Big rock candy science | Science News for Students

Big rock candy science

Every sugar crystal needs to grow from a ‘seed’
Mar 5, 2019 — 6:30 am EST
a photo of green, purple, pink, red and light blue rock candy sticks

Does dipping sticks in sugar make a difference when make rock candy? Here’s an experiment to help you find out.


Making rock candy at home is a tasty way to show chemistry in action. But the instructions contain a step that seems a bit odd. You’re supposed to dip your candy stick or string in sugar at the start of the process. Doesn’t that seem like cheating somehow? And is it really necessary? I did an experiment to find out. It turns out that that sugar dip is definitely required. If you want any rock candy to eat, anyway.

It’s easy to make rock candy. All you need is a lot of sugar, some water and a little patience. Pour three cups of sugar into one cup of water, and bring your mix to a boil while you stir. Once the mix boils, the sugar will dissolve into the water. It quickly forms a clear solution. Pour the syrupy mixture into a glass. Hang a stick or string in the mix. Then walk away.

After a few days or a week, sugar crystals will have built up on the string, making a sticky-sweet candy. But the candy doesn’t look like the sugar you started with. The sugar molecules have instead become highly organized into a crystal structure.

A key step in this process is wetting the string or stick and then dipping it in sugar. The sugar clinging to the string or stick serves as a seed crystal. This is a crystal that promotes the growth of the bigger crystals of the rock candy.

Sugar molecules crystallize in a solution when they bump into each other and stick together. This first phase is called nucleation. Once a tiny crystal forms, it serves as a nucleation point. Other sugar molecules then glom on to it and make the crystal bigger. Seed crystals in the rock candy mix serve as this nucleation point, making the rock candy form faster.

How important are those seed crystals, though? To find out, I ran an experiment.

Seedy science

Every experiment starts with a hypothesis — a statement that can be tested. In this case, I’m testing whether seed crystals promote more rock candy formation. My hypothesis will be that using sticks with seed crystals will produce more rock candy than sticks without.

To test this hypothesis, I made two batches of rock candy. One batch, colored blue, will have no crystal seeding. I just put a clean stick into my sugar solution. This batch was my control — where nothing changes. The other batch, colored red, had sticks dipped in sugar before I put them in the sugar solution. To be able to measure if the seed crystals make a difference, I weighed the sticks (and the sugar on them) at the beginning and end of the experiment.

I wanted to make sure I had enough candy to be able to detect a difference in my samples. To do this, I would need to make 26 rock candy cups for each condition, for a total of 52 cups. That’s a lot. Unfortunately, I didn’t have enough sugar. I ended up with nine cups in each group.

a short animated image showing how to create seed crystals on a rock candy stick
This is how you create seed crystals on your rock candy stick. 
B. Brookshire/SSP

Here’s how to make this rock candy:

  • Take 18 clean pieces of string or wooden skewers, like the ones used to grill kebabs. Set half aside. For the other half, dip the last 12.7 centimeters (5 inches) of the end of the skewer or string into a cup of clean water, then roll it in a small pile of sugar. Set each aside to dry. (If you want to eat your experimental results, make sure you use the blunt ends of the skewers, so you don’t end up poking yourself in the mouth.)
  • Set out 18 clear plastic or glass cups.
  • Meanwhile, bring 4 cups (512 grams) of water and 12 cups (1.5 kilograms) of sugar to a boil in a pot, stirring. Keep an eye on your mix. I walked out on mine, and my sugary solution boiled over and soaked my floor in a sticky mess. Lesson learned.
  • Once the solution is clear, add food coloring to get the desired color. I used blue for my control, and red for my seed crystal-covered skewers.
  • Using a measuring cup, pour 250 milliliters (8.4 fluid ounces) of the solution into each cup. You should have enough for about nine cups of blue.
  • Use a scale to find the mass of each stick in grams (each of mine weighed about two grams). Once you have noted the mass, dip the stick carefully into a cup of the sugar solution, and secure it in place. Make sure the stick does not touch the bottom or sides of the cup. I taped my grill skewer to another skewer placed across each cup. But you can also use pieces of string that are tied to a skewer and dangle down into the solution.
  • Make another batch of your solution, this time coloring it red, and using your seeded skewers. Make sure to weigh each skewer before you dip it in the solution.
  • Put all your cups in a cool dry place where they will not be disturbed.
  • Wait.

ingredients needed to make rock candy

a photo of sugar boiling on a stovetop

rock candy growth experiment set up

finished rock candy

After a day or so, you might be able to see crystals beginning to grow. The longer you leave the experiment, the bigger your crystals will get, but three days is enough to detect a difference.

