Experiment 11: Rock Candy — Supersaturation

Experiment 10 grew crystals by evaporation. Rock candy uses the other door into supersaturation: temperature. Dissolve sugar in hot water until it can hold no more, then simply let it cool — cold water cannot keep the sugar, and the excess must go somewhere. Given a rough string and a quiet week, it goes into the oldest sweet in the world: crystals you can eat.

The chemistry behind it

Sugar's solubility climbs steeply with temperature:

A syrup made near boiling with 400 g per 100 mL is comfortably dissolved while hot. Cooled to 20 °C it can only hold 200 g — the other ~200 g per 100 mL is excess solute that will crystallise out over the following days. That difference, the gap between the hot and cold solubility, is the theoretical maximum yield; everything in this experiment is about how much of it you can persuade onto the stick, in crystals big enough to admire. Two controls decide that: nucleation (a sugar-rolled stick offers ready-made seed surfaces) and cooling speed. Cool slowly and few nuclei form, so the excess sugar adds onto a handful of growing crystals — big, clear, regular. Cool fast (fridge) and nuclei appear everywhere at once: the same excess is shared among thousands of tiny crystals, and much of it snows down as sediment instead of growing on the stick.

Variables. Independent variable: cooling speed (or, in the virtual lab, starting concentration). Dependent variables: crystal yield on the stick (g) and average crystal size (mm). Controlled variables: syrup batch, jar, stick preparation, position, growing time.

Safety first

• An adult does all heating and handling of hot syrup — syrup burns are nasty because the sticky liquid clings to skin. Never taste syrup until it is fully cooled.
• This is the course's edible experiment, so kitchen hygiene applies properly: clean jar, clean stick, fresh ingredients, jar covered while growing, and discard the batch if anything fuzzy grows on it or it smells off.
• Let the jar cool before moving it; hot glass plus syrup is heavy and slippery.

Doing the investigation

Hands-on. With an adult: heat 1 cup of water to just below boiling and stir in sugar a spoonful at a time until no more dissolves (about 2–3 cups). Add colouring if you like, let the syrup cool slightly, and pour it into a clean jar. Wet a wooden skewer, roll it in sugar to seed it, and let the coating dry. Lower the skewer into the syrup — touching neither base nor walls — cover the jar loosely with a paper towel, and leave it completely undisturbed for 5–7 days. Photograph daily (through the glass), then lift the stick, let it drip, and weigh it.

In the virtual lab below (Rock Candy — Supersaturation) set the Sugar concentration in g per 100 mL and the Cooling speed (Slow / Medium / Fast), press Grow candy, and read how much sugar stayed dissolved, the crystal yield and the average crystal size. Follow its guided procedure: 200 g/100 mL slow first, then drop to 100 g/100 mL and explain the zero, then hold 220 g/100 mL and change only the cooling. Study the yield chart — below about 170 g/100 mL, slow cooling grows nothing at all. Log your best recipe in the observation log.

Worked investigation: how much of the excess can you catch?

One batch of syrup (400 g per 100 mL) split between three jars, grown 7 days:

And in the virtual lab (slow cooling throughout), concentration changed the catch:

Step-by-step processing:

Theoretical excess at 400 g/100 mL = 400 - 200 = 200 g per 100 mL
Best catch (slow cooling) = 46 g -> 46/200 = 23% of the excess
Crystal size, slow vs fast = 8 mm vs 2 mm -> 4x larger for the same sugar

Conclusion. The chemistry guarantees the excess *will* crystallise; your technique decides *where* and *how big*. Slow cooling caught the most sugar on the stick in the fewest, largest crystals, because few nuclei formed and each grew fat. Fast cooling spent the excess on a blizzard of tiny crystals and sediment. High starting concentration gives more total candy but a *worse catch rate* — the more violent the supersaturation, the more nucleation escapes your control and happens everywhere at once.

Uncertainty. Stick yield on a kitchen scale is ±1 g (about ±2% at 46 g) — but crystal size is the soft number: "average" over a cluster of uneven crystals is ±1 mm at best, ±12% on an 8 mm crystal. Weighing beats measuring for comparing recipes; use size as supporting evidence, ideally the mean of the ten largest crystals.

Evaluation: how good is this evidence?

