloride) has a cubic crystal structure. Half of the balls are sodium atoms, and half are chlorines. These shapes are just building blocks. A real crystal would have zillions of these blocks, all exactly the same, all piled up one on top of another. Table sugar, or sucrose, is made up of monoclinic blocks, with the green balls representing individual sugar molecules.

When you made jack wax and nut brittle, you learned two ways to keep large crystals from forming - by cooling quickly, or by adding some other substance. Let's talk about why these methods prevent crystallization. Imagine that you're a young child again, trying to make a tower out of building blocks. It's easy to get the first few blocks in place, but then it gets harder. If you had to construct the tower very quickly, it would probably fall down sooner than if you could build it slowly because you would be able to place each block just where you wanted it. In the same way, cooling a liquid very quickly doesn't let big crystals grow. Now imagine trying to build a tower with some cubical blocks and some that had a triangular shape! This wouldn't work very well either, because it's harder to fit the triangles into your pile. So, adding a molecule of another shape (like glucose and fructose in corn syrup) also makes it hard for crystals to get big.

So, was this explanation crystal clear???