Cooling Rate and Crystal Size

This demonstration shows how the rate at which a saturated copper(II) sulfate solution cools affects the size of the crystals that form. When a hot saturated solution of copper sulfate is cooled slowly, large and well-defined blue crystals form. Rapid cooling produces many small, less regular crystals. The experiment illustrates how crystal size depends on the rate of nucleation and growth.

1. Dissolve approximately 40 g of copper(II) sulfate crystals in 100 mL of hot distilled water in a beaker, stirring until no more dissolves (a saturated solution).
2. Filter the hot solution into two clean beakers to remove any undissolved solids.
3. Label one beaker A (slow cooling) and the other B (rapid cooling).
4. Beaker A: Allow the hot solution to cool slowly to room temperature undisturbed. Optionally cover it with a watch glass to minimize dust.
5. Beaker B: Place the second beaker in an ice bath for rapid cooling.
6. Leave both beakers for several hours or overnight.
7. Observe and compare the crystals formed in each beaker, noting their size, shape, and clarity.

Effect of cooling rate on crystal size - Stuart Kerr:

Junior Science - Igneous Rocks & Crystal Size | Does cooling rate make a difference? - The Crazy Scientist:

📄 Cooling Rate and Crystal Size - Seth Stein: https://sites.northwestern.edu/sethstein/a-small-is-beautiful-approach-to-upgrading-a-beginning-geophysics-course/cooling-rate-and-crystal-size/

• Try repeating the experiment with other soluble salts (e.g., alum or sodium thiosulfate).
• Compare results using controlled cooling at different rates (room temperature, warm water bath, refrigerator, ice bath).
• Evaporate a copper sulfate solution at room temperature without heating to see the effect of evaporation rate.

• Copper(II) sulfate is harmful if ingested, handle with care and wash hands after use.
• Avoid contact with skin and eyes; wear gloves and safety goggles.
• Use caution when heating solutions, handle hot glassware with tongs or heatproof gloves.
• Dispose of copper sulfate waste in a labeled container as per school chemical disposal rules.

• Why does slow cooling produce larger crystals than rapid cooling?
• What is the relationship between nucleation rate and crystal size?
• How does supersaturation affect crystal growth?
• What industrial processes rely on controlling crystal size (e.g., sugar refining, pharmaceuticals)?
• How might impurities affect the crystal growth process?