• Syrup skin and surface crust. A crust on the surface seals in vapour and steals solute. Improvement: cover with a paper towel (done) and never stir after pouring.
• The stick competes with the jar exactly as in Experiment 10. Improvement: centre the stick, use a smooth clean jar, and weigh the sediment too so the mass balance is honest.
• "Slow" cooling is not a controlled rate. A jar beside a radiator and a jar in a cold pantry are different experiments. Improvement: cool all jars in the same insulated box so they share one temperature history.
• Opening the jar to peek drops dust (nuclei!) and germs into an edible product. Improvement: observe and photograph through the glass only.

Common mistakes

• Growing in a jar the syrup never supersaturated in — under about 200 g/100 mL at room temperature there is no excess and no candy, however long you wait.
• Fishing the stick out daily to admire it: every removal breaks crystals and seeds new ones in the syrup.
• Crediting the fridge batch's small crystals to "cold being bad for crystals" — the variable is cooling *speed* (nucleation rate), not cold itself.
• Eating a batch that grew mould or smells fermented — when in doubt, throw it out.

Exam Experience

Question 1 — calculation [4 marks]. A syrup is made at 350 g of sugar per 100 mL of water and cooled slowly to 20 °C, where solubility is 200 g per 100 mL. (a) Calculate the excess sugar per 100 mL that must crystallise. (b) The stick collects 45 g of crystals per 100 mL of syrup. What percentage of the excess was caught? (c) State one change that would increase the *size* of the crystals without changing the concentration.

Scoring guide:

• Excess = 350 − 200 = 150 g per 100 mL [1]
• Caught = 45 ÷ 150 = 30% [1]
• Change: cool more slowly (or seed more effectively / insulate the jar) [1]
• Reasoning: slower cooling means fewer nuclei, so the excess adds onto fewer crystals, which grow larger [1]

Question 2 — design [4 marks]. Design a fair test of whether cooling speed affects crystal size, using one batch of syrup. Include how you will measure "crystal size" fairly.

Scoring guide:

• One syrup batch split equally between identical jars with identical seeded sticks [1]
• Only the cooling regime differs (e.g. insulated box / open bench / fridge), same growing time and position otherwise [1]
• Size measured consistently — e.g. mean length of the 10 largest crystals per stick, same ruler, or photographs against a scale [1]
• Yield weighed as supporting data; decision rule stated before looking at results [1]

Multiple choice 1. Nothing grows on the stick when the syrup concentration is 150 g per 100 mL because:

• A. The stick was not seeded properly
• B. 150 g is below the 20 °C solubility (200 g), so the cooled syrup is not supersaturated at all
• C. Crystals only grow above 300 g per 100 mL
• D. The sugar has been destroyed by heating

Answer: B. Crystallisation needs excess solute over the cold solubility limit. At 150 g per 100 mL everything stays dissolved forever.

Multiple choice 2. Fast cooling produces many small crystals rather than a few large ones because:

• A. Cold sugar molecules cannot join a lattice
• B. Rapid supersaturation triggers nucleation in many places at once, so the excess is shared among countless crystals
• C. The fridge light damages growing crystals
• D. Cold syrup is thicker, and thick syrup makes small crystals

Answer: B. Crystal size is a competition between nucleation (starting new crystals) and growth (enlarging existing ones). Fast cooling floods the system with nuclei.

Going deeper: crystallisation you can eat (and spread)

Rock candy's slow patience is one extreme of a dial that food makers turn both ways. Fondant — the smooth filling in chocolate creams — is the fast extreme: a supersaturated syrup beaten vigorously as it cools, forcing nucleation in millions of places so the crystals stay microscopic and the texture reads as creamy rather than crunchy. Same sucrose, opposite cooling-and-disturbance regime, opposite crystal size: the Experiment 10 rule (few nuclei = big crystals) run deliberately in reverse.

Creamed honey is the seeded version. Liquid honey is mostly fructose and glucose; left alone, the glucose crystallises slowly into coarse, gritty grains. Beekeepers instead stir in a spoonful of finely crystalline honey as a *seed* and hold the batch cool — exactly your sugar-rolled stick, at jar scale — so the glucose copies the fine seed texture and sets smooth and spreadable. And your pre-seeded stick is doing professional work too: crystal growers call it secondary nucleation, offering a ready lattice so the solute never has to start from scratch. Whether the product is a sweet on a string, a jar of set honey or a silicon wafer, the craft is identical: choose your nuclei before the solution chooses them for you.