What makes the crystals grow in rock candy?

Crystals form in rock candy due to the process of crystallization. Here's what happens:

1. Supersaturation: The starting point for creating rock candy is a supersaturated sugar solution. Sugar is heated in water until it completely dissolves, resulting in a clear solution. When you reach the point where no more sugar can be dissolved at a given temperature, you have a supersaturated solution.

2. Cooling: The key to forming crystals is controlled cooling. The supersaturated solution is allowed to cool slowly. As the temperature drops, the solubility of sugar decreases, meaning that the water can no longer hold all the dissolved sugar.

3. Nucleation: As the solution cools, sugar molecules start to clump together and form tiny solid particles called nuclei. These nuclei serve as the centers where crystals will begin to grow.

4. Crystal Growth: Once nuclei are formed, sugar molecules from the solution start attaching themselves to the nuclei. Gradually, the crystals grow in size as more and more sugar molecules accumulate on their surface.

5. String or Stick: A string or stick is suspended in the solution, providing a surface for the sugar crystals to adhere to. The string acts as a support structure, allowing the crystals to grow larger without settling at the bottom of the container.

6. Evaporation: Evaporation also plays a role in crystal growth. As the water content in the solution reduces, the concentration of sugar increases, further promoting crystal formation.

The slow cooling and controlled evaporation process allows sugar crystals to form and grow on the string or stick, resulting in the beautiful, sugar crystal formations known as rock candy.