P27 Rock Candy

Crystals as we saw in the last Project have regular shapes. Not everything we look at as a crystal is really a crystal like sugar. Can we grow sugar crystals?


The sugar crystals are called rock candy. If you used clean string and jars, you can eat it.


Question: How do different crystals grow large?




Measuring cups



Piece of charcoal


2 Jars




Growing sugar crystals:

Pour 1c water into a medium sized saucepan

Add 2 1/2c sugar

Heat as you stir to dissolve the sugar

Stop stirring when the sugar dissolves

Heat the solution to boiling [Be careful as it will quickly foam up and out of the pan when it boils.]

jars of sugar solution

Sugar solution fills both jars. Strings hang down from pencils. The sugar crystals are supposed to form on the strings.

Cut a length of string long enough to tie to a pencil and reach to the bottom of the jar

Tie the string to the pencil

Tie a little weight to the other end [I used a tiny nail.]

Wet the string

Pour the sugar solution into the jar almost to the top

Dangle the string in the solution so it doesn’t quite touch the bottom of the jar

Set the jar aside to let the sugar crystals form [This can take a week.}

Growing salt crystals:

Put a layer of salt 1cm deep on the bottom of the jar

charcoal after two days

In two days the salt layer has a smooth salt slurry on top. The charcoal is soaked.

Add water so it is 1cm over the salt

Set the piece of charcoal on the salt layer

Set the jar aside for the salt crystals to form [This can take a week.]


Describe the sugar solution you pour into the jar

Describe the sugar crystals each day for a week

sugar crystals

By day two sugar crystals covered the top of the solution but none were on the string.

Describe what happens in the jar of salt and charcoal every day for a week



Why do you put a weight on the end of the string?

Are the sugar crystals true crystals? Why do you think this?

In the last Project salt formed cubes as crystals. Does it form cubes on the charcoal?

charcoal pieces after three days

Look closely for thin white patches started on the pieces of charcoal. How does the salt get on top of the charcoal?

What is happening to the salt crystals on the charcoal?


What I Found Out:

Hot water dissolves a lot of sugar. The solution gets thick. I could see swirls as I stirred.

When I stopped stirring, the solution became very clear but slightly brown in color. It suddenly foamed up and out of the pan.

Plain string floats on the sugar solution. I used a tiny nail to add weight to the string so it dangled down into the solution in my two jars. The solution was too much for one jar.

In a few hours clear sugar crystals formed across the tops of my two jars. These had some hard bubbles or foam on top.

By the next day these top layers were thick. One jar had crystals forming on the bottom. No crystals were forming on the strings.

jar of rock candy

My rock candy formed across the top of the solution and the bottom of the jar. A few crystals are forming on the string and nail.

On the third day some crystals were forming on the string and nail in one jar and the string in the other one.

Nothing seemed to happen on the first day in the jar with the salt and charcoal. The charcoal was very wet and shiny.

By the second day I could see little white spots on the charcoal. These began to look like spider webs on the charcoal. Then little mounds started building up on the charcoal.

salt on charcoal

Water and salt seep up through the charcoal. The water evaporates leaving the salt behind. After four days there are little salt piles several places on the pieces of charcoal.

These salt crystals looked entirely different from last Project. They look like stalagmites in a cave. It looks like the salt water pulls up over the charcoal and the water evaporates leaving a mound of salt behind.


Having Fun

When you set up the charcoal, put food coloring on the charcoal. You can put several drops of different colors in different places or several drops of one color in different places. How does this change the salt crystals?

Look up the Devil’s Golf Course in Death Valley. This is the charcoal set up by nature on a gigantic scale.