adventures in growing sugar crystals

Lately I’ve been busy helping start-up a new science and engineering outreach program at my school. It’s been a ton of work, but it’s also been a fun and hugely rewarding experience. Earlier this year, we worked with a fifth-grade class in South Central LA and did everything from build speakers out of plastic cups to test the pH of common household liquids. We even helped the students learn about minerals and crystallization by growing sugar crystals.

Or rather, we tried to grow sugar crystals. You’d think that a hands-on activity aimed at grade-school students would be fairly fool-proof, right? Wrong! Growing sugar crystals turns out to be deceptively tricky, and not a single crystal grew. Luckily, the fifth-graders didn’t seem to mind. But I took it as a personal challenge to grow the largest sugar crystals I possibly could. It probably didn’t help that I’d experienced a similar sugar crystal fiasco in my own eighth-grade science class.

Well, three attempts and a minor sugar burn later…

So what was the secret? I think my sugar crystal success ultimately came down to the following three factors:

1. A (ridiculously) saturated sugar-water solution

Although it’s impossible to dissolve an infinite amount of sugar into a fixed volume of water, you can get a lot more sugar to dissolve in water when it’s hot. A solution (in this case the sugar-water) is said to be saturated when as much solute (sugar) has been dissolved as possible. The saturation point of a solution varies with temperature. At higher temperatures, more sugar will dissolve. As the solution is cooled, the sugar precipitates back out of the solution as a solid and can begin to form crystals.

2. “Seeding” the crystal with a sugar-coated string

Crystals form when atoms or molecules arrange into specific repeating patterns. A new crystal grows most easily by attaching itself to the surface of an existing crystal. These existing crystals are commonly referred to as “seeds” and act as a kind of molecular template as new molecules are added to the crystal. To create seeds for my sugar crystals, I dipped my string into a sugar-water solution and then let it dry. The solid bits of sugar left behind on the dried string acted as very small seeds and helped my much larger sugar crystal start to grow.

3. Cooling the sugar-water solution very very very slowly

Slowly cooling a hot, saturated sugar-water solution will cause sugar crystals to grow more slowly. Slower crystal formation gives each additional sugar molecule more time to settle into the most uniform arrangement on the growing crystal. You can slow down the cooling process by wrapping the sugar-water solution in towels or by placing it in a larger container full of hot water. This will give the sugar crystals more time to grow before the solution reaches room temperature, which will lead to the formation of bigger and more perfect sugar crystals.