Rock Candy: Recovering Solute Crystals
It's easy to recover a solute if you don't care about keeping the solvent. Leave a water solution open to the air and the solvent will evaporate, leaving the solute behind. To recover the solute from all kinds of beverages, put small amounts in shallow dishes. The water evaporates from the large surface area.
Some solutes form crystals as the solvent evaporates. Crystals are solids that have a regular geometric shape, with many sides or faces. take a close look at sugar and salt crystals with a magnifying lens. They have very different shapes. Rock candy is simply very large sugar crystals. Grow some in this experiment. Materials & Equipment
Procedure
Observations Watch the solution as it cools. Be careful not to jolt it or disturb it in any way. Does the solution remain clear? If it becomes cloudy, take a close look at it with a magnifying glass. A clear solution that contains more solute than would normally dissolve at that temperature is said to be super saturated. Supersaturated solutions are very unstable and the slightest disturbance will cause crystals to form, removing them from the solution. Some candy, like fudge, depends on the formation of millions of tiny crystals. When you beat fudge, tiny crystals form quickly from a supersaturated solution. If you don't beat fudge hard enough, the crystals will be larger and the fudge will feel grainy in your mouth. To make rock candy, you want large crystals to grow, and this takes time, sometimes weeks. Let the solution strand undisturbed at room temperature for a week or more. Every day, carefully break off the crust of crystals that forms at the surface so that the water can continue to evaporate. Rock candy crystals will form around any small object you put in the solution. Make rock candy lollipops by putting a swizzle stick in a class of supersaturated sugar solution. they will also quickly form around a crystal of sugar dropped into the solution. Such a crystal is called a seed crystal. You might want to try using a colored crystal sprinkle as a seed crystal in a supersaturated solution. The formation of crystals is one way chemists know when they've got pure matter, either an element or a compound. Crystals are also a clue to the structure of a substance. Scientists figure that the perfect shape of crystals is not an accident but is the result of a regular arrangement of the smallest particles of a substance. These particles, atoms and molecules, have a size (although it's way too small to see even with the strongest microscope) and a shape. The arrangement of the atoms or molecules of a crystal produces the shape of the crystal much as closely stacked bricks can produce only a rectangle stack. Compare the shape of the rock candy crystals to the crystals of granulated sugar with a magnifying glass. Are they the same shape? Do sugar crystals have the same shape as salt crystals? Would you expect sugar molecules to have the same shape as salt molecules? |