Growing rock candy is a simple way to see how dissolved particles become solid crystals. In this project, students prepare sugar solutions at three saturation levels and watch crystals form on a skewer or string over one week. The experiment connects everyday candy making to chemistry ideas such as solubility, saturation, temperature, and crystal growth.
Careful measurements help show why some jars make many small crystals while others grow fewer, larger crystals.
Sugar dissolves better in hot water because higher temperature allows more sugar particles to spread through the liquid. As the solution cools or water slowly evaporates, the liquid can become supersaturated, meaning it holds more dissolved sugar than it normally can at that temperature. Extra sugar molecules then attach to rough spots on the skewer, string, or seed crystals, building an orderly crystal structure.
Comparing three concentrations and tracking crystal size over seven days lets students study how changing one variable affects the final crystal growth.
Key Facts
- Concentration = mass of solute / volume of solution, such as g/mL.
- Percent by mass = (mass of sugar / total mass of solution) x 100%.
- A saturated solution contains the maximum amount of dissolved sugar at a given temperature.
- Sugar solubility increases as temperature increases.
- Supersaturation happens when a solution contains more dissolved sugar than it can normally hold at that temperature.
- Crystal growth rate can be estimated with growth rate = change in crystal length / time.
Vocabulary
- Solute
- The substance that dissolves in a solvent, such as sugar dissolving in water.
- Solvent
- The substance that does the dissolving, such as water in a sugar solution.
- Saturated solution
- A solution that contains as much dissolved solute as possible at a certain temperature.
- Supersaturated solution
- An unstable solution that contains more dissolved solute than it normally can at its current temperature.
- Nucleation
- The first step of crystal formation when dissolved particles begin attaching to a surface or to each other.
Common Mistakes to Avoid
- Changing both sugar concentration and jar temperature at the same time makes the results hard to interpret because you cannot tell which variable caused the crystal differences.
- Adding undissolved sugar grains to the jar can create many extra nucleation sites, which may cause clumps instead of clear growth on the skewer or string.
- Moving or shaking the jars during the week can break small crystals loose, which changes the growth pattern and makes measurements less reliable.
- Comparing crystal size without using the same measurement method each day is misleading because length, mass, and visual estimates do not describe growth in the same way.
Practice Questions
- 1 A student dissolves 180 g of sugar in enough water to make 300 mL of solution. What is the sugar concentration in g/mL?
- 2 A crystal on a skewer is 4 mm long on day 2 and 18 mm long on day 7. What is its average growth rate in mm/day during that time?
- 3 Three jars contain low, medium, and high sugar concentrations, but only the high concentration jar forms crystals quickly after cooling. Explain why concentration and temperature together can make this jar more likely to become supersaturated.