After three or more days, get out your scale again. Carefully crack the sugary film on top of each cup with a spoon (this part is very satisfying). Remove the stick or string in the cup, make sure it’s not dripping, and weigh it.

Sweet, sweet results

a table that shows a tally of growth on seeded and unseeded sticks
This table tallies crystal growth on unseeded (control) and seeded sticks.  
B. Brookshire/SSP

To find out how much rock candy I got in each group, I subtracted the weight of the stick at the beginning of the experiment from the weight of the stick and candy at the end. This gave me a measure of crystal growth in grams. I made a spreadsheet with the average mass of crystals from both conditions. At the bottom of each column, I calculated the mean — the average crystal mass — for each group.

My unseeded sticks grew 1.3 grams of rock candy on average. It didn’t look like a very tasty treat.

My seeded sticks, however, grew about 4.8 grams of rock candy on average. It wasn’t a lot, but it definitely looked like dessert.

But were these two groups truly different? To find out, I needed to run some statistics — tests to interpret the meaning of my results. I used a t test. This is a test that finds differences between two groups. There are free programs that will let you put in your data and run these tests. I used one from Graphpad Prism.

A t test will give you a p value. This is a probability measure. In this case, it’s a measure of how likely it is that I would find by accident alone a difference as big as the one I found. A p value of less than 0.05 (or five percent) is considered by many scientists to be statistically significant. My p value was 0.00003. That’s a 0.003 percent chance that this difference happened by chance. That seemed pretty good.

But I also wanted to find out how big the difference was. I used a measure called Cohen’s d. For this, I needed a standard deviation — a measure of how much my data spread out around the mean (a previous post has more detail). I used another free online calculator for this calculation.

My Cohen’s d for this experiment was 2.19. Generally, scientists count any Cohen’s d above 0.8 as a large difference. So my difference was pretty large. I made a graph of my results. 

a graph showing that my seeded sticks grew larger crystals than my unseeded sticks
This is a graph showing that my seeded sticks grew larger crystals than my unseeded sticks.   
B. Brookshire/SSP

Based on the results of my experiment, it’s clear that those tiny seed crystals are an important rock candy hack. My hypothesis was that using sticks with seed crystals will produce more rock candy than sticks without. This experiment supports that hypothesis. 

This study had limitations, though — things I could have done better. I had only nine cups per groups, which is definitely not enough. Next time, I need more sugar and more cups. In addition, while I looked at the total mass of the rock candy, I didn’t look at how fast it formed. I would need to weigh my candy every day of the experiment to look at the speed of my candy crystal formations. I clearly need to do more experiments. I guess I’ll just have to make more rock candy.

Materials List

Granulated sugar (3 bags, $6.36 each)

Grill skewers (pack of 100, $4.99)

Clear plastic cups (pack of 100, $6.17)

Large pot (4 quarts, $11.99)

Measuring cups ($7.46)

Scotch tape ($1.99)

Food coloring ($3.66)

Roll of paper towels ($0.98)

Nitrile or latex gloves ($4.24)

Small digital scale ($11.85)

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Power Words

(more about Power Words)

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

boil     To heat a liquid to the temperature at which it turns to vapor.

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.

control     A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that gives scientists something against which they can compare their experimental data.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

dissolve     To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)

glass     A hard, brittle substance made from silica, a mineral found in sand. Glass usually is transparent and fairly inert (chemically nonreactive). Aquatic organisms called diatoms build their shells of it.

hypothesis     (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

mean     One of several measures of the “average size” of a data set. Most commonly used is the arithmetic mean, obtained by adding the data and dividing by the number of data points.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nucleation     (in chemistry) The process of new structures forming by self-assembly, or the start of a new phase in a chemical process that happens spontaneously. For instance, it can mark the initial stage of crystal formation, when a liquid or gas begins to form organized crystals.

p value    (in research and statistics) This is the probability of seeing a difference as big or bigger than the one observed if there is no effect of the variable being tested. Scientists generally conclude that a p value of less than five percent (written 0.05) is statistically significant, or unlikely to occur due to some factor other than the one tested.

probability     A mathematical calculation or assessment (essentially the chance) of how likely something is to occur.

solution     A liquid in which one chemical has been dissolved into another.

standard deviation     (in statistics) The amount that each a set of data varies from the mean.

statistics     The practice or science of collecting and analyzing numerical data in large quantities and interpreting their meaning. Much of this work involves reducing errors that might be attributable to random variation. A professional who works in this field is called a statistician.