Fig. 148. Alum crystals.
In both of the above experiments, the hot water was able to dissolve more of the alum than the cold water could possibly hold. So when the water cooled it could no longer hold the alum in solution. Therefore part of the alum turned to solid particles.
When the string was in the cooling liquid, it attracted the particles of alum as they crystallized out of the solution. The force of adhesion drew the near-by molecules to the string, then these drew the next, and these drew more, and so on until the crystals were formed. But when you kept stirring the liquid while it cooled, the crystals never had time to grow large before they were jostled around to some other part of the liquid or were broken by your stirring rod. Therefore they were small instead of large. Stirring or shaking a solution will always make crystals form more quickly, but it will also make them smaller.
How rock candy is made. Rock candy is made by hanging a string in a strong sugar solution or syrup and letting the water evaporate slowly until there is not enough water to hold all the sugar in solution. Then the sugar crystals gather slowly around the string, forming the large, clear pieces of rock candy. The sugar around the mouth of a syrup jug is formed in the same way.
You always get crystallization when you make a liquid too cool to hold the solid thing in solution, or when you evaporate so much of the liquid that there is not enough left to keep the solid thing dissolved.
When you make fudge, the sugar forms small crystals as the liquid cools. When a boat has been on the ocean, salt crystals form on the sails when the spray that has wet them evaporates.
But crystals may form also in the air. There is always some moisture in the air, and when it becomes very cold, some of this moisture forms crystals of ice. If they form up in the clouds, they fall as snow. If they form around blades of grass or on the sidewalk, as the alum crystals formed on the string, we have frost.
Still another place that crystals occur is in the earth. When the rocks in the earth were hot enough to be melted and then began to cool, certain substances in the rocks crystallized. Some of these crystals that are especially hard and clear constitute precious and semi-precious stones.
Application 64. Explain why you beat fudge as it cools; why the paper around butter becomes encrusted with salt if it is exposed to the air for some